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 (b...

/pdf/carnitine-metabolism-and-functions-19pl58f13m.pdf

Carnitine--metabolism and functions

Recent studies in patients with long-term heart failure have suggested that intrinsic abnormalities in skeletal muscle can contribute to the development of early lactic acidosis and fatigue during exercise. The present study provides an analysis of substrate and enzyme content, fiber typing, and capillarization in skeletal muscle biopsy samples obtained at rest from the vastus lateralis in 11 patients with long-term heart failure (left ventricular ejection fraction, 21 +/- 8%) and nine normal subjects. Patients demonstrated a reduced peak exercise oxygen consumption (13.0 +/- 3.3 ml/kg/min) when compared with normals (30.2 +/- 8.6 ml/kg/min, p less than 0.001) and had an accelerated rise in blood lactate levels during exercise. In mixed fiber skeletal muscle, total phosphorylase and glycolytic enzyme activities were not different in the two groups, whereas mitochondrial enzymes involved in terminal oxidation were decreased in patients as compared with normal subjects as indicated by reductions in succinate dehydrogenase (51 +/- 15 vs. 81 +/- 17 microM/g protein/min, p less than 0.001) and citrate synthetase (26 +/- 7 vs. 43 +/- 20 microM/g protein/min, p less than 0.05). 3-Hydroxyacyl-CoA-dehydrogenase, an important enzyme mediating beta-oxidation of fatty acids, was also reduced in patients as compared with normals (18 +/- 7 vs. 27 +/- 10 microM/g protein/min, p less than 0.05). There was no difference in high-energy phosphagens or lactate concentration of mixed muscle in the two groups, whereas glycogen content was decreased in patients (262 +/- 29 vs. 298 +/- 35 microM glucosyl units/kg dry wt, p = 0.01). Patients demonstrated a reduced percentage of slow twitch type I fibers (36 +/- 7% vs. 52 +/- 22%, p less than 0.05) and had a higher percentage of type IIb fast twitch fibers (24 +/- 9% vs. 11 +/- 12%, p = 0.02), which were smaller than the type IIb fibers seen in normal subjects (p less than 0.05). In patients, the number of capillaries per fiber was decreased for type I and type IIa fibers (both, p less than 0.03), but the ratio of capillaries to cross-sectional fiber area was not different for the two groups. These data demonstrate major alterations in skeletal muscle histology and biochemistry in patients with long-term heart failure, including fiber atrophy, a decrease in percentage of composition of type I fibers, and an increase in type IIb fibers accompanied by a decrease in oxidative enzyme capacity.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle biochemistry and histology in ambulatory patients with long-term heart failure.

The intracellular concentration of free unbound acyl-CoA esters is tightly controlled by feedback inhibition of the acyl-CoA synthetase and is buffered by specific acyl-CoA binding proteins. Excessive increases in the concentration are expected to be prevented by conversion into acylcarnitines or by hydrolysis by acyl-CoA hydrolases. Under normal physiological conditions the free cytosolic concentration of acyl-CoA esters will be in the low nanomolar range, and it is unlikely to exceed 200 nM under the most extreme conditions. The fact that acetyl-CoA carboxylase is active during fatty acid synthesis (Ki for acyl-CoA is 5 nM) indicates strongly that the free cytosolic acyl-CoA concentration is below 5 nM under these conditions. Only a limited number of the reported experiments on the effects of acyl-CoA on cellular functions and enzymes have been carried out at low physiological concentrations in the presence of the appropriate acyl-CoA-buffering binding proteins. Re-evaluation of many of the reported effects is therefore urgently required. However, the observations that the ryanodine-senstitive Ca2+-release channel is regulated by long-chain acyl-CoA esters in the presence of a molar excess of acyl-CoA binding protein and that acetyl-CoA carboxylase, the AMP kinase kinase and the Escherichia coli transcription factor FadR are affected by low nanomolar concentrations of acyl-CoA indicate that long-chain acyl-CoA esters can act as regulatory molecules in vivo. This view is further supported by the observation that fatty acids do not repress expression of acetyl-CoA carboxylase or Delta9-desaturase in yeast deficient in acyl-CoA synthetase.

Role of long-chain fatty acyl-CoA esters in the regulation of metabolism and in cell signalling.

Coenzyme A: back in action.

beta-oxidation of fatty acids in mitochondria, peroxisomes, and bacteria: a century of continued progress.

Determination of adenine nucleotides and inosine in human myocard by ion-pair reversed-phase high-performance liquid chromatography.

Subcellular fractionation studies of rat liver localized the activity of palmitoyl-L-carnitine hydrolase to the microsomal fraction whereas palmitoyl-CoA hydrolase activity was found both in the microsomal fraction and in mitochrondria. An unusual biphasic sataration curve for palmitoyl-CoA was observed when intact mitochondrial hydrolase activity. Disruption of the mitochondrial structure doubled the palmitoyl-CoA hydrolysis. Discontinuous sucrose gradient centrifugation and digitonin fractionation of rat liver mitochondria demonstrated that a palmitoyl-CoA hydrolase was associated with the matrix fraction. Pure matrix and microsomal fractions showed that the two hydrolase activities were differently affected by the presence of divalent cations. Both the specific activity and the saturation concentration of palmitoyl-CoA were higher for the microsomal enzyme than for the matrix-associated enzyme.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1979.tb12942.x

Dual Localization of Long‐Chain Acyl‐CoA Hydrolase in Rat Liver: One in the Microsomes and One in the Mitochondrial Matrix

An enzyme able to hydrolyze long-chain acyl-CoA esters has been purified from mitochondrial matrix of rat liver by ammonium sulfate extraction, gel chromatography, and isoelectric focusing. The specific activity of the purified enzyme represented approx. 53-fold and 700-fold enrichment over the initial extract in the absence and presence of serum albumin, respectively.



The enzyme has been purified to homogeneity as judged by polyacrylamide-gel electrophoresis in the absence and presence of sodium dodecyl sulfate. The enzyme has a sedimentation rate of 2.1 S in a sucrose gradient, a Stokes radius of 1.9 nm, and consists of a single polypeptide with a molecular weight of 19000 determined by gel chromatography. The isoelectric point (pI) of the enzyme is 6.0.



The enzyme is specific for long-chain (over C10) fatty acyl-CoA esters with maximal activity for palmitoyl-CoA. At physiological substrate concentrations the enzyme did not hydrolyze palmitoyl-l-carnitine, tripalmitoyl-glycerol, dipalmitoyl-phosphatidylcholine, or cholesterol-oleate. The enzyme did not possess any palmitoyl-CoA synthetase or carnitine palmitoyltransferase activity.



The hydrolase activity was strongly influenced by the acyl-CoA/protein ratio as serum albumin increased the activity. As this effect was also seen at acyl-CoA concentrations under the critical micelle concentration it seems likely that serum albumin not only prevents the inhibition of the enzyme by binding the inhibiting micellar form of the substrate, but also stabilizes the enzyme. Similar effects were found with the nonionic detergent, Triton X-100.

Mikael Farstad

Papers

Determination of adenine nucleotides and inosine in human myocard by ion-pair reversed-phase high-performance liquid chromatography.

Dual Localization of Long‐Chain Acyl‐CoA Hydrolase in Rat Liver: One in the Microsomes and One in the Mitochondrial Matrix

Purification and Characterization of Long-Chain Acyl-CoA Hydrolase from Rat Liver Mitochondria

Sixty minutes of normothermic ischemia in the rat liver: Correlation between adenine nucleotides and bile excretion

Hepatic enzymes, CoASH and long-chain acyl-CoA in subcellular fractions as affected by drugs inducing peroxisomes and smooth endoplasmic reticulum.