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Biomedical subjects

L Lumeng

Publications and source records attributed to L Lumeng.

At least 199 records · Page 11Linked to original sources
PubMed1980

Plasma content of B6 vitamers and its relationship to hepatic vitamin B6 metabolism.

The plasma content of B6 vitamers is governed by, among other factors, dietary supply and metabolic interconversion. This study examines the effect of pyridoxine supplementation on the plasma content of B6 vitamers and pyridoxic acid in man, and the metabolic conversion and release of B6 compounds in isolated rat hepatocytes. Six healthy human subjects were given 100 mg pyridoxine-HCl/d orally for 1--3 wk. Before pyridoxine supplementation, the mean total plasma level of B6 vitamers was 114 +/- 9 nM; and pyridoxal-P, pyridoxamine-P, pyridoxal, pyridoxine, and pyridoxamine accounted for 54, 3, 11, 27, and 5%, respectively. Plasma level of pyridoxic acid was 40 +/- 7 nM. Thus, pyridoxal-P is the principal B6 vitamer in plasma. During pyridoxine supplementation, mean plasma levels of the B6 vitamers and pyridoxic acid increased to 655 +/- 122 and 222 +/- 55 nM, respectively. The plasma content of pyridoxal-P and pyridoxic acid increased 6--7-fold and that of pyridoxal, 12-fold, but the pyridoxine level did not increase. Isolated hepatocytes, 1 g/15 ml, were incubated for 2 h with 3.33 microM [14C]pyridoxine (6 microCi/mumol). At zero time, the cells contained about 35 nmol pyridoxal-P and 25 nmol pyridoxamine-P. After 2 h incubation, the cellular content of pyridoxal-P and pyridoxamine-P did not change significantly, but the medium contained 5.9 nmol pyridoxal-P, 0.3 nmol pyridoxamine-P, 7.2 nmol pyridoxal, 26.6 nmol pyridoxine, 0.3 nmol pyridoxamine, and 7.5 nmol pyridoxic acid. Whereas the specific radioactivity of pyridoxal-P, pyridoxal, and pyridoxic acid in the medium approached that of [14C]pyridoxine, the specific radioactivity of cellular pyridoxal-P and pyridoxamine-P was only 20% of that of pyridoxine. Thus, newly synthesized pyridoxal-P is not freely exchangeable with endogenous pyridoxal-P, but is preferentially released or degraded to pyridoxal and pyridoxic acid. The latter B6 compounds are also released. These results suggest that orally ingested pyridoxine is rapidly metabolized in liver and its products are released into the circulation in the form of pyridoxal-P, pyridoxal, and pyridoxic acid.

Adult↗
PubMed1979

Hormonal control of ornithine decarboxylase in isolated liver cells and the effect of ethanol oxidation.

The regulation of ornithine decarboxylase activity was studied in freshly isolated rat hepatocytes incubated in a chemically defined medium for 5 h. Glucagon, dibutyryl cyclic AMP, insulin and dexamethasone produced dramatic increases in ornithine decarboxylase activity, 6--100-times the basal activity. Actinomycin D inhibited completely the stimulatory action of these substances. With glucagon, dibutyryl cyclic AMP and insulin, the rise in ornithine decarboxylase activity was rapid but transient, peaking at 200 min and then declining rapidly. By contrast, the response to dexamethasone was gradual and sustained in the 5 h incubation. The transient nature of the response to glucagon was unaltered by repeated additions of optimally effective doses of glucagon suggesting the development of 'refractoriness' to the actions of this hormone. Ethanol oxidation inhibited by 50% the stimulation of ornithine decarboxylase by glucagon and dexamethasone and this effect was blocked by 4-methylpyrazole, an inhibitor of alcohol dehydrogenase. Acetate (2.5--20 mM), the metabolic product of hepatic ethanol oxidation, was also effective. The data indicate that glucagon, insulin and glucocorticoids are all effective in stimulating the activity of ornithine decarboxylase in isolated hepatocytes but they differ in their duration and time of peak of action. Additionally, the inhibitory effect of ethanol on the hormonal stimulation of ornithine decarboxylase is dependent on its oxidation and may be mediated by acetate.

Animals↗
PubMed1979

Transport and metabolism of thiamin in isolated rat hepatocytes.

This study examines thiamin transport in isolated rat hepatocytes and its relationship to thiamin phosphorylation. In an Na+ medium, [35S]thiamin, 3 microM, was accumulated rapidly by the cells, and a near study state intra-/extracellular distribution ratio of 3 was attained in 1 min. However, the uptake of radioactivity continued to increase with time owing principally to the accumulation of [35S]thiamin pyrophosphate (TPP). In a choline, Li+ or K+ medium, the steady state intra-/extracellular distribution ratio of [35S]thiamin was decreased to less than or equal to 1.1. Accordingly, the rate of formation of [35S]TPP also decreased. Ouabain and uncouplers of oxidative phosphorylation significantly lowered the distribution ratio of intra-/extracellular [35S]thiamin. These data indicate that thiamin transport in liver is concentrative, Na+-dependent, and dependent on biological energy. Additionally, they suggest that thiamin transport plays a significant role in governing the rate of synthesis of TPP. Neither pyrithiamin, an inhibitor of thiamin pyrophosphokinase nor o-benzoylthiamin disulfide, a permeable thiamin analog, affected the distribution ratio of intra-/extracellular [35S]thiamin, but preferentially inhibited the phosphorylation of [35S]thiamin. By contrast, amprolium primarily inhibited uptake. These data suggest that thiamin transport and phosphorylation can be differentiated by the action of appropriate inhibitors.

Animals↗
PubMed1978

Neurochemical and operant behavioral studies of a strain of alcohol-preferring rats.

The levels of serotonin (5-HT), 5-hydroxyindoleacetic acid, tyrosine (TYR), norepinephrine (NE), acetylcholine, GABA, glutamate (GLU), aspartate (ASP), alanine, glycine (GLY) and taurine were measured in the CNS of adult male rats selectively inbred to the F8 generation for alcohol preference (P) and non-preference (NP). With respect to the values found in the NP group, higher levels of 5-HT, GABA, GLU and GLY and lower levels of ASP were found in the diencephalon-mesencephalon (D-M) and higher levels of NE were found in the telencephalon (TEL) of the P group. The animals in the P and NP strains were further subdivided into two additional groups, one given only H2O (W) and the second given 10% ethanol (A) during a one week period, thereby producing four groups (NP-W, NP-A, P-W and P-A). With these conditions, the level of (a) TYR in the D-M was higher in the P-A and NP-A animals than in the P-W and NP-W groups respectively; (b) 5-HT in the TEL was higher in the NP-A group than NP-W group; and (c) GABA in the TEL was higher in the P-A than P-W animals. No differences were observed in the cerebellum between the two strains or between the subgroups within each strain. The present study also demonstrated that the P animals will work in an operant situation to obtain 10% ethanol, even when H2O is freely available, and will voluntarily bar-press up to 6-7 times for each ethanol reinforcement.

Alcohol Drinking↗
PubMed1978

Validation of the diagnostic value of plasma pyridoxal 5'-phosphate measurements in vitamin B6 nutrition of the rat.

The relationship of plasma pyridoxal 5'-phosphate (PLP) to PLP content of tissues and activities of PLP-dependent enzymes was examined to establish its value in assessing vitamin B6 nutrition. Weanling rats were fed ad libitum for 9 weeks purified diets which supplied 0, 4, 12, 24, and 100 micrograms of pyridoxine daily. Growth increased with increasing pyridoxine intake, reaching a maximum at 24 micrograms/day. Liver and brain PLP also increased, attaining maximal values at 12 micrograms. By contrast, muscle and plasma PLP did not saturate when vitamin B6 intake was increased to 100 micrograms. Erythrocytic holoenzyme activity of aspartate (Asp) aminotransferase became maximal with 24 micrograms but that of alanine (Ala) aminotransferase did not. Hepatic holoenzyme activities of Ala, Asp and tyrosine aminotransferases reached maximal values with only 4 micrograms vitamin B6 but that of serine dehydratase became maximal with 12 micrograms. Measurement of coenzyme saturability suggested that apoenzyme degradation, coenzyme affinity and PLP transfer determine the activities of these enzymes. It is concluded that plasma and muscle PLP behave as mobilizable storage pools and that plasma PLP is a sensitive and reliable indicator of vitamin B6 nutrition.

Animals↗
PubMed1978

The role of acetaldehyde in mediating the deleterious effect of ethanol on pyridoxal 5'-phosphate metabolism.

Previous studies in vivo and with isolated perfused rat livers have suggested that the deleterious effect of ethanol on hepatic pyridoxal 5'-phosphate metabolism is mediated by acetaldehyde. Inasmuch as acetaldehyde has no effect on the synthesis of pyridoxal phosphate, it has also been postulated that acetaldehyde accelerates pyridoxal phosphate degradation by displacing this coenzyme from binding proteins, which protect it against hydrolysis. To test these hypotheses, studies have been performed with isolated rat hepatocytes, subcellular fractions of rat liver, and human erythrocytes. Ethanol oxidation lowered the pyridoxal phosphate content of isolated liver cells when acetaldehyde oxidation was inhibited by either disulfiram or prior treatment of rats with cyanamide. Additions of 7.5 mM acetaldehyde alone at 40-min intervals to cell suspensions decreased hepatic pyridoxal phosphate content only slightly because acetaldehyde was rapidly metabolized. However, when acetaldehyde oxidation and reduction were inhibited by cyanamide treatment and by 4-methyl-pyrazole and isobutyramide, respectively, a 40% decrease in hepatic pyridoxal phosphate content was observed in 80 min of incubation. In equilibrium dialysis experiments, acetaldehyde, 7.5 and 15 mM, displaced protein-bound pyridoxal phosphate in undialyzed hepatic cytosol and in hemolysate supernate containing added pyridoxal phosphate. In the presence of alkaline phosphatase, acetaldehyde accelerated the degradation of pyridoxal phosphate in dialyzed hemolysate supernate and hepatic cytosol with added pyridoxal phosphate. Acetaldehyde also inhibits tyrosine aminotransferase. The kinetics of inhibition were mixed competitive-noncompetitive with respect to pyridoxal phosphate. These observations support the hypothesis that the deleterious effect of ethanol oxidation on pyridoxal phosphate metabolism is mediated at least in part by acetaldehyde which displaces this coenzyme from protein binding, thereby enhancing its degradation.

Acetaldehyde↗
PubMed1976

Suppression of the mitochondrial oxidation of (-)-palmitylcarnitine by the malate-aspartate and alpha-glycerophosphate shuttles.

Palmitylcarnitine oxidation by isolated liver mitochondria has been used to investigate the interaction of fatty acid oxidation with malate, glutamate, succinate, and the malate-aspartate shuttle. Mitochondria preincubated with fluorocitrate were added to a medium containing 2mM ATP and ATPase. This system, characterized by a high energy change, allowed titration of respiration to any desired rate between States 4 and 3 (Chance, B., and Williams, G. R. (1956) Adv. Enzymol. Relat. Areas Mol. Biol. 17, 65-134). When respiration (reference, with palmitylcarnitine and malate as substrates) was set at 75% of State 3, the oxidation of palmitylcarnitine was limited by acetoacetate formation. The addition of malate or glutamate approximately doubled the rate of beta oxidation. Malate circumvented this limitation by citrate formation, but the effect of glutamate apparently was due to enhancement of the capacity for ketogenesis. The rate of beta oxidation was curtailed when malate and glutamate were both present. This curtailment was more pronounced when the malate-aspartate shuttle was fully reconstituted. Among the oxidizable substrates examined, succinate was most effective in inhibiting palmitylcarnitine oxidation. Mitochondrial NADH/NAD+ ratios were correlated positively with suppression of beta oxidation. The degree of suppression of beta oxidation by the malate-aspartate shuttle (NADH oxidation) or by succinate oxidation was dependent on the respiratory state. Both substrates extensively reduced mitochondrial NAD+ and markedly suppressed beta oxidation as respiration approached State 4. Calculations of the rates of flux of hydrogen equivalents through beta oxidation show that the suppression of beta oxidation by glutamate or by the malate-aspartate shuttle is accounted for by increased flux of reducing equivalents through mitochondrial malic dehydrogenase. This increased Flux is accompanied by an increase in the steady state NADH/NAD+ ratio and a marked decrease in the synthesis of citrate. The alpha-glycerophosphate shuttle was reconstituted with mitochondria isolated from rats treated with L-thyroxine. This shuttle was about equal to the reconstructed malate-aspartate shuttle in supression of palmitylcarnitine oxidation. This interaction could not be demonstrated in euthyroid animals owing to the low activity of the mitochondrial alpha-glycerol phosphate dehydrogenase. It is concluded that beta oxidation can be regulated by the NADH/NAD+ ratio. The observed stimulation of flux through malate dehydrogenase both by glutamate and by the malate-aspartate shuttle results in an increased steady state NADH/NAD+ ratio, and is linked to a stoichiometric outward transport of aspartate. We suggest, therefore, that some of the reducing pressure exerted by the malate-aspartate shuttle and by glutamate plus malate is provided through the energy-linked, electrogenic transport of aspartate out of the mitochondria. These results are discussed with respect to the mechanism of the genesis of ethanol-induced fatty liver.

Animals↗
PubMed1976

Adequacy of vitamin B6 supplementation during pregnancy: a prospective study.

This prospective study assesses the effect of 2.5, 4, and 10 mg of pyridoxine supplementation during pregnancy on maternal and fetal plasma levels of pyridoxal 5'-phosphate (PLP) and on the degree of coenzyme saturation (activation factor) of aspartate aminotransferase and alanine aminotransferase (alphaEGOT and alphaEGPT) in maternal erythrocytes. More than 4 mg of pyridoxine supplementation daily was required for most pregnancies to maintain maternal plasma PLP levels within the range observed during the first trimester and in the nonpregnant state. The plasma PLP concentrations in maternal and cord blood were highly correlated and indicated a dependence of fetal vitamin B6 nutrition on maternal circulating PLP. Measurements of alphaEGOT and alphaEGPT were not as reproducible as plasma PLP assays and were less sensitive and quantitative indicators. In the majority of subjects, the changes in alphaEGOT and alphaEGPT with time correlated poorly with the changes in plasma PLP. However, when the data were analyzed without regard for their dependence on time, they demonstrated a negative, linear correlation between alphaEGOT and log plasma PLP and between alphaEGPT and log plasma PLP for the group on 2.5 mg of pyridoxine and for all the subjects combined. Finally, the dietary records showed that most of the subjects consumed less than 2 mg of vitamin B6 daily from their food. The results indicate that the current Recommended Dietary Allowance for vitamin B6 during pregnancy (2.5 mg) is too low and that supplementation of this vitamin in an amount more than 4 mg daily is recommended.

Alanine Transaminase↗
PubMed1975

Characterization of the pyridoxal 5'-phosphate and pyridoxamine 5'-phosphate hydrolase activity in rat liver. Identity with alkaline phosphatase.

The tissue content of pyridoxal 5'-phosphate is controlled principally by the protein binding of this coenzyme and its hydrolysis by a cellular phosphatase. The present study identifies this enzyme and its intracellular location in rat liver. Pyridoxal-P is not hydrolyzed by the acid phosphatase of intact lysosomes. At pH 7.4 and 9.0, the subcellular distribution of pyridoxal-P phosphatase activity is similar to the for p-nitrophenyl-P, and the major portion of both activities is found in the plasma membrane fraction. The ratio of specific activities for pyridoxal-P and p-nitrophenyl-P hydrolysis remains relatively constant during the isolation of plasma membranes. These activities also behave concordantly with respect to pH rate profile, pH-Km profile, and response to chelating agents, Zn2+, Mg2+, and inhibitors. Kinetic studies indicate that pyridoxal-P binds to same enzyme sites as beta-glycerophosphate and phosphorylcholine. The data strongly favor alkaline phosphatase as the enzyme which functions in the control of pyridoxal-P and pyridoxamine-P metabolism in rat liver. Alkaline phosphatase was solubilized from isolated plasma membranes. The kinetic properties of the enzyme are not markedly altered by its dissociation from the membrane matrix. However, there are significant differences in its behavior toward Mg2+ which suggest a structural role for Mg2+ in liver alkaline phosphatase.

Alkaline Phosphatase↗
PubMed1975

Maternal and fetal plasma levels of pyridoxal phosphate at term: adequacy of vitamin B6 supplementation during pregnancy.

The effect of different amounts of vitamin B6 supplementation during pregnancy on maternal and fetal plasma levels of pyridoxal phosphate (PLP) at term has been studied. Ten of 13 subjects given 2 to 2.5 mg. of vitamin B6 daily exhibited plasma PLP levels lower than 4.7 ng. per milliliter (the lower limit of normal for nonpregnant subjects). In contrast, only 4 of 11 subjects given a supplement of 10 mg. of vitamin B6 daily had plasma PLP less than this value. The mean plasma PLP level (64.4 ng. per milliliter) of 10 cord blood samples from newborn infants whose mothers exhibited plasma PLP levels greater than 4.7 ng. per milliliter was significantly higher (P less than or equal 0.005) than that (34.2 ng. per milliliter) from 14 newborn infants whose mothers had abnormally lowered plasma PLP concentrations. In cord plasma, an average venous-arterial gradient of 10.6 ng. per milliliter was observed, indicating that the fetus retains and/or degrades PLP. These results suggest that more than 2 to 2.5 mg. of vitamin B6 supplement daily is required for most pregnancies to restore normal vitamin B6 nutrition in the mother and, perhaps, also in the fetus.

Adult↗
PubMed1975

Vitamin B6 metabolism in chronic alcohol abuse The effect of ethanol oxidation on hepatic pyridoxal 5'-phosphate metabolism.

Individuals with chronic alcohol abuse frequently exhibit lowered plasma levels of pyridoxal 5'-phosphate, the coenzyme form of vitamin B6. Because the liver is the primary source of this coenzyme in plasma and also the principal organ that oxidizes ethanol, the effect of ethanol on hepatic pyridoxal phosphate metabolism was studied in the rat. The chronic feeding of ethanol (36 percent of the total dietary calories) for 6 wk significantly decreased the hepatic pyridoxal phosphate content both in animals given a sufficient amount of vitamin B6 in their diet and in those rendered vitamin B6 deficient. In isolated perfused livers, the addition of 18 mM ethanol lowered the pyridoxal phosphate content of livers from vitamin B6-sufficient animals and deceased the net synthesis of pyridoxal phosphate from pyridoxine by the livers of vitamin B6-deficient animals. Ethanol also diminished the rate of release of pyridoxal phosphate into the perfusate by the livers of vitamin B6-deficient rats. These effects of ethanol, in vitro, were abolished by 4-methyl pyrazole, an inhibitor of alcohol dehydrogenase. Thus the derangement of pyridoxal phosphate metabolism produced by ethanol is dependt upon its oxidation. These data support previous findings whic indicate that acetaldehyde is the responsible agent which acts by accelerating the degradation of intracellular pyridoxal phosphate.

Acute Disease↗