Carnitine palmitoyltransferase in human erythrocyte membrane. Properties and malonyl-CoA sensitivity

Physiology of L-carnitine in plants in light of the knowledge in animals and microorganisms

L-carnitine is present in all living kingdoms where it acts in diverse physiological processes. It is involved in lipid metabolism in animals and yeasts, notably as an essential cofactor of fatty acid intracellular trafficking. Its physiological significance is poorly understood in plants, but L-carnitine may be linked to fatty acid metabolism among other roles. Indeed, carnitine transferases activities and acylcarnitines are measured in plant tissues. Current knowledge of fatty acid trafficking in plants rules out acylcarnitines as intermediates of the peroxisomal and mitochondrial fatty acid metabolism, unlike in animals and yeasts. Instead, acylcarnitines could be involved in plastidial exportation of de novo fatty acid, or importation of fatty acids into the ER, for synthesis of specific glycerolipids. L-carnitine also contributes to cellular maintenance though antioxidant and osmolyte properties in animals and microbes. Recent data indicate similar features in plants, together with modulation of signaling pathways. The biosynthesis of L-carnitine in the plant cell shares similar precursors as in the animal and yeast cells. The elucidation of the biosynthesis pathway of L-carnitine, and the identification of the enzymes involved, is today essential to progress further in the comprehension of its biological significance in plants.

Cellular properties of human erythrocytes preserved in saline-adenine-glucose-mannitol in the presence of L-carnitine

L-Carnitine (LC) in the preservation medium during storage of red blood cells (RBC) can improve the mean 24-hr percent recovery in vivo and increase RBC life-span after reinfusion. The purpose of the study was to investigate the differences in the biochemical properties of RBCs stored in the presence or absence of LC, and the cell-age related responses to storage conditions and to LC. RBC concentrates in saline-adenine-glucose-mannitol (SAG-M) were stored in the presence or absence of 5 mM LC at 4 degrees C for up to 8 weeks. RBC subpopulations of different densities were prepared by centrifugation on Stractan density gradient. Cells were sampled at 0, 3, 6, and 8 weeks, and hematological and cellular properties analyzed (MCV, MCHC, 4.1a/4.1b ratio as a cell age parameter, intracellular Na(+) and K(+)). After 6 weeks, MCV of RBC stored in the presence of LC was lower than that of controls (6 weeks MCV: controls 95.4 +/- 1.8 fl; LC 91.5 +/- 2.0 fl; n = 6; P < 0.005). This was due to swelling of control cells, and affected mainly older RBCs. LC appeared to reduce or retard cell swelling. Among the osmotically active substances whose changes during storage could contribute to cell swelling, only intracellular Na(+) and K(+) differed between stored control RBCs and LC-treated cells. LC reduces the swelling of older cells during storage at 4 degrees C in SAG-M, possibly by acting on the permeability of cell membrane to monovalent cations.

Natural phosphatidylcholine is actively translocated across the plasma membrane to the surface of mammalian cells

The cell surface of eukaryotic cells is enriched in choline phospholipids, whereas the aminophospholipids are concentrated at the cytosolic side of the plasma membrane by the activity of one or more P-type ATPases. Lipid translocation has been investigated mostly by using short chain lipid analogs because assays for endogenous lipids are inherently complicated. In the present paper, we optimized two independent assays for the translocation of natural phosphatidylcholine (PC) to the cell surface based on the hydrolysis of outer leaflet phosphoglycerolipids by exogenous phospholipase A2 and the exchange of outer leaflet PC by a transfer protein. We report that PC reached the cell surface in the absence of vesicular traffic by a pathway that involved translocation across the plasma membrane. In erythrocytes, PC that was labeled at the inside of the plasma membrane was translocated to the cell surface with a half-time of 30 min. This translocation was probably mediated by an ATPase, because it required ATP and was vanadate-sensitive. The inhibition of PC translocation by glibenclamide, an inhibitor of various ATP binding cassette transporters, and its reduction in erythrocytes from both Abcb1a/1b and Abcb4 knockout mice, suggest the involvement of ATP binding cassette transporters in natural PC cell surface translocation. The relative importance of the outward translocation of PC as compared with the well characterized fast inward translocation of phosphatidylserine for the overall asymmetric phospholipid organization in plasma membranes remains to be established.

Control of mitochondrial beta-oxidation flux

The control of mitochondrial beta-oxidation, including the delivery of acyl moieties from the plasma membrane to the mitochondrion, is reviewed. Control of beta-oxidation flux appears to be largely at the level of entry of acyl groups to mitochondria, but is also dependent on substrate supply. CPTI has much of the control of hepatic beta-oxidation flux, and probably exerts high control in intact muscle because of the high concentration of malonyl-CoA in vivo. beta-Oxidation flux can also be controlled by the redox state of NAD/NADH and ETF/ETFH(2). Control by [acetyl-CoA]/[CoASH] may also be significant, but it is probably via export of acyl groups by carnitine acylcarnitine translocase and CPT II rather than via accumulation of 3-ketoacyl-CoA esters. The sharing of control between CPTI and other enzymes allows for flexible regulation of metabolism and the ability to rapidly adapt beta-oxidation flux to differing requirements in different tissues.

Evaluation of brain long-chain acylcarnitines during cerebral ischemia

Concentration and distribution of long-chain acylcarnitines in control microwaved and ischemic (decapitated) rat brain were measured by electrospray ionization tandem mass spectrometry. The total acylcarnitine concentration from control rat brains equaled 7-8 nmol/g wet weight brain, about one-fourth the total concentration of long-chain acyl-CoA, indicating a small role in buffering the total acyl-CoA pool concentration. Furthermore, acylcarnitine did not differ between ischemic and control rat brain with regard to total concentration or concentrations of molecular species of acylcarnitine. Therefore, the size of the acylcarnitine pool in brain is not affected by the dramatic increase in unesterified fatty acids (approximately 4x) that occurs in ischemia.

The malonyl-CoA-long-chain acyl-CoA axis in the maintenance of mammalian cell function

Long-chain acyl-CoA esters have potent specific actions (e.g. on gene transcription, membrane trafficking) as well as non-specific ones (e.g. on phospholipid bilayers). They are synthesized on the cytosolic aspects of several intracellular membranes, to give rise to (a) cytosolic pool(s) to which a variety of enzymes and processes have access, including some localized in the nucleus. Their concentration in cells is highly regulated, interconversion with corresponding acylcarnitines being the most important mechanism involved. This reaction is catalysed by cytosol-accessible carnitine long-chain acyl (palmitoyl) transferase activities that are themselves located on multiple membrane systems. Regulation of these activities is through the inhibitory action of malonyl-CoA. Hence the existence of a potent malonyl-CoA-acyl-CoA axis through which many processes involved in the maintenance of mammalian cell function are regulated. The molecular, topographical and physiological interactions that make this possible are described and discussed.

Erythrocyte membrane reacylation in juvenile neuronal ceroid-lipofuscinosis: measurement of membrane-bound carnitine palmitoyl transferase, acyl-CoA synthetase, and lysophospholipid: acyl-CoA acyltransferase activities

In order to study the biochemical mechanisms responsible for the membrane fatty acid deficiency in juvenile neuronal ceroid-lipofuscinosis, we have analyzed the reacylation pathway in isolated erythrocyte membranes in 5 patients. We studied membrane carnitine palmitoyl transferase, and developed a combined assay to study acyl-CoA synthetase and lysophospholipid acyl-CoA acyltransferase activities. There were no significant differences between control and patient membranes, suggesting that abnormalities in these 3 putative candidate enzymes are not responsible for the disease.

Substrate specificity of rat liver mitochondrial carnitine palmitoyl transferase I: evidence against alpha-oxidation of phytanic acid in rat liver mitochondria

The two branched chain fatty acids pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) and phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) were converted to co-enzyme A thioesters by rat liver mitochondrial outer membranes. However, these branched chain fatty acids could not be converted to pristanoyl and phytanoyl carnitines, respectively, by mitochondrial outer membranes. As expected, the unbranched long chain fatty acids, stearic acid and palmitic acid, were rapidly converted to stearoyl and palmitoyl carnitines, respectively, by mitochondrial outer membranes. These observations indicate that the branched chain fatty acids could not be transported into mitochondria. The data presented strongly suggest that in rat liver, alpha-oxidation of phytanic acid occurs in organelles other than mitochondria.

Evidence for the involvement of carnitine-dependent long-chain acyltransferases in neuronal triglyceride and phospholipid fatty acid turnover

This study focuses on the potential involvement of carnitine palmitoyltransferase (CPT) on the phospholipid and triglyceride fatty acid turnover in neurons. This category of enzymes, which has been identified in several rat brain tissues, is well known for its role in modulating cellular fatty acid oxidation. Neuronal cell cultures from rat brain cortex incorporated radioactive palmitate or oleate into phospholipids and triglycerides. The largest fraction of radioactive fatty acids was recovered in phosphatidylcholine followed by triglycerides and, to a lesser extent, phosphatidylethanolamine. CPT activity measured in neuronal lysates obtained from neurons treated with 40 microM 2-tetradecylglycidic acid (TDGA) was almost completely abolished. Furthermore, between 2 and 10 microM TDGA CPT activity dropped more rapidly than between 10 and 40 microM. When the cells were pretreated with TDGA, the incorporation process of either radioactive fatty acid into triglycerides was dose-dependently suppressed. Radioactive fatty acid incorporation into phosphatidylcholine was significantly decreased in cells treated with TDGA. In contrast, phosphatidylethanolamine reacylation was essentially not affected by the CPT inhibitor. Similar results on the fatty acid incorporation into triglycerides and phospholipids were observed with neurons treated with palmitoyl-DL-aminocarnitine (PAC), a reversible CPT inhibitor, which does not consume free CoA. These effects do not seem to be the result of an inhibitory activity toward one of the steps involved in the acylation-deacylation process of triglycerides or phospholipids, as cellular lysates from TDGA-treated cells or lysates containing PAC incorporated radioactive fatty acids at rates comparable to controls. Our results suggest that CPT may be an important partner in the pathway of phospholipid and triglyceride fatty acid turnover in neurons.

Acyl-trafficking in membrane phospholipid fatty acid turnover: the transfer of fatty acid from the acyl-L-carnitine pool to membrane phospholipids in intact human erythrocytes

In this work we have investigated the transfer of radioactive palmitic acid between membrane phospholipids and acyl-L-carnitines in intact human erythrocytes. During the incubation period of labeled erythrocyte in non-defatted bovine serum albumin, radioactivity in phosphatidylcholine and phosphatidylethanolamine increased. On the contrary, a decrease of radioactivity in erythrocyte palmitoyl-L-carnitine was observed. 2-Tetradecylglycidic acid, an irreversible erythrocyte carnitine palmitoyltransferase inhibitor, abolished any radioactivity changes in both phospholipids and palmitoyl-L-carnitine. Similar findings were obtained by using erythrocytes labeled with radioactive oleic acid. Our data suggest that in human erythrocytes a carnitine palmitoyltransferase-catalyzed acyl transfer from acyl-L-carnitine to phospholipids, rather than a previously described fatty acid transfer from phosphatidylcholine to phosphatidylethanolamine, is operative.

Regulation of mitochondrial carnitine palmitoyl transferases from liver and extrahepatic tissues

Developments in our understanding of the complex CPT enzyme system over the past ten years have been reviewed. Liver CPT1, which is probably distinct from that in several extrahepatic tissues, is subject to up- or down-regulation of its activity and kinetic properties with changing physiological state. Evidence is now accumulating to support the notion that the catalytic and malonyl-CoA-binding entities of CPT1 are separate polypeptides.