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V A Zammit

Publications and source records attributed to V A Zammit.

At least 127 records · Page 7Linked to original sources
PubMed1982

Changes in the proportion of acetyl-CoA carboxylase in the active form in rat liver. Effect of starvation, lactation and weaning.

1. The activity of acetyl-CoA carboxylase (EC 6.4.1.2) in extracts of freeze-clamped liver samples from fed or 24 h-starved virgin, pregnant, lactating and weaned rats was measured (i) immediately after preparation of extracts (;I activity'), (ii) after incubation of extracts with partially purified preparations of either rabbit muscle protein phosphatase 1 [Antoniw, Nimmo, Yeaman & Cohen (1977) Biochem. J.162, 423-433] or rabbit liver phosphatase [Brandt, Capulong & Lee (1975) J. Biol. Chem.250, 8038-8044] (;A activity') and (iii) after incubation with 20mm-potassium citrate before or after incubation with phosphatases (;C activity'). 2. Incubation of liver extracts at 30 degrees C without any additions resulted in activation of acetyl-CoA carboxylase that was shown to be due to dephosphorylation of the enzyme by endogenous protein phosphatase activity. This latter activity was not stimulated by Ca(2+) and/or Mg(2+) but was stimulated by 1 mm-Mn(2+). Incubation of extracts with either of the partially purified phosphatases (0.2-0.5 unit) resulted in faster dephosphorylation and activation. The activity achieved after incubation with either of the exogenously added phosphatases was similar. 3. The A and C activities increased during late pregnancy, were lower than in the virgin rat liver during early lactation and increased by 2-fold in liver of mid-lactating rats. Weaning of mid-lactating rats for 24 h resulted in no change in A and C activities but after 48 h weaning they were significantly lower than those in livers from suckled mothers. 4. The I activity followed a similar pattern of changes as the A and C activities during pregnancy and lactation such that, although the I/A and I/C activity ratios tended to be lower during late pregnancy and early lactation, there were no significant changes in I/A and I/C ratios between lactating and virgin animals. However, these ratios were significantly higher in liver from fed 24 h-weaned animals. 5. Starvation (24 h) resulted in a marked decrease in I activity for all animals studied except early-lactating rats. This was due to a combination of a decrease in the concentration of acetyl-CoA carboxylase in liver of starved animals (A and C activities) and a decrease in the fraction of the enzyme in the active form (lower I/C and I/A ratios). The relative importance of the two forms of regulation in mediating the starvation-induced fall in I activity was about equal in livers of virgin, pregnant and lactating animals. However, the decrease in I/A and I/C ratios was of dominating importance in livers of weaned animals. The A/C activity ratios were the same for livers from all animals studied. 6. The maximal activity of fatty acid synthase was also measured in livers and was highly and positively correlated with the A and C activities of acetyl-CoA carboxylase, suggesting that the concentrations of the two enzymes in the liver were controlled coordinately. 7. It is suggested that the lack of correlation between plasma insulin levels and rates of lipogenesis in the transition from the virgin to the lactating state may be explained by different effects of insulin and prolactin on the concentration of acetyl-CoA carboxylase in the liver and on the fraction of the enzyme in the active form.

Acetyl-CoA Carboxylase↗
PubMed1982

Regulation of acetyl-CoA carboxylase in rat mammary gland. Effects of starvation and of insulin and prolactin deficiency on the fraction of the enzyme in the active form in vivo.

The ;initial' (I), endogenous phosphatase-activated (A) and citrate-activated (C) activities of acetyl-CoA carboxylase were measured in mammary-gland extracts of pregnant and lactating rats. There was a 10-fold increase in the A and C enzyme activities in the transition from early to peak lactation [cf. data of Mackall & Lane (1977) Biochem. J.162, 635-642], but there was no significant increase in the ratio of the initial activity to the A and C activities of the enzyme. Starvation (24h) or short-term (3h) streptozotocin-induced diabetes both resulted in a 40% decrease in I/A and I/C activity ratios. In starvation this was accompanied by a decrease in the absolute values of the A and C activities such that the initial activity in mammary glands of starved animals was 45% that in glands from fed animals. Insulin treatment of starved or diabetic animals 60min before killing increased the I activity without affecting the A or C enzyme activities. Removal of the pups for 24h from animals in peak lactation (weaning) resulted in a marked but similar decrease in all three activities such that, although the initial activity was only 10% of that in suckled animals, the I/A and I/C activity ratios remained high and unaltered. Inhibition of prolactin secretion by injection of 2-bromo-alpha-ergocryptine gave qualitatively similar results to those during weaning. Simultaneous administration of ovine prolactin completely prevented the effects of bromoergocryptine. It is suggested that the initial activity of acetyl-CoA carboxylase in rat mammary gland is regulated by at least two parallel mechanisms: (i) an acute regulation of the proportion of the enzyme in the active state and (ii) a longer-term modulation of enzyme concentration in the gland. Insulin appeared to mediate its acute effects through mechanism (i), whereas prolactin had longer-term effects on enzyme concentration in the gland. A comparison of initial enzyme activities (I) obtained in the present study with rates of lipogenesis measured in vivo [Agius & Williamson (1980) Biochem. J.192, 361-364; Munday & Williamson (1981) Biochem. J.196, 831-837] gave good agreement between the two sets of data for all conditions studied except for 24h-starved and streptozotocin-diabetic animals. It is suggested that acetyl-CoA carboxylase activity is rate-limiting for lipogenesis in the mammary gland in normal, fed, suckled or weaned animals but that in starved and short-term diabetic animals changes in the activity of the enzyme by covalent modification alone may not be sufficient to maintain the enzyme in its rate-limiting role.

Acetyl-CoA Carboxylase↗
PubMed1981

Regulation of acetyl-CoA carboxylase in rat mammary gland. Effects of incubation with Ca2+, Mg2+ and ATP on enzyme activity in tissue extracts.

1. The effect of preincubation of extracts of lactating rat mammary gland with ATP, Mg2+ and micromolar concentrations of Ca2+ on the activity of acetyl-CoA carboxylase was studied. 2. Both Mg2+ and Ca2+ activated the enzyme. Activation with Mg2+ (5 mM) was larger than that with Ca2+ (calculated free Ca2+ concentration = 20-50 microM), but the activity decreased after reaching a peak. The activation obtained with Ca2+ was stable for up to 180 min. 3. Incubation with Ca2+ and Mg2+ together resulted in an activation that was slightly higher than that with Mg2+ only and was stable (compare the results for Ca2+ alone). 4. Preincubation in the absence of Mg2+, but not in the absence of Ca2+, resulted in the impairment of subsequent activation with either Mg2+ (when preincubation was with Ca2+ alone) or Mg2+ plus Ca2+. 5. KF (50 mM) prevented the activation of acetyl-CoA carboxylase by Ca2+ and Mg2+. 6. MgATP2- reversed (Mg2+ + Ca2+)-mediated activation and decreased the activity of acetyl-CoA carboxylase to about 10% of initial activity. Inhibition by ATP was unaffected by addition of cyclic AMP or cyclic AMP-dependent protein kinase inhibitor. 7. 32P was incorporated into acetyl-CoA carboxylase when incubations were carried out in the presence of [gamma-32P]ATP. Subsequent removal of ATP from the incubation medium resulted in rapid loss of 32P from acetyl-CoA carboxylase. 8. It is suggested that extracts of rat mammary gland contain endogenous protein kinase and phosphatase activities that modulate acetyl-CoA carboxylase activity through reversible phosphorylation and dephosphorylation. The phosphatase activity is sensitive to both Mg2+ and micromolar concentrations of Ca2+, whereas the kinase does not appear to be cyclic AMP-dependent.

Acetyl-CoA Carboxylase↗
PubMed1981

Regulation of hepatic fatty acid metabolism. The activities of mitochondrial and microsomal acyl-CoA:sn-glycerol 3-phosphate O-acyltransferase and the concentrations of malonyl-CoA, non-esterified and esterified carnitine, glycerol 3-phosphate, ketone bodies and long-chain acyl-CoA esters in livers of fed or starved pregnant, lactating and weaned rats.

1. The concentrations of malonyl-CoA, glycerol 3-phosphate, non-esterified carnitine, acid-soluble and acid-insoluble acylcarnitines, acetoacetate, 3-hydroxybutyrate and acid-insoluble acyl-CoA were measured in rapidly-frozen liver samples from fed or starved (24h) virgin, pregnant (19-20 days), lactating (2, 10-12 and 18-20 days) and weaned (for 24h, on 10th day of lactation) rats. The activities of total and N-ethylmaleimide-sensitive and -insensitive glycerophosphate acyltransferase (acyl-CoA:sn-glycerol 3-phosphate O-acyltransferase; EC 2.3.1.15) were also measured. 2. The concentration of malonyl-CoA was significantly higher in liver of fed pregnant, mid- and late-lactating rats than in liver of fed virgin rats. After starvation for 24h hepatic malonyl-CoA concentrations were higher in mid-lactating rats and lower in pregnant and weaned rats than in virgin animals. 3. After starvation for 24h the hepatic concentrations of glycerol 3-phosphate, ketone bodies, acid-soluble acylcarnitines and the value for the [3-hydroxybutyrate]/[acetoacetate] ratio were all highest in pregnant rats, intermediate in virgin, 2-day lactating and weaned animals and lowest in mid- and late-lactating rats. The concentrations of acid-insoluble acylcarnitines also increased most in pregnant rats, after starvation. The concentration of acid-insoluble acyl-CoA increased equally after starvation in virgin and pregnant animals but did not increase significantly in all other animals studied. 4. The total concentration of carnitine was similar in livers of fed virgin, pregnant and 2-day lactating animals but fell markedly by the 10th day of lactation and remained low in late-lactating animals. The concentration of non-esterified carnitine followed the same pattern. After starvation for 24h the hepatic concentration of non-esterified carnitine decreased significantly in virgin, pregnant and 2-day lactating animals, but remained unchanged in mid- and late-lactating or weaned animals. 5. The activities of N-ethylmaleimide-sensitive and -insensitive glycerophosphate acyltransferase both increased significantly in livers of mid-lactating animals. After starvation for 24h the activity of the N-ethylmaleimide-insensitive O-acyltransferase decreased in livers of virgin, pregnant and mid-lactating animals, whereas the activity of the N-ethylmaleimide-sensitive O-acyltransferase was unchanged in virgin animals but decreased markedly in livers of pregnant and lactating rats. 6. The results are discussed in relation to the importance of different metabolic parameters in the regulation of long-chain acyl-CoA metabolism in the liver.

Acyl Coenzyme A↗
PubMed1980

The effect of glucagon treatment and starvation of virgin and lactating rats on the rates of oxidation of octanoyl-L-carnitine and octanoate by isolated liver mitochondria.

1. Oxygen-consumption rates owing to oxidation of octanoate or octanoylcarnitine by isolated mitochondria from livers of fed, starved and glucagon-treated virgin or 12-day-lactating animals were measured under State-3 and State-4 conditions, in the presence or absence of l-malate and inhibitors of tricarboxylic acid-cycle activity (malonate and fluorocitrate). 2. Mitochondria from fed lactating animals had a slightly lower rate of octanoylcarnitine oxidation than did those of fed virgin animals, whereas the rates of octanoate oxidation were unaffected. 3. Starvation of virgin animals for 24h or 48h resulted in a large (70-100%) increase in mitochondrial octanoylcarnitine oxidation; rates of octanoate oxidation were either unaffected (24 and 48h starvation in the absence of malonate and fluorocitrate) or diminished by 30% (48h starvation in the presence of inhibitors). In lactating animals, 24h starvation resulted in a smaller increase in the rate of octanoylcarnitine oxidation than that obtained for mitochondria from virgin rats. 4. Glucagon treatment (by intra-abdominal injection) of fed virgin and lactating rats increased the rate of mitochondrial oxidation of both octanoylcarnitine and octanoate. Injection of glucagon into 48h-starved virgin rats did not increase further the already elevated rate of octanoylcarnitine oxidation, but reversed the inhibition of octanoate beta-oxidation observed for these mitochondria in the presence of malonate and fluorocitrate. 5. It is suggested that glucagon activates octanoylcarnitine oxidation by increasing the activity of the carnitine/acylcarnitine transport system [Parvin & Pande (1979) J. Biol. Chem.254, 5423-5429] and that the increase in octanoate oxidation by mitochondria from glucagon-treated animals is caused by the increased rate of ATP synthesis in these mitochondria. 6. The results are discussed in relation to the increased capacity of the liver to oxidize long-chain fatty acids and carnitine esters of medium-chain fatty acids under conditions characterized by increased ketogenesis.

Animals↗
PubMed1980

Activities of 3-hydroxybutyrate dehydrogenase, 3-oxoacid CoA-transferase and acetoacetyl-CoA thiolase in relation to ketone-body utilisation in muscles from vertebrates and invertebrates.

1. The activities of 3-hydroxybutyrate dehydrogenase were non-detectable in muscles of invertebrates and marine teleost fish; activities were found in muscles of amphibia, reptiles and mammals and also in an elasmobranch fish. Muscles were classified into three groups according to the activities of 3-oxoacid CoA-transferase: muscles with very low activities (less than 0.01 mumol x min-1 x g-1) which obtain energy for contraction from anaerobic glycolysis; muscles with low activities (greater than 0.01 less than 5 mumol x min-1 x g-1) which include insect flight muscles, muscles of other invertebrates and skeletal muscles of higher vertebrates; muscles with high activities of 3-oxoacid CoA-transferase (greater than 5 mumol x min-1 x g-1) which are characterised by continuous mechanical activity for long periods of time, e.g. heart, diaphragm, postural and some smooth muscles of mammals. 2. It is suggested that ketone bodies may be important fuels for muscles in the very low and low activity groups during starvation, when the muscle is at rest. The muscles in the high activity group may use ketone bodies when they are available in the blood to provide energy for mechanical activity. Since these muscles provide a continuous vital physiological function, they must always be provided with a fuel for respiration and, in a similar manner to brain, they may utilise either glucose or ketone bodies.

Acetyl-CoA C-Acetyltransferase↗
PubMed1979

Activities of enzymes of fat and ketone-body metabolism and effects of starvation on blood concentrations of glucose and fat fuels in teleost and elasmobranch fish.

1. Activities of 3-oxo acid CoA-transferase and carnitine palmitoyltransferase together with tri- and di-acylglycerol lipase were present in red and heart muscles of the teleost fish. However, d-3-hydroxybutyrate dehydrogenase activity was not detectable. These results suggest that the heart and red muscles of the teleosts should be able to utilize the fat fuels triacylglycerol, fatty acids or acetoacetate, but not hydroxybutyrate. The muscles from the elasmobranchs differed in that d-3-hydroxybutyrate dehydrogenase and 3-oxo acid CoA-transferase activities were present, but carnitine palmitoyltransferase activity was not detectable. This suggests that ketone bodies are the most important fat fuels in elasmobranchs. 2. The concentrations of acetoacetate, 3-hydroxybutyrate, glycerol, non-esterified fatty acids and triacylglycerols were measured in blood or plasma of several species of fish (teleosts and elasmobranchs) in the fed state. Teleosts have a 10-fold higher concentration of plasma non-esterified fatty acids, but a lower blood concentration of ketone bodies; both acetoacetate and 3-hydroxybutyrate are present in blood of elasmobranchs, whereas 3-hydroxybutyrate is absent from that of the teleosts. 3. The effects of starvation (up to 150 days) on the concentrations of blood metabolites were studied in a teleost (bass) and an elasmobranch (dogfish). In the bass there was a 60% decrease in blood glucose after 100 and 150 days starvation. In dogfish there was a large increase in the concentration of ketone bodies, whereas in bass the concentration of acetoacetate (the only ketone body present) remained low (<0.04mm) throughout the period of starvation. The concentration of plasma non-esterified fatty acids increased in bass, but decreased in dogfish. These changes are consistent with the predictions based on the enzyme-activity data. 4. Starvation did not change the activities of ketone-body-utilizing enzymes or that of phosphoenolpyruvate carboxykinase in heart and red skeletal muscles of both fish, but it decreased markedly the activity of phosphoenolpyruvate carboxykinase in white skeletal muscle of both fish. However, in the liver of the dogfish, starvation resulted in a twofold increase in the activities of 3-hydroxybutyrate dehydrogenase and acetoacetyl-CoA thiolase, whereas in bass liver it decreased the activity of acetoacetyl-CoA thiolase and increased that of 3-oxo acid CoA-transferase. The activity of phosphoenolpyruvate carboxykinase was increased twofold in the liver of bass, but was unchanged in that of the dogfish. 5. The difference in changes in concentrations of blood metabolites and enzyme activities in the two fish support the suggestion that, in starvation, ketone bodies, but not non-esterified fatty acids, are an important fuel for muscle in elasmobranchs, whereas non-esterified fatty acids, but not ketone bodies, are an important fuel in teleosts. The results are discussed in relation to the evolution of a discrete lipid-storing adipose tissue in teleosts and higher vertebrates.

Animals↗
PubMed1979

Use of enzyme activities as indices of maximum rates of fuel utilization.

It can be shown theoretically and experimentally that the maximum activities in vitro of enzymes that catalyse near-equilibrium reactions in vivo must be considerably higher than the maximum flux through that pathway. Consequently, the activities of such enzymes cannot provide quantitative information on the maximum possible flux through a pathway. On the other hand, the maximum activity of an enzyme that catalyses a non-equilibrium reaction in vivo may provide quantitative information. Such possibilities must be tested experimentally. Thus the maximum flux through a given metabolic pathway is measured (or calculated) and compared with the maximum in vitro activities of enzymes that catalyse non-equilibrium reactions in that pathway. Catalytic activities similar to the flux suggest that such enzymes may be useful as flux indicators. For example, phosphorylase or phosphofructokinase activities provide a quantitative indication of maximum flux through glycolysis-from-glycogen (i.e. anaerobic glycolysis); hexokinase activities provide a quantitative indication of maximum flux through glycolysis-from-glucose; 2-oxoglutarate dehydrogenase activities provide a quantitative indication of maximum flux through the citric acid cycle. The advamtages of the use of enzyme activities in this manner include simplicity, general applicability to pathways, tissues and animals, and minimum intervention (particularly in larger animals including the human species). One disadvantage is that the properties of the enzyme must be known in detail before an assay that gives maximum activities can be developed, and the properties of enzymes that catalyse non-equilibrium reactions may be complex. These considerations emphasize the dangers of quantitative interpretation of the maximum flux through pathways from 'near-equilibrium' enzymes or from 'non-equilibrium' enzymes whose properties have been inadequately studied.

Adenosine Triphosphate↗
PubMed1979

Photodynamic action of bonellin, an integumentary chlorin of Bonellia viridis, Rolando (Echiura, Bonelliidae).

1. The photodynamic activity of bonellin, an integumentary chlorin of Bonellia viridis, is investigated. 2. 10(-6) M bonellin solutions haemolyze erythrocytes only in the presence of light. Previous illumination (tungsten lamp 2000-4000 lux for 1 hr) of the bonellin solutions does not affect the results. Under lowered oxygen tensions the bioactivity is depressed. Benzoquinone and singlet oxygen quenchers delay the photodynamic effect. 3. Bonellin (2 x 10(-6) M) destroys echinoid gamete function, depresses oxygen uptake of spermatozoa, and arrests development of echinoid and Bonellia eggs. These effects are produced only in the presence of light. 4. Copper bonellin is not photodynamically active. The role of copper may be to protect against photosensitization.

Animals↗
PubMed1978

Properties of pyruvate kinase and phosphoenolpyruvate carboxykinase in relation to the direction and regulation of phosphoenolpyruvate metabolism in muscles of the frog and marine invertebrates.

1. The properties of pyruvate kinase and, if present, phosphoenolpyruvate carboxykinase from the muscles of the sea anemone, scallop, oyster, crab, lobster and frog were investigated. 2. In general, the properties of pyruvate kinase from all muscles were similar, except for those of the enzyme from the oyster (adductor muscle); the pH optima were between 7.1 and 7.4, whereas that for oyster was 8.2; fructose bisphosphate lowered the optimum pH of the oyster enzyme from 8.2 to 7.1, but it had no effect on the enzymes from other muscles. Hill coefficients for the effect of the concentration of phosphoenolpyruvate were close to unity in the absence of added alanine for the enzymes from all muscles except oyster adductor muscle; it was 1.5 for this enzyme. Alanine inhibited the enzyme from all muscles except the frog; this inhibition was relieved by fructose bisphosphate. Low concentrations of alanine were very effective with the enzyme from the oyster (50% inhibition was observed at 0.4mm). Fructose bisphosphate activated the enzyme from all muscles, but extremely low concentrations were effective with the oyster enzyme (0.13mum produced 50% activation). 3. In general, the properties of phosphoenolpyruvate carboxykinase from the sea anemone and oyster muscles are similar: the K(m) values for phosphoenolpyruvate are low (0.10 and 0.13mm); the enzymes require Mn(2+) in addition to Mg(2+) for activity; and ITP inhibits the enzymes and the inhibition is relieved by alanine. These latter compounds had no effect on enzymes from other muscles. 4. It is suggested that changes in concentrations of fructose bisphosphate, alanine and ITP produce a coordinated mechanism of control of the activities of pyruvate kinase and phosphoenolpyruvate carboxykinase in the sea anemone and oyster muscles, which ensures that phosphoenolpyruvate is converted into oxaloacetate and then into succinate in these muscles under anaerobic conditions. 5. It is suggested that in the muscles of the crab, lobster and frog, phosphoenolpyruvate carboxykinase catalyses the conversion of oxaloacetate into phosphoenolpyruvate. This may be part of a pathway for the oxidation of some amino acids in these muscles.

Alanine↗
PubMed1978

Maximum activities and effects of fructose bisphosphate on pyruvate kinase from muscles of vertebrates and invertebrates in relation to the control of glycolysis.

1. Comparison of the maximum activities of pyruvate kinase with those of phosphofructokinase in a large number of muscles from invertebrates and vertebrates indicates that, in general, in any individual muscle, the activity of pyruvate kinase is only severalfold higher than that of phosphofructokinase. This is consistent with the suggestion, based on mass-action ratio data, that the pyruvate kinase reaction is non-equilibrium in muscle. However, the range of activities of pyruvate kinase in these muscles is considerably larger than that of phosphofructokinase. This difference almost disappears if the enzyme activities from muscles that are known to possess an anaerobic ;succinate pathway' are excluded. It is suggested that, in these muscles, phosphofructokinase provides glycolytic residues for both pyruvate kinase (i.e. glycolysis) and phosphoenolpyruvate carboxykinase (i.e. the succinate pathway). This is supported by a negative correlation between the activity ratio, pyruvate kinase/phosphofructokinase, and the activities of nucleoside diphosphokinase in these muscles, since high activities of nucleoside diphosphokinase are considered to indicate the presence of the succinate pathway. 2. The effect of fructose bisphosphate on the activities of pyruvate kinase from many different muscles was studied. The stimulatory effect of fructose bisphosphate appears to be lost whenever an efficient system for supply of oxygen to the muscles is developed (e.g. insects, squids, birds and mammals). This suggests that activation of pyruvate kinase is important in the co-ordinated regulation of glycolysis in anaerobic or hypoxic conditions, when the change in glycolytic flux during the transition from rest to activity needs to be large in order to provide sufficient energy for the contractile activity. However, lack of this effect in the anaerobic muscles of the birds and mammals suggests that another metabolic control may exist for avian and mammalian pyruvate kinase in these muscles.

Animals↗
PubMed1978

The role of creatine kinase and arginine kinase in muscle.

Arginine and creatine kinase activities in different muscles are compared with calculated maximum rates of ATP turnover. The magnitude of the kinase activities decreases in the following order: anaerobic muscles and vertebrate skeletal muscles greater than heart muscle greater than insect flight muscle. The maximum activity of phosphagen kinases (i.e. creatine kinase and arginine kinase), in the direction of phosphagen formation, is lower than the calculated maximum rate of ATP turnover in insect flight muscle or rat heart.

Adenosine Triphosphate↗
PubMed1976

The maximum activities of hexokinase, phosphorylase, phosphofructokinase, glycerol phosphate dehydrogenases, lactate dehydrogenase, octopine dehydrogenase, phosphoenolpyruvate carboxykinase, nucleoside diphosphatekinase, glutamate-oxaloacetate transaminase and arginine kinase in relation to carbohydrate utilization in muscles from marine invertebrates.

Comparison of the activities of hexokinase, phosphorylase and phosphofructokinase in muscles from marine invertebrates indicates that they can be divided into three groups. First, the activities of the three enzymes are low in coelenterate muscles, catch muscles of molluscs and muscles of echinoderms; this indicates a low rate of carbohydrate (and energy) utilization by these muscles. Secondly, high activities of phosphorylase and phosphofructokinase relative to those of hexokinase are found in, for example, lobster abdominal and scallop snap muscles; this indicates that these muscles depend largely on anaerobic degradation of glycogen for energy production. Thirdly, high activities of hexokinase are found in the radular muscles of prosobranch molluscs and the fin muscles of squids; this indicates a high capacity for glucose utilization, which is consistent with the high activities of enzymes of the tricarboxylic acid cycle in these muscles [Alp, Newsholme & Zammit (1976) Biochem. J. 154, 689-700]. 2. The activities of lactate dehydrogenase, octopine dehydrogenase, phosphoenolpyruvate carboxykinase, cytosolic and mitochondrial glycerol 3-phosphate dehydrogenase and glutamate-oxaloacetate transaminase were measured in order to provide a qualitative indication of the importance of different processes for oxidation of glycolytically formed NADH. The muscles are divided into four groups: those that have a high activity of lactate dehydrogenase relative to the activities of phosphofructokinase (e.g. crustacean muscles); those that have high activities of octopine dehydrogenase but low activities of lactate dehydrogenase (e.g. scallop snap muscle); those that have moderate activities of both lactate dehydrogenase and octopine dehydrogenase (radular muscles of prosobranchs), and those that have low activities of both lactate dehydrogenase and octopine dehydrogenase, but which possess activities of phosphoenolpyruvate carboxykinase (oyster adductor muscles). It is suggested that, under anaerobic conditions, muscles of marine invertebrates form lactate and/or octopine or succinate (or similar end product) according to the activities of the enzymes present in the muscles (see above). The muscles investigated possess low activities of cytosolic glycerol 3-phosphate dehydrogenase, which indicates that glycerol phosphate formation is quantitatively unimportant under anaerobic conditions, and low activities of mitochondrial glycerol phosphate dehydrogenase, which indicates that the glycerol phosphate cycle is unimportant in the re-oxidation of glycolytically produced NADH in these muscles under aerobic conditions. Conversely, high activities of glutamate-oxaloacetate transaminase are present in some muscles, which indicates that the malate-aspartate cycle may be important in oxidation of glycolytically produced NADH under aerobic conditions. 3. High activities of nucleoside diphosphate kinase were found in muscles that function for prolonged periods under anaerobic conditions (e.g...

Adenosine Triphosphate↗
PubMed1976

Effects of calcium ions and adenosine diphosphate on the activities of NAD+-linked isocitrate dehydrogenase from the radular muscles of the whelk and flight muscles of insects.

1. The activity of NAD+-linked isocitrate dehydrogenase from the radular muscle of the whelk is higher than those in many vertebrate muscles and only slightly lower than in the flight muscles of insects. The enzyme activity from the whelk (Buccinum undatum) is stable for several hours after homogenization of the radular muscle, whereas that from insect flight muscle is very unstable. Consequently, the enzyme from the whelk muscle is suitable for a systematic investigation of the effects of Ca2+ and ADP. 2. The sigmoid response of the enzyme activity to isocitrate concentration is markedly increased by raising the Ca2+ concentration from 0.001 to 10 muM, but it is decreased by ADP. The inhibitory effect of Ca2+ is most pronounced at pH7.1; it is not observed at pH 6.5. Similar effects are observed for the enzyme from the flight muscle of the locust (Schistocerca gregaria) and the water bug (Lethocerus cordofanus). The percentage activation by ADP of the enzyme from either the whelk or the insects is greater at 10 muM-Ca2+, and 50% of the maximum activation is obtained at 0.10 and 0.16 mM-ADP for the enzyme from whelk and locust respectively at this Ca2+ concentration. At 10 muM-Ca2+ in the absence of added ADP, the apparent Km for isocitrate is markedly higher than in other conditions. Ca2+ concentrations of 0.01, 0.1 and 0.2 muM cause 50% inhibition of maximum activity of the enzyme from the muscles of the whelk, locust and water bug respectively. 3. Recent work has indicated that mitochondria may play a complementary role to the sarcoplasmic reticulum in the control of the distribution of Ca2+ in muscle. The opposite effects of Ca2+ on the activities of isocitrate dehydrogenase and mitochondrial glycerol phosphate dehydrogenase from muscle tissue are consistent with the hypothesis that changes in the intracellular distribution of Ca2+ control the activities of these two enzymes in order to stimulate energy production for the contraction process in the muscle. Although both enzymes are mitochondrial, glycerol phosphate dehydrogenase resides on the outer surface of the inner membrane and responds to sarcoplasmic changes in Ca2+ concentration (i.e. an increase during contraction), whereas the isocitrate dehydrogenase resides in the matrix of the mitochondria and responds to intramitochondrial concentrations of Ca2+ (i.e. a decrease during contraction). It is suggested that changes in intramitochondrial Ca2+ concentrations are primarily responsible for regulation of the activity of NAD+-isocitrate dehydrogenase in order to control energy formation for the contractile process. However, when the muscle is at rest, changes in intramitochondrial concentrations of ADP may regulate energy formation for non-contractile processes.

Adenosine Diphosphate↗