Transport of fatty acids across the membrane of human erythrocyte ghosts

Interaction of acyl-CoA binding protein (ACBP) on processes for which acyl-CoA is a substrate, product or inhibitor

It is shown that acyl-CoA binding protein (ACBP), in contrast with fatty acid binding protein (FABP), stimulates the synthesis of long-chain acyl-CoA esters by mitochondria. ACBP effectively opposes the product feedback inhibition of the long-chain acyl-CoA synthetase by sequestration of the synthesized acyl-CoA esters. Feedback inhibition of microsomal long-chain acyl-CoA synthesis could not be observed, due to the formation of small acyl-CoA binding vesicles during preparation and/or incubation. Microsomal membrane preparations are therefore unsuitable for studying feedback inhibition of long-chain acyl-CoA synthesis. ACBP was found to have a strong attenuating effect on the long-chain acyl-CoA inhibition of both acetyl-CoA carboxylase and mitochondrial adenine nucleotide translocase. Both processes were unaffected by the presence of long-chain acyl-CoA esters when the ratio of long-chain acyl-CoA to ACBP was below 1, independent of the acyl-CoA concentration used. It is therefore not the acyl-CoA concentration as such which is important from a regulatory point of view, but the ratio of acyl-CoA to ACBP. The cytosolic ratio of long-chain acyl-CoA to ACBP was shown to be well below 1 in the liver of fed rats. ACBP could compete with the triacylglycerol-synthesizing pathway, but not with the phospholipid-synthesizing enzymes, for acyl-CoA esters. Furthermore, in contrast with FABP, ACBP was able to protect long-chain acyl-CoA esters against hydrolysis by microsomal acyl-CoA hydrolases. The results suggest that long-chain acyl-CoA esters synthesized for either triacylglycerol synthesis or beta-oxidation have to pass through the acyl-CoA/ACBP pool before utilization. This means that acyl-CoA synthesized by microsomal or mitochondrial synthetases is uniformly available in the cell. It is suggested that ACBP has a duel function in (1) creating a cytosolic pool of acyl-CoA protected against acyl-CoA hydrolases, and (2) protecting vital cellular processes from being affected by long-chain acyl-CoA esters.

Transfer of long-chain fluorescent fatty acids between small and large unilamellar vesicles

Transfer of 12-(9-anthroyloxy)stearic acid (12AS) was measured between small unilamellar vesicles (SUV) and between large unilamellar vesicles (LUV), over a temperature range of 5-50 degrees C. The results of this study clearly establish the biexponential nature of the time dependence of the transfer in a variety of vesicle types and confirm our previous results using egg phosphatidylcholine (EPC) SUV at 25 degrees C (Storch & Kleinfeld, 1986). In our previous study we developed a kinetic model of the transfer process and concluded that the observed time dependence of the transfer of long-chain 12-(9-anthroyloxy) fatty acids (AOFA) was due to transbilayer flip-flop that was much slower than the rate at which the fatty acids (FA) move from the vesicle and into the surrounding aqueous phase (the off step). In the present study, experimental and theoretical advances have allowed us to examine, in detail, predictions of the kinetic model that critically depend upon the slow rate of flip-flop. The current results verify these predictions and demonstrate that slow AOFA flip-flop is rate limiting in at least three different vesicle systems and at all temperatures studied. Moreover, both flip-flop and the off rate constants were almost an order of magnitude smaller in EPC-LUV than in EPC-SUV. Flip-flop was found to be asymmetric (the rate constant for transfer from the inner to outer hemileaflet of the bilayer is approximately twice that from the outer to inner hemileaflet) in SUV but virtually symmetric in LUV. The temperature dependence of transfer was used to determine the thermodynamic activation potentials for the flip-flop and off rate constants.(ABSTRACT TRUNCATED AT 250 WORDS)

Fast translocation of phosphatidylcholine to the outer membrane leaflet after its synthesis at the inner membrane surface in human erythrocytes

The translocation rate of [14C]phosphatidylcholine to the outer membrane leaflet of human erythrocytes after its primary synthesis from lysophosphatidylcholine by acylation with 14C-labeled oleic or palmitic acid in the inner leaflet has been measured by following the time-dependent increase of cleavability of 14C-labeled phospholipids by external phospholipase A2 (5 min, 37 degrees C). Immediately after a short acylation time period of 10 min about 20% of the newly synthesized [14C]phosphatidylcholine are already detectable in the outer leaflet. After an incubation of 1 h at 37 degrees C following 10 min of acylation the fractions of labeled and native phosphatidylcholine accessible to the lipase are identical, which demonstrates that [14C]phosphatidylcholine has attained the same asymmetric distribution as its endogenous analogue. The calculated halftime of the outward translocation is about 20 min and its activation energy is low, 30 kJ/mol. Translocation is inhibited by a 5 min treatment with phenylglyoxal following acylation. A fast translocation is not observed for newly synthesized phosphatidylethanolamine. Results suggest a selective, protein-mediated outward translocation of newly synthesized phosphatidylcholine.

Oxidation of vitamin E in red cell membranes by fatty acids, hydroperoxides and selected oxidants

Our laboratory previously reported that vitamin E (tocopherol) in human blood platelets was oxidized in vitro by various oxidants. This paper shows that diamide, superoxide, hydroperoxides and polyunsaturated fatty acids induce oxidation of tocopherols in red cell membranes. In contrast to platelets, red cell membrane tocopherol was oxidized by hydrogen peroxide and tertiary butyl hydroperoxide. Alpha tocopherolquinone was one of the products of oxidation. Among the fatty acids, the cis polyunsaturated acids were the most potent oxidizing agents with monounsaturated and trans compounds relatively ineffective. The oxidation is not a detergent effect of the fatty acids since neither the detergents Brij and Lubrol, when present in concentrations under 0.5 mM, nor sodium arachidonate (1.25 mM), could oxidize the membrane tocopherol. When red cell membrane samples were incubated with 0.5 mM arachidonate, 47 +/- 11% (S.D.) of the tocopherol lost was converted to tocopherolquinone. Unlike arachidonate, oxidants such as diamide, hydrogen peroxide and tertiary butylhydroperoxide are unable to oxidize all of the membrane tocopherol and produce less tocopherolquinone from oxidation (10-15%) under the experimental conditions of this study. Linoleic acid hydroperoxide is a much more potent oxidant and produces less quinone than arachidonate. The mechanisms of tocopherol oxidations induced by the various compounds seem to be different since the yields of quinone during oxidation vary with the nature of the oxidant. Tocopherol is consumed by oxidation as it protects the membrane from oxidant damage induced by compounds such as unsaturated fatty acids and hydroperoxides.

The Wellcome Trust lecture. Mechanisms of molecular trafficking in malaria

The asexual stages of Plasmodium living within the erythrocyte result in growth-related changes in the permeability properties of the red cell for substances such as glucose, amino acids, purine nucleosides, sodium, potassium, calcium, zinc, iron and several antimalarial drugs such as chloroquine, amodiaquine and mefloquine. In most cases such changes do not appear to be due to a modification in the affinity or specificity of red cell transporters; indeed, for most substances the membrane-associated transporters are either unaffected or are partially inactivated. In malaria-infected erythrocytes, where a striking increase in influx has been observed, it has been attributed to the insertion of parasite-encoded transporters into the red cell membrane or the formation of aqueous leaks and/or pores. Leak formation, in the vast majority of cases, does not appear to be dependent on the insertion of plasmodial proteins into the red cell membrane. However, since the data presently available are less than satisfactory for discriminating amongst the various possible transport mechanisms future studies will require painstaking efforts and carefully controlled conditions to discriminate amongst the various transport systems which are operational in the malaria-infected red cell and the parasite.

Rat hepatocyte plasma membrane acyl:CoA synthetase activity

The presence of long chain acyl:CoA synthetases in mammalian microsomes and mitochondria has been established previously by Aas (Biochim. Biophys. Acta 231, 32-47 [1971] ). The presence of a plasma membrane-associated enzyme was investigated in rat hepatocyte plasma membranes, where an enzyme exhibiting high activity and with a preferred substrate of 18-carbon chain length was discovered. The results are consistent with the presence of a single enzyme. The effect of the degree of unsaturation of the fatty acid substrates was not as pronounced as that arising from the length of the carbon chain. The pattern of substrate preference of the enzyme was omega 3 polyenoic fatty acids greater omega 6 polyenoic acids greater than omega 9 monoenoic acids greater than saturated acids. This may relate to the similar substrate preference pattern exhibited by the fatty acyl desaturase enzymes. The role played by long chain acyl:CoA synthetase in hepatocyte metabolism is uncertain, but it may relate to the incorporation of polyenoic fatty acids from the circulation into cell membranes and the trapping of other fatty acids within the cell for further metabolism.

Metabolic incorporation of 9-(2-anthryl)-nonanoic acid, a new fluorescent and photoactivable probe, into the membrane lipids of Chinese hamster ovary cells

9-(2-Anthryl)-nonanoic acid is a new fluorescent and photoactivable probe, which has been designed for studying the lateral diffusion rate and the lateral distribution of lipids in biological membranes by means of the anthracene photodimerization reaction. It is shown that this anthracene fatty acid is metabolically incorporated into the glycerophospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol) of the eukaryotic Chinese hamster ovary cells in culture. Under our culture conditions (Eagle's minimal essential medium plus delipidized fetal calf serum) this incorporation proceeded with a very good rate (up to 45 mol/100 mol, after two days culture) and could be easily modulated depending on the way the cells were fed with the anthracene fatty acid. It occurred to a similar extent at the sn-1 (55 +/- 5%) or at the sn-2 (45 +/- 5%) position on the phospholipid glycerol backbone, without any degradation or elongation. No double labelling at the sn-1 and sn-2 positions was detected. Although incorporation of the anthracene fatty acid affected the cell growth rate (generation time of 48 h compared to a generation time of 21 h for control cells) it did not bring about cell mortality. This incorporation took place not only into the phospholipids but also into the triglycerides with, as a consequence, the appearance of strongly fluorescent lipid vesicles inside the cells. It affected the whole cell fatty acid composition by slightly increasing the amount of palmitic acid and markedly decreasing the amount of stearic and oleic acids.

Erythrocyte membrane acyl:CoA synthetase activity

The presence of long-chain acyl:CoA synthetases in mammalian microsomes and mitochondria has been established previously [(1971) Biochim. Biophys. Acta 231, 32-47]. The presence of a plasma membrane-associated enzyme was investigated in human erythrocyte ghost plasma membranes, where an enzyme exhibiting high activity, and with a preferred substrate of 18 carbon chain length, was discovered. The results are consistent with the presence of a single enzyme. The effect of the degree of unsaturation of the fatty acid substrates was not as pronounced as that arising from the length of the carbon chain. The pattern of substrate preference of the enzyme was omega 3 polyenoics greater than omega 6 polyenoics greater than omega 9 monoenoics greater than saturated fatty acids. This may relate to the similar substrate preference pattern exhibited by the fatty acyl desaturase enzymes. However, the role played by long-chain acyl:CoA synthetase in erythrocyte metabolism is uncertain, but may relate to the transportation of polyenoic fatty acids in the circulation.