Lipid requirement of membrane-bound enzymes

Changes by progesterone derivatives in fatty acids from phosphatidylcholine and phosphatidylethanolamine fractions in rat liver endoplasmic reticulum

The effects of two progesterone metabolites on fatty acid composition of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) from total liver and liver microsomes were studied in female rats. 16 alpha-Hydroxyprogesterone significantly increased the amount of fatty acids esterified to PC and PE fractions in total liver and liver microsomes. Both saturated and unsaturated fatty acyl components were enhanced. In contrast, 5 beta-pregnan-3 alpha-ol-20-one caused a reduction of fatty acids bound to PC and PE fractions from total liver and liver microsomes. Pregnanolone decreased both saturated and unsaturated fatty acids. Changes in specific fatty acids occurred in palmitic and stearic acids among saturated components, and palmitoleic, oleic, linoleic, eicosatrienoic, arachidonic and docosahexenoic acids among unsaturated ones. The unsaturated: saturated fatty acid ratio was raised by 16-alpha-hydroxyprogesterone and lowered by 5 beta-pregnan-3 alpha-ol-20-one in all phospholipid fractions. The induction of drug metabolizing enzymes by 16 alpha-hydroxyprogesterone may be related to an enhanced synthesis of microsomal phospholipids containing unsaturated fatty acids, particularly arachidonic and docosahexenoic acids. In contrast, the inhibition of drug metabolism by 5 beta-pregnan-3 alpha-ol-20-one is associated with reduced formation of unsaturated fatty acyl side chains.

Effect of dietary fats on some membrane-bound enzyme activities, membrane lipid composition and fatty acid profiles of rat heart sarcolemma

The effect of various dietary fats on membrane lipid composition, fatty acid profiles and membrane-bound enzyme activities of rat cardiac sarcolemma was assessed. Four groups of male weanling Charles Foster Young rats were fed diets containing 20% of groundnut, coconut, safflower or mustard oil for 16 weeks. Cardiac sarcolemma was prepared from each group and the activities of Na+, K(+)-ATPase, 5'-nucleotidase, Ca(2+)-ATPase and acetylcholinesterase were examined. ATPase activities were similar in all groups except the one fed coconut oil, which had the highest activities. Acetylcholinesterase activity was also similar in all the groups, however, it was significantly higher in the group fed mustard oil. No significant changes were observed among the groups in 5'-nucleotidase activity, in the cholesterol-to-phospholipid molar ratio and in sialic acid content. The coconut, safflower and mustard oil diets significantly increased cholesterol and phospholipid contents and the lipid-to-protein ratio of cardiac sarcolemma as compared to feeding the groundnut oil diet. The fatty acid composition of membrane lipids was quite different among the various groups, reflecting the type of dietary fat given. The total unsaturated-to-saturated fatty acid ratio was not different among the various groups; however, the levels of some major fatty acids such as palmitic (16:0), oleic (18:1) and linoleic (18:2) acids were significantly different. Cardiac sarcolemma of the group fed safflower oil had the highest polyunsaturated fatty acid content. The results suggest that dietary fats induce changes not only in the fatty acid composition of the component lipids but also in the activities of sarcolemmal enzymes involved in the regulation of cardiac function.

Effect of dietary fats on erythrocyte membrane lipid composition and membrane-bound enzyme activities

Four different oil-based diets were used in a feeding study involving rats to assess the relationship between the fatty acid composition of the dietary fat and its influence on erythrocyte membrane (EM) lipid composition and the activities of membrane-bound enzymes. Nutritionally adequate diets containing 20% groundnut (GNO), coconut (CO), safflower (SO), or mustard oil (MO) were fed to weanling CFY rats for 4 months. EMs were analyzed for total cholesterol, phospholipids, fatty acid profiles, and sialic acid content. Activities of membrane-bound enzymes such as Na+, K(+)-adenosine triphosphatase (ATPase), Mg(2+)-ATPase, Ca2+, Mg(2+)-ATPase, and acetylcholinesterase were also assayed. The activities of all membrane-bound enzymes, except Mg(2+)-ATPase, and sialic acid content were higher in the MO-fed group than in the rest of the groups. Ca2+, Mg(2+)-ATPase activity was distinctly lower in the SO-fed group than in the other groups. Cholesterol to phospholipid molar ratio was similar in all the groups. However, SO- and MO-fed groups displayed an increased cholesterol content and a higher degree of unsaturation in the membrane fatty acid composition. The higher membrane fatty acid unsaturation in the SO-fed group was principally due to linoleic (18:2) and arachidonic (20:4) acids, while in the MO-fed group it was mainly due to oleic (18:1), eicosenoic (20:1), erucic (22:1), and linoleic (18:2) acids. These results suggest a relationship between the quality of dietary fat, EM fatty acyl composition, and the activities of membrane-bound enzymes.

Pathobiochemical mechanisms of hepatocellular damage following lipid peroxidation

In hepatocytes, cytotoxic events induced by haloalkanes or acute iron-overload exhibit neither a quantitative nor a temporal correlation to lipid peroxidation or covalent binding. Thus, secondary pathological mechanisms have been postulated linking initial focal reactions of free radicals and end stage pathological consequences. Due to the crucial role of plasma-membrane integrity in the cytotoxic process it has to be supposed that relevant secondary pathological mechanisms finally impair the physico-chemical and functional properties of this membrane. Based on recent developments a chain of causality is proposed as a two-step activation of phospholipase A2 producing cytolytic amounts of lysophosphatides. In this cascade, the initial activating step is a decrease of membrane lipid fluidity induced by lipid peroxidation and/or by calcium binding and intramembranous formation of 4-hydroxynonenal. This enzyme activation is further amplified by the early rise of cytosolic calcium. Consequently, increasing amounts of lysophosphatides progressively impair membrane configuration thus improving the substrate accessibility for phospholipase A2 in a second activation step. Finally, the lysophosphatides reach plasma membrane-lytic concentrations by this autocatalytic enzyme activation.

Role of fatty acid structure in the reversible activation of phosphatidylcholine synthesis in lymphocytes

Fatty acids rapidly accelerate (1.5-7.0-fold) the incorporation of [methyl-3H]choline chloride into the phosphatidylcholine fraction of bovine lymphocyte lipids. This ability of fatty acids to activate choline phospholipid synthesis has been correlated with certain structural features of fatty acids. Mono- and polyenoic unsaturated fatty acids of 18 and 20 carbons in length are highly active, whereas their saturated analogues are nearly inactive. Among the unsaturated fatty acids, the cis-isomers are active, while the trans-isomers are relatively ineffective. The delayed addition of bovine serum albumin (5 mg/ml) and other lipid-binding proteins to activated cells rapidly counteracts the lipid effects. The activated state of the cell membrane thus appears to be a dynamic one, requiring the continued interaction of the fatty acid with a lipid-sensitive target molecule of the cell surface that in turn appears to coordinate the enzymatic components of this pathway.

Phosphatidylglycerol stimulates cholinephosphate cytidylyltransferase activity and phosphatidylcholine synthesis in type II pneumocytes

Phosphatidylcholine synthesis in type II pneumocytes is stimulated by inclusion of phosphatidylglycerol and other phospholipids in the culture medium (Gilfillan, A.M., Chu, A.J. and Rooney, S.A. (1984) Biochim. Biophys. Acta 794, 269-273). We have now examined the effect of phosphatidylglycerol in the medium on enzymes of de novo phosphatidylcholine synthesis in adult rat type II cells. Activities of choline kinase, cholinephosphate cytidylyltransferase and cholinephosphotransferase in homogenates of whole lung and type II cells were generally similar. Phosphatidate phosphatase activity in type II cells, however, was only 16% that in whole lung. Addition of phosphatidylglycerol (10 microM) to the culture medium had no effect on choline kinase, cholinephosphotransferase or phosphatidate phosphatase activities in type II cells but it increased the activity of cholinephosphate cytidylyltransferase by 56%. Since it is known that cholinephosphate cytidylyltransferase is stimulated in vitro by addition of phospholipids to the assay mixture, we also measured its activity in the presence of sufficient phosphatidylglycerol (1.1 mM) to maximally stimulate in vitro. Even under these conditions cholinephosphate cytidylyltransferase activity in type II cells cultured in the presence of phosphatidylglycerol was 32% greater than in control cells. These data show that the stimulatory effect of phospholipid in the culture medium on phosphatidylcholine synthesis in type II cells is mediated by increased cholinephosphate cytidylyltransferase activity. The mechanism of increased cytidylyltransferase activity remains to be elucidated but it is not due to direct in vitro activation by the phospholipid.

Evidence against an abnormal hepatic microsomal lipid matrix as the primary genetic defect in the jaundiced Gunn rat

The congenitally jaundiced Gunn rat does not conjugate bilirubin but does conjugate bilirubin dimethyl diester. Partial defects in conjugating p-nitrophenol and demethylating aminopyrine are also evident. A proposed mechanism to explain this combination of findings is a defective microsomal membrane. To examine the 'matrix' of Gunn microsomal membranes, hepatic microsomes were isolated from Gunn (jj) and outbred Wistar (JJ) rats and were studied by electron paramagnetic resonance spectroscopy of 7-doxylstearic and 12-doxylstearic acid probes, fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene, glucose-6-phosphatase activity vs. temperature, and lipid analysis. The data indicate several factors related to lipid bilayer order do not differ in microsomes from jj and JJ.

Incorporation of bovine thyroid peroxidase in liposomes

Bovine thyroid peroxidase (TPO), an enzyme requiring lipids for demonstrating catalytic activity, was incorporated in liposomes made of pure phospholipids. The enzyme did not show high differences in activity when bilayer thickness was changed, but dipalmitoyl phosphatidyl choline (DPPC) seemed to be more appropriate for activity. The perturbation caused on lipid fluidity by enzyme incorporation was studied by differential scanning calorimetry (DSC) and fluorescence polarization of the apolar probe 1,6-diphenyl-1,3,5-hexatriene (DPH). The complexes of TPO with dimyristoyl phosphatidyl choline (DMPC), DPPC, and distearoyl phosphatidyl choline (DSPC) bilayers showed transition temperatures (Tc) which were lower than the characteristic ones shown by liposomes with the respective phospholipids alone. The microsomal fraction from which TPO was extracted was in the fluid state at 37 degrees C, the temperature at which thyroid peroxidase works 'in vivo'. Since the effect of the protein in lowering the transition temperature of the phospholipids was so low, the contribution of phospholipids containing unsaturated fatty acids has to be essential for obtaining a fluid bilayer at body temperature.

Incorporation of bovine thyroid peroxidase in liposomes

In the microsomal fraction of thyroid glands, the temperature dependence of DPH fluorescence polarization showed a discontinuity in the range of 29-33 degrees C. The transition temperatures of DMPC, DPPC and DSPC are near to the observed for the microsomal fraction. So that, thyroid peroxidase (TPO) was incorporated into liposomes made with these phospholipids. When DPH was incorporated in this peroxidase-liposome complex, a less pronounced phase transition was observed in the profiles of temperature dependence of DPH polarization, and the incorporation of the enzyme decreased the Tc. Arrhenius plots of TPO incorporated into liposomes showed discontinuities at similar temperatures observed by fluorescence polarization. The decrease of transition temperature of liposomes induced by thyroid peroxidase incorporation suggests that this enzyme seems to need a fluid medium for its enzyme activity.

Phospholipid fatty acid modification of rat liver microsomes affects acylcoenzyme A:cholesterol acyltransferase activity

The effect of phospholipid fatty acyl composition on the activity of acylcoenzyme A:cholesterol acyltransferase was investigated in rat liver microsomes. Specific phosphatidylcholine replacements were produced by incubating the microsomes with liposomes and bovine liver phospholipid-exchange protein. Although the fatty acid composition of the microsomes was modified appreciably, there was no change in the microsomal phospholipid or cholesterol content. As compared to microsomes enriched for 2 h with dioleoylphosphatidylcholine, those enriched with dipalmitoylphosphatidylcholine exhibited 30-45% less acyl-CoA:cholesterol acyltransferase activity. Enrichment with 1-palmitoyl-2-linoleoylphosphatidylcholine increased acyl-CoA:cholesterol acyltransferase activity by 20%. By contrast, dilinoleoylphosphatidylcholine abolished microsomal acyl-CoA:cholesterol acyltransferase activity almost completely. Addition of cofactors that stimulated microsomal lipid peroxidation inhibited acyl-CoA:cholesterol acyltransferase activity by only 10%, however, and did not increase the inhibition produced by submaximal amounts of dilinoleoylphosphatidylcholine. Certain of the phosphatidylcholine replacements produced changes in palmitoyl-CoA hydrolase, NADPH-dependent lipid peroxidase, glucose-6-phosphatase and UDPglucuronyl transferase activities, but they did not closely correlate with the alterations in acyl-CoA:cholesterol acyltransferase activity. Electron spin resonance measurements with the 5-nitroxystearate probe indicated that microsomal lipid ordering was reduced to a roughly similar extent by dioleoyl- or by dilinoleoylphosphatidylcholine enrichment. Since these enrichments produce widely different effects on acyl-CoA:cholesterol acyltransferase activity, changes in bulk membrane lipid fluidity cannot be the only factor responsible for phospholipid fatty acid compositional effect on acyl-CoA:cholesterol acyltransferase. The present results are more consistent with a modulation resulting from either changes in the lipid microenvironment of acyl-CoA:cholesterol acyltransferase or a direct interaction between specific phosphatidylcholine fatty acyl groups and acyl-CoA:cholesterol acyltransferase.

Ca2+- and Mg2+-dependent ATPase activities in the deoxycholate-treated rat heart sarcolemma

1. Isolated rat heart sarcolemma was treated with different concentrations of an ionic detergent, deoxycholate (DOC) and ATP hydrolysis in the presence of Ca2+ or Mg2+ was determined. 2. Both Ca2+-dependent ATPase and Mg2+-dependent ATPase activities were decreased in the DOC-treated membranes; however, the depression of Mg2+-dependent ATPase activity was greater than that of Ca2+-dependent ATPase. 3. The differential changes in Ca2+-dependent ATPase and Mg2+-dependent ATPase activities were apparent when incubations with DOC were carried out for different time intervals and at different temperatures. 4. In DOC-treated preparations, the Km value for Ca2+-dependent ATPase was decreased whereas that for Mg2+-dependent ATPase was increased. The half maximal velocities of the Ca2+-dependent ATPase and Mg2+-dependent ATPase enzyme reactions in the treated preparations were obtained at a DOC: membrane protein ratio of 3.0 and 0.6, respectively. 5. In the DOC-treated membranes exhibiting the half maximal velocities of enzyme reactions, the Ka value for Ca2+-dependent ATPase was drastically reduced but remained unchanged for Mg2+-dependent ATPase. 6. The DOC treatment was associated with a loss of protein as well phospholipids and resulted in changes in the ultrastructural integrity of the membrane. 7. Varying degrees of decreases in the activities of sarcolemmal adenylate cyclase. (Na+-K+)-ATPase, 5'-nucleotidase and calcium binding were seen upon DOC treatment. 8. The extent of reduction in Ca2+-dependent ATPase and Mg2+-dependent ATPase activities were also different when the membrane was treated with a non-ionic detergent, Lubrol PX. 9. These data suggest that Ca2+-dependent ATPase in heart sarcolemma is more resistant than Mg2+-dependent ATPase to detergent treatments and further indicate some differences in the properties of these enzymes.

Studies on (K+ + H+)-ATPase. IV. Effects of phospholipase C treatment

(1) The total phospholipid content of a gradient purified (K+ + H+)-ATPase preparation from pig gastric mucosa is 105 mumol per 100 mg protein, and consists of 29% sphingomyelin, 29% phosphatidylcholine, 28% phosphatidylethanolamine, 10% phosphatidylserine and 4% phosphatidylinositol. The cholesterol content corresponds to 50 mumol per 100 mg protein. (2) Treatment with phospholipase C (from Clostridium welchii and Bacillus cereus) results in an immediate decrease of the phosphate content. Up to 50% of the phospholipids are hydrolyzed by each phospholipase C preparation alone, without further hydrolysis by increased phospholipase concentration or prolonged incubation time. Combined treatment with the two phospholipase C preparations, sequentially or simultaneously, hydrolyzes up to 65% of the phospholipids. (3) The (K+ + H+)-ATPase and K+ stimulated p-nitrophenylphosphatase activities are decreased proportionally with the total phospholipid content, indicating that these enzyme activities are dependent on phospholipids. (4) Phospholipase C treatment does not change optimal pH, Km value for ATP and temperature dependence of the gastric (K+ + H+)-ATPase, but slightly decreases the Ka value for K+. (5) Phospholipase C treatment lowers the AdoPP[NH]P binding and phosphorylation capacities, suggesting that inactivation occurs primarily on the substrate binding level. (6) Most of the results can be understood by assuming that hydrolysis of the phospholipids by phospholipase C leads to aggregation of the membrane protein molecules and complete inactivation of the aggregated ATPase molecules.

The effect of hypercholesterolaemia on the lipid composition and ability of human platelets to accumulate cyclic AMP

Washed human platelets from seventeen hypercholesterolaemic subjects were compared to those from seventeen age and sex matched controls with respect to lipid composition and ability to accumulate cyclic AMP. No significant difference in platelet lipid composition was found in terms of total cholesterol, phospholipid or the ratio of cholesterol to phospholipid. There was no significant different in basal levels of platelet cyclic AMP between the two groups. Platelets from hypercholesterolaemic subjects showed an impaired ability to accumulate cyclic AMP compared with controls when challenged with prostaglandins (PG) I2, D2 and E1. The platelet inhibitory actions of PGI2, PGD2 and PGE1, which are mediated by cyclic AMP are thought to be important mechanisms for the control of platelet-platelet and platelet-vessel wall interaction. Thus, an impaired response to these three prostaglandins may be relevant to our understanding of the role played by the platelet in mediating vessel wall pathology caused by hypercholesterolaemia.