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Biomedical subjects

J Bremer

Publications and source records attributed to J Bremer.

At least 91 records · Page 5Linked to original sources
PubMed1987

Methionine metabolism by rat muscle and other tissues. Occurrence of a new carnitine intermediate.

Perfused rat hindquarter preparations were shown to incorporate radioactivity from [U-14C]methionine into citrate-cycle intermediates, lactate, alanine, glutamate, glutamine and CO2. During perfusion, large amounts of methionine were also oxidized to methionine sulphoxide. The capacity for transamination of methionine or its oxo analogue, 4-methylthio-2-oxobutyrate, by muscle extracts was demonstrated. Rat skeletal muscle, heart, liver and kidney mitochondria, when incubated with the latter plus radiolabelled carnitine, formed a newly identified carnitine derivative, 3-methylthiopropionylcarnitine. It is concluded that the capacity for oxidation of methionine by a trans-sulphuration-independent pathway occurs in several mammalian tissues. The extent of inter-organ handling of intermediates in this pathway(s) is discussed.

Acyltransferases↗
PubMed1987

Role of carnitine-dependent metabolic pathways in heart disease without primary ischemia.

1. Carnitine and carnitine palmitoyltransferase are active in the transfer of fatty acids into the mitochondria for oxidation. Very long chain fatty acids (C22) are poorly oxidized by mitochondria. Lack of carnitine or overloading with C22 fatty acids leads to lipidosis in heart and other tissues. 2. The oxidation of fatty acids (including C22 fatty acids) in the peroxisomes is not dependent on carnitine. However, carnitine acetyltransferase and carnitine medium chain acyltransferase are presumably auxiliary enzymes in the oxidation of acetyl-CoA and shortened fatty acids formed in the peroxisomes. 3. Branched-chain acylcarnitines may be formed in the mitochondria from branched-chain amino acids. They are also metabolized in the mitochondria. When formed in large amounts, they are released into the circulation and urine by the liver and kidney. 4. The mechanisms leading to secondary carnitine deficiency because of branched-chain acylcarnitine formation in metabolic disturbances are discussed.

Amino Acids, Branched-Chain↗
PubMed1986

Carnitine palmitoyltransferase: activation and inactivation in liver mitochondria from fed, fasted, hypo- and hyperthyroid rats.

The sensitivity of carnitine palmitoyltransferase to malonyl-CoA is lost when liver mitochondria are preincubated in a KCl-containing medium. This loss of sensitivity is slowed down in mitochondria from hypothyroid rats and accelerated in mitochondria from fasted and hyperthyroid rats. Glucagon seems to enhance the effect of fasting. The loss of sensitivity is significantly slowed down by 50-500 nM malonyl-CoA and accelerated by small amounts of palmitoyl-CoA in the preincubation medium.

Acyltransferases↗
PubMed1986

Metabolism and excretion of carnitine and acylcarnitines in the perfused rat kidney.

Rat kidneys were perfused for 30 min with a Krebs-Henseleit bicarbonate buffer with 5 mM glucose. Albumin proved superior to pluronic polyols as oncotic agent with regard to carnitine reabsorption in the perfused kidney. The reabsorption of 30 microM (-)-[methyl-3 H]carnitine was approx. 96% during the first 10 min. At 750 microM the reabsorption decreased to 40%. The tubular reabsorptive maximum (Tmax) was approx. 170 nmol/min per kidney. The fractional reabsorption and clearance of (+)-carnitine, gamma-butyrobetaine, and carnitine esters did not deviate significantly from that of (-)-carnitine. (+)-Carnitine was not metabolized by the perfused kidney. In perfusions with (-)-carnitine or (-)-carnitine plus 10 mM alpha-ketoisocaproate or alpha-ketoisovalerate increased amounts of acetylcarnitine, isovalerylcarnitine and isobutyrylcarnitine were found. Propionate (5 mM) inhibited acetylcarnitine formation. Isovalerylcarnitine, isobutyrylcarnitine and propionylcarnitine were actively degraded to free (-)-carnitine. In urine, we found a disproportionally high excretion of carnitine or carnitine esters formed in the kidney, compared to the same derivatives when ultrafiltrated. Leakage of metabolites formed in the kidney into preurine may explain this phenomenon.

Absorption↗
PubMed1986

The effect of feeding fish oils, vegetable oils and clofibrate on the ketogenesis from long chain fatty acids in hepatocytes.

Groups of rats were fed diets containing 25% fish oil (FO), 25% soybean oil, 25% partially hydrogenated fish oil (PHFO), 25% partially hydrogenated soybean oil (PHSO), 25% partially hydrogenated coconut oil or 0.3% clofibrate for 3 wk. After the animals were fasted for 24 hr, hepatocytes were isolated and ketogenesis from added palmitate, linoleate cis and trans, arachidonate and docosahexaenoate was measured. Ketogenesis after oil feeding was significantly stimulated (two- to threefold) only in cells from the PHFO- and PHSO-fed rats. The stimulation was most apparent with the long chain unsaturated fatty acids as substrates. These fatty acids were relatively poor ketone body precursors in control hepatocytes. Essential fatty acid deficiency did not seem to be the reason for this stimulation. Clofibrate also stimulated ketogenesis significantly (1.5- to 3-fold). The degree of stimulation increased with chain length and degree of unsaturation of the substrate. The activity of the enzyme 2,4-dienoyl-CoA reductase was also studied in the same groups. Its activity was stimulated about fourfold in the clofibrate-treated rats and to a lesser extent by the PHFO, PHSO and FO diets. The activity showed no correlation with the content of unsaturated fatty acids in the diet or their oxidation in isolated hepatocytes. The 2,4-dienoyl-CoA reductase, therefore, does not seem to be a regulatory enzyme in the metabolism of dietary polyunsaturated fatty acids. It is concluded that an induction of the peroxisomal beta-oxidation system most likely is involved in the reported increases in ketogenesis from very long chain polyunsaturated fatty acids.

Animals↗
PubMed1985

Substrate inhibition of carnitine palmitoyltransferase by palmitoyl-CoA and activation by phospholipids and proteins.

When carnitine palmitoyltransferase is purified it shows increasing substrate inhibition by palmitoyl-CoA as the protein content of the assay mixture is decreased. The purified enzyme is stimulated by addition of phospholipids (phosphatidylcholine, cardiolipin) and proteins (albumin, fatty acid-binding protein, lambda-globulin) to the reaction mixture. The effects of phospholipid and protein are more than additive, particularly with relatively high concentrations of palmitoyl-CoA. It is suggested that the enzyme contains hydrophobic sites which require phospholipid to prevent spurious binding of palmitoyl-CoA and which normally anchor the enzyme to the mitochondrial membrane.

Acyl Coenzyme A↗
PubMed1985

Carnitine palmitoyltransferase. Activation by palmitoyl-CoA and inactivation by malonyl-CoA.

Extraction of rat liver mitochondria twice with 0.5% Triton X-100 in a salt-free medium leaves less than 10% of the carnitine palmitoyltransferase membrane bound. The remaining membrane-bound enzyme is inhibited virtually completely by 10 microM malonyl-CoA. Preincubation of the extracted membranes with palmitoyl-CoA and salts (KCI) for several minutes activates the enzyme and makes it increasingly insensitive to malonyl-CoA. Addition of malonyl-CoA to the preincubation reverses this desensitization. In albumin-containing media salts also decrease the binding of palmitoyl-CoA to albumin and stimulate carnitine palmitoyltransferase by increasing substrate availability in free solution. The reverse reaction shows accelerated desensitization by palmitoylcarnitine and resensitization by malonyl-CoA.

Acyl Coenzyme A↗
PubMed1984

Determination of malonyl-coenzyme A in rat heart, kidney, and liver: a comparison between acetyl-coenzyme A and butyryl-coenzyme A as fatty acid synthase primers in the assay procedure.

The malonyl-CoA assay was nonlinear at low malonyl-CoA concentrations when labeled acetyl-CoA was used as fatty acid synthase primer. Linearity was obtained with low concentrations of both fatty acid synthase and labeled acetyl-CoA, but then the assay was disturbed by the diluting effect of endogenous acetyl-CoA. The problems of nonlinearity and dilution of radioactivity by endogenous compounds were absent when labeled butyryl-CoA was used as primer. The levels of malonyl-CoA in rat heart, kidney, and liver were determined. The use of butyryl-CoA gave higher values of malonyl-CoA.

Acetyl Coenzyme A↗
PubMed1984

The effect of glucagon on the carbon flux from palmitate into glucose, lactate and ketone bodies, studied with isolated hepatocytes.

Glucagon induced a rapid (within 3 min) increase in glucose radioactivity and a decrease in the labeling of ketone bodies when isolated hepatocytes were incubated in the presence of [1-14C]palmitate. Simultaneously, the hormone induced a decrease in the levels of pyruvate and Krebs cycle intermediates and an increase in the level of phosphoenolpyruvate (PEP). The glucagon-induced increase in glucose radioactivity was much larger than the simultaneous decrease in lactate labeling. A comparison of the incorporation of labeled carbon from [1-14C]palmitate and [U-14C]palmitate into glucose and CO2 indicates a selective stimulatory action of glucagon on the flux through the phosphoenolpyruvate carboxykinase (PEPCK) reaction.

Animals↗
PubMed1983

Beta-oxidation of polyunsaturated fatty acids having double bonds at even-numbered positions in isolated rat liver mitochondria.

beta-Oxidation of polyunsaturated fatty acids was studied with isolated rat liver mitochondria in state 3 or uncoupled conditions. 1. Incubation of mitochondria with docosahexaenoyl-, linolenoyl- or gamma-linolenoylcarnitine resulted in an increase of the absorbance at 340 minus 385 nm. This increased absorbance was due to an accumulation of beta-oxidation intermediates of the polyunsaturated fatty acids, and not to the reduction of nicotinamide nucleotides. 2. Experiments carried out with soluble fractions of liver mitochondria incubated with docosahexaenoyl-CoA and gamma-linolenoyl-CoA indicated that this ultraviolet light-absorption was at least partly caused by acyl-CoA esters having a 2,4(,7)-di(tri)enoyl-CoA structure. 3. The addition of glutamate to mitochondria oxidizing gamma-linolenoylcarnitine decreased the absorbance at 340 minus 385 nm, and simultaneously stimulated respiration. With liver mitochondria isolated from fasted rats, 6 mM glutamate increased the rate of acetoacetate production from gamma-linolenoylcarnitine by 130 and 210% under state 3 and uncoupled conditions, respectively. Glutamate did not have any significant effect on the degradation of oleoylcarnitine. The proposed explanation for these findings is that the glutamate dehydrogenase reaction can function as a source of NADPH for 2,4-dienoyl-CoA reductase. 4. The degradation of gamma-linolenoylcarnitine to ketone bodies was augmented in mitochondria isolated from rats treated with clofibrate or partially hydrogenated marine oil. 5. We conclude that 2,4-dienoyl-CoA reductase is an important auxiliary enzyme in the beta-oxidation of polyunsaturated fatty acids. Induction of this enzyme by clofibrate or by certain high-fat diets increases mitochondrial capacity for the degradation of polyunsaturated fatty acids.

Animals↗
PubMed1983

The outer carnitine palmitoyltransferase and regulation of fatty acid metabolism in rat liver in different thyroid states.

The activity of the outer carnitine palmitoyltransferase (EC 2.3.1.21) and the carnitine-dependent oxidation of palmitoyl-CoA was increased 3-4 fold in liver mitochondria from hyperthyroid rats as compared with hypothyroid rats. The inhibitory effect of malonyl-CoA on carnitine-dependent fatty acid oxidation was preserved in all thyroid states, but decreased in hyperthyroid mitochondria. Fasting for 24 h increased the activity of the outer carnitine palmitoyltransferase about 50% in hypothyroid liver mitochondria, whereas it had no significant effect in hyperthyroid mitochondria. The thyroid state had no significant effect on total carnitine palmitoyltransferase in liver mitochondria. Fasting stimulated fatty acid oxidation 3-4-fold in isolated hepatocytes from hypothyroid rats, whereas it had no effect in hyperthyroid rats. Feeding 0.3% clofibrate to euthyroid rats about doubled the activity of the total carnitine palmitoyltransferase, whereas it had no effect on the outer transferase. The regulation of fatty acid oxidation in the liver is discussed.

Acyltransferases↗
PubMed1983

Carnitine acetyltransferase. Effect of malonyl-CoA, fasting and clofibrate feeding in mitochondria from different tissues.

Malonyl-CoA sensitivity of carnitine acetyltransferase (EC 2.3.1.7) activities in isolated intact mitochondria from rat liver, heart and white and brown adipose tissue, and rabbit liver and heart were studied with a radioisotopic assay using [3H]acetyl-CoA. Malonyl-CoA inhibited [3H]acetylcarnitine formation in intact rat liver mitochondria (overt activity), while acetyltransferase in heart and white and brown adipose tissue were malonyl-CoA-insensitive. Acetyltransferase from rabbit liver and heart was only weakly inhibited by malonyl-CoA. Acetyltransferase in intact rat liver peroxisomes was malonyl-CoA-insensitive. Lysis of the mitochondria with (+)palmitoylcarnitine increased acetyltransferase activities (total activity) in all mitochondria studied and abolished malonyl-CoA-sensitivity, showing the necessity of intact mitochondrial membranes for malonyl-CoA-sensitivity. A differential increase in overt and total mitochondrial carnitine acetyltransferase was observed in rat liver upon fasting and clofibrate feeding. Fasting increased overt activity more than total activity and diminished malonyl-CoA-sensitivity. Clofibrate increased mainly total activity and also reduced malonyl-CoA-sensitivity. A similar pattern of increased activities was observed with palmitoyl-CoA and octanoyl-CoA as substrates.

Acetyltransferases↗
PubMed1983

Carnitine--metabolism and functions.

Carnitine was detected at the beginning of this century, but it was nearly forgotten among biochemists until its importance in fatty acid metabolism was established 50 years later. In the last 30 years, interest in the metabolism and functions of carnitine has steadily increased. Carnitine is synthesized in most eucaryotic organisms, although a few insects (and most likely some newborn animals) require it as a nutritional factor (vitamin BT). Carnitine biosynthesis is initiated by methylation of lysine. The trimethyllysine formed is subsequently converted to butyrobetaine in all tissues; the butyrobetaine is finally hydroxylated to carnitine in the liver and, in some animals, in the kidneys (see Fig. 1). It is released from these tissues and is then actively taken up by all other tissues. The turnover of carnitine in the body is slow, and the regulation of its synthesis is still incompletely understood. Microorganisms (e.g., in the intestine) can metabolize carnitine to trimethylamine, dehydrocarnitine (beta-keto-gamma-trimethylaminobutyric acid), betaine, and possibly to trimethylaminoacetone. In some insects carnitine can be converted to methylcholine, presumably with trimethylaminoacetone as an intermediate (see Fig. 3). In mammals the unphysiological isomer (+) carnitine is converted to trimethylaminoacetone. The natural isomer (-)carnitine is excreted unchanged in the urine, and it is still uncertain if it is degraded in mammalian tissues at all (Fig. 2). The only firmly established function of carnitine is its function as a carrier of activated fatty acids and activated acetate across the inner mitochondrial membrane. Two acyl-CoA:carnitine acyltransferases with overlapping chain-length specificities have been isolated: one acetyltransferase taking part in the transport of acetyl and short-chain acyl groups and one palmitoyltransferase taking part in the transport of long-chain acyl groups. An additional octanoyltransferase has been isolated from liver peroxisomes. Although a carnitine translocase that allows carnitine and acylcarnitine to penetrate the inner mitochondrial membrane has been deduced from functional studies (see Fig. 5), this translocase has not been isolated as a protein separate from the acyltransferases. Carnitine acetyltransferase and carnitine octanoyltransferase are also found in the peroxisomes. In these organelles the enzymes may be important in the transfer of acyl groups, which are produced by the peroxisomal beta-oxidation enzymes, to the mitochondria for oxidation in the citric acid cycle. The carnitine-dependent transport of activated fatty acids across the mitochondrial membrane is a regulated process. Malonyl-CoA inh

Amino Acids, Branched-Chain↗
PubMed1982

A role for 2,4-enoyl-CoA reductase in mitochondrial beta-oxidation of polyunsaturated fatty acids. Effects of treatment with clofibrate on oxidation of polyunsaturated acylcarnitines by isolated rat liver mitochondria.

1. beta-Oxidation of gamma-linolenoylcarnitine, arachidonoylcarnitine and docosahexaenoylcarnitine by isolated rat liver mitochondria is inhibited by uncoupling conditions. Partial re-activation is obtained with added ATP. With mitochondria from clofibrate-treated rats ATP-stimulated rates of beta-oxidation of docosahexaenoylcarnitine are higher than ADP-stimulated rates. This is not observed with the beta-oxidation of oleoylcarnitine. 2. beta-Oxidation of docosahexaenoylcarnitine, in the presence of rotenone, is inhibited by added oxaloacetate, analogous to previous findings with pent-4-enoylcarnitine [see Osmundsen (1978) FEBS Lett. 88, 219-222]. In the absence of rotenone added oxaloacetate stimulates the beta-oxidation of docosahexaenoylcarnitine, but has the opposite effect on the beta-oxidation of palmitoylcarnitine. 3. beta-Oxidation of polyunsaturated acylcarnitines by isolated rat liver mitochondria is selectively increased after treatment of the animals with a low dietary dose (0.2%, w/w) of clofibrate. Treatment with a higher dose of clofibrate (0.5%, w/w) resulted in a general stimulation of beta-oxidation. 4. The results presented suggest that long-chain fatty acids possessing a delta 4-double bond are not readily beta-oxidized unless the 2,4-enoyl-CoA reductase (EC 1.3.1.-) is operating.

2,4-Dinitrophenol↗