Microsomal Phospholipids Research Articles

Changes in some rat hepatic microsomal components induced by prolonged administration of chloroquine

Alterations in microsomal drug metabolizing enzymes and phospholipids following prolonged exposure to chloroquine have been investigated. The levels of microsomal aminopyrine- N-demethylase, aniline hydroxylase and both microsomal and cytosolic glutathione- S-transferase are reduced in treated rats. Microsomal epoxide hydrolase is unaffected by the treatment. An increase in the cholesterol-phospholipid ratio and a decrease in the phosphocholine-sphingomyelin ratio occur. There is a general reduction in the total microsomal phospholipid level though the percentage content of sphingomyelin is higher in all cases. The reduction in microsomal phospholipid level is probably due to a reduced incorporation of acetate into microsomal phospholipids as shown in this study using radioactive sodium acetate.

Effects of dietary linoelaidate on fatty acid composition and phospholipase activity in rat liver microsomes

The activity of calcium-stimulated microsomal phospholipase towards endogenous membrane bound phospholipids was monitored by analyzing the fatty acids released using gas chromatography. Activation by calcium (10 mM) resulted in the release of 2.1 nmoles fatty acid/10 mg microsomal protein/hr. The pattern of release of particular fatty acids reflected their concentration in the microsomal phospholipids. The effects of feeding 10 and 50% dietary linoelaidate (t,t18:2) and hydrogenated tallow (i.e., essential fatty acid deficient) on microsomal fatty acids and phospholipase activity was determined. The levels of t,t18:2 in the microsomal phospholipids increased with dietary intake and duration of feeding and attained 8.4% of total fatty acids after 11 weeks. There was a concomitant increase in linoleic acid and a decrease in arachidonic acid levels in microsomal lipids. The microsomal phospholipids of animals receiving exclusively hydrogenated tallow contained only trace amounts of linoleic and arachidonic acid after four weeks. The amount of eicosatrienoic acid increased dramatically in these animals. Palmitoleic and oleic acid also increased. The calcium-stimulated phospholipase activity in liver microsomes was unchanged by alterations in the fatty acid composition of the phospholipids.

Thigmomorphogenesis: membrane lipid and protein changes in bean plants as affected by mechanical perturbation and Ethrel

When bean plants (Phaseolus vulgaris L. cv. Red Cherokee Bush) are mechanically perturbed by rubbing, their stem elongation is inhibited and the stem thickness increases. The decrease in cell elongation and the increase in lateral cell divisions, which are partially responsible for this syndrome, were correlated with a decrease in the tree fatty acids and in the phospholipids of the membranes of microsomal fractions of first and second internodes of mechanically stimulated plants. This was true even though only the first internode was mechanically stimulated.Of the microsomal free fatty acids, mechanical perturbation induced an increase in myristic acid and large decreases in stearic, oleic, linoleic and linolenic acids. It also reduced the unsaturated:saturated ratio of the fatty acids. It induced a decrease in phosphatidyl choline but an increase in phosphatidyl ethanolamine. When the fatty acids were cleaved from the microsomal phospholipids, mechanical perturbation caused only a slight decrease in the unsaturated:saturated ratio and no significant changes in the double bond index.Mechanical perturbation induced an increase in the total microsomal protein and of membrane‐associated latent IDPase. However, the activity of membrane‐associated KCN‐insensitive NADPH cytochrome‐c reductase was decreased by mechanical perturbation.Treatment of the first internode with exogenous Ethrel produced results that were very similar in all respects to those obtained by mechanical perturbation. The factors inducing hardening against frost and drought, as achieved by mechanical perturbation and Ethrel treatment, are not only related to sterols or the polar head‐groups of phospholipids but may also be related to the protein components, and all may have an effect on the fluidity of a bilayer membrane model. These data support the hypothesis that ethylene mediates thigmomorphogenesis and that mechanical perturbation of the first internode results in the acropetal transport of a translocatable thigmomorphogenetic factor.

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 acyltransf erase 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.

Microsomal 5-ane-3β-hydroxysteroid oxidoreductase from pubertal rat leydig cells: Partial purification and characterization

A 3β-hydroxysteroid oxidoreductase which acts on 5α(β)-reduced C 19 and C 21 steroids (5-ane-3β-hydroxysteroid oxidoreductase; 5-ane-3β-HSO) has been solubilized from pubertal rat Leydig cell microsomes and purified 300-fold by ion exchange and gel filtration chormatography. The partially purified enzyme is stable only in the presence of 0.4 M NaCl and appears to exist as a molecule having a molecular weight of 35,000 or as aggregates with a molecular weight in excess of 150,000. NAD + and NADH + are used exclusively as cofactors. The velocity of the steroid oxidation reaction was unaffected by either Ca 2+ or Mg 2+. The steroid oxidation reaction has a pH optimum between 8.0 and 8.5, a temperature optimum at 35°C and an activation energy of 12,850 cal/mol. The pH optimum of the steroid reduction reaction is 6.6. A variety of 5α-reduced C 19 and C 21 steroids can be utilized as substrates. Treatment of microsomes with phospholipase A; resulted in a 26 to 90% loss of enzyme activity, paralleling decreased microsomal phospholipid content, and suggesting a role for phospholipids in 5-ane-3β-HSO activity. Assays with combined substrates indicate that one enzyme is responsible for activities observed with 5α- and 5β-reduced C 19- and 5α-reduced C 21-3β-hydroxysteroids. Purification data indicate that the 5-ane-3β-HSO and the 5-ene-3β-hydroxysteroid oxidoreductase: isomerase are distinct enzymes.

Long-term effects of a single oral dose of polybrominated biphenyls on serum and liver lipids in rats

Adult (5 months) male Sherman strain rats received a single dose of either 0 or 500 mg polybrominated biphenyls (PBB) in corn oil/kg body weight by stomach tube. After an 18-month recovery period, serum and liver samples were examined. The primary serum lipid response was an increase in cholesterol (both free and esterified) and in total phospholipids. The percentage of esterified cholesterol was not significantly different from that of the controls, and no significant differences in the cholesterol ester fatty acid composition were observed. Serum triglycerides were also unaffected. In the PBB-dosed animals, the total hepatic fatty acids contained significantly less palmitic acid and more stearic acid, consistent with an increase in palmitic acid chain elongation activity. No significant differences could be detected in the n−3 or n−6 acids except for a slight decline in the content of 22:6 ( n−3). Hepatic microsomal phospholipids were slightly higher (per milligram protein) in the PBB-dosed animals, and the cholesterol content was lower. Consequently, the cholesterol-phospholipid ratio was reduced, and microsomes from the latter group appeared to have an altered lipid domain on the basis of steady-state fluorescence anisotropy measurements. In addition, total hepatic thiobarbituric acid-reactive substances (assayed as malondialdehyde) were significantly increased in the PBB-dosed animals. This observation appeared to reflect an increased susceptibility to peroxidative stress in the latter group, probably resulting from reduced membrane antioxidant concentrations. The PBB-dosed rats had significantly lower serum retinol levels and a reduced content of this vitamin in liver microsomes. Microsomes were also deficient in α-tocopherol in the PBB-dosed animals, although serum levels were normal.

Altered microsomal phospholipid composition in the streptozotocin diabetic rat.

Streptozotocin diabetes in the rat alters liver microsomal membrane fatty acid composition. The present study was undertaken to determine if such changes in fatty acid composition were due to changes in the amount of individual phosphoglycerides or to disproportionate changes in fatty acid composition in any of the individual phosphoglycerides. The diabetic animals showed a small increase in total microsomal phospholipid, which is due to a selective increase in the phosphatidylethanolamine fraction. The changes in fatty acid composition in the total lipid extract (decreased palmitoleic, oleic and arachidonic acids and increased linoleic and docosahexaenoic acids) from the diabetic animals were present in both the major phosphoglycerides, phosphatidylcholine and phosphatidylethanolamine, with very little change in fatty acid composition in the phosphatidylserine and inositol fraction. Further studies are necessary to delineate the cause of the abnormal membrane phospholipid composition in the diabetic animal.

Changes in fatty acid distribution and thermotropic properties of phospholipids following phosphatidylcholine depletion in a choline-requiring mutant of Neurospora crassa

Growth of a choline requiring auxotroph of Neurospora crassa on medium lacking exogenous choline produces large changes in the levels of phosphatidylethanolamine and phosphatidylcholine. Whole cell fatty acid distributions were found to vary widely between different phospholipid species of normally growing, choline-supplemented cultures with phosphatidylcholine showing the highest levels of unsaturation and anionic phospholipids and cardiolipin having the lowest. In these lipids, choline deprivation produced little change in fatty acid profiles of phosphatidylethanolamine, whereas changes in fatty acids of phosphatidylcholine and acidic phospholipids resulted in increased levels of unsaturation at both growth temperatures. Microsomal phospholipids also showed fatty acid variability with sharp decreases in phosphatidylcholine unsaturates and increases in acidic phospholipid unsaturated fatty acids at low growth temperatures. Fluorescence polarization of 1,6-diphenylhexatriene in vesicles formed from total cellular and microsomal lipids showed that choline deprivation produces changes in thermotropic properties in the lipids in deprived cultures at either growth temperature. The effective differences in fluorescence polarization between choline-deprived and supplemented cultures grown at a given temperature were found to be comparable to those produced by temperature acclimation in normally growing cultures over a temperature range of 22 K.

Studies of the biochemical basis of steroid sulphatase deficiency—III. Phospholipid composition of microsomes from normal and steroid sulphatase deficient placentas

The phospholipid composition has been determined for placental microsomes from 11 normal and eight pregnancies complicated by steroid sulphatase deficiency. Phosphatidylcholine, phosphatidylethanolamine and sphingomyelin were found to be the major phospholipids of normal placental microsomes, comprising respectively 41.6 ± 4.6% (mean ± SD), 30 ± 5.7% and 22.5 ± 4.9% of the total phospholipid content. There was no correlation between the steroid sulphatase activity of the microsomes and the content of any of the three phospholipids. Though their contents were significantly decreased, ( P < 0.001) phosphatidylcholine, phosphatidylethanolamine and sphingomyelin similarly constituted the major portion of the total phospholipids in sulphatase deficient microsomes, representing 36 ± 4.2%, 34 ± 6.1% and 22.4 ± 6.7% respectively. Only the percentage of phosphatidylcholine was significantly different ( P < 0.02) from normal microsomes. The results show that the decreased phospholipid content of steroid sulphatase deficient placental microsomes reflects a lower content of all major classes of phospholipids, particularly phosphatidylcholine.

The effect of dietary partially hydrogenated marine oils on desaturation of fatty acids in rat liver microsomes.

The influence of dietary partially hydrogenated marine oils on distribution of phospholipid fatty acids in rat liver microsomes was studied with particular reference to the metabolism of linoleic acid. Five groups of weanling rats were fed diets containing 20% (w/w) peanut oil (PO), partially hydrogenated peanut oil (HPO), partially hydrogenated Norwegian capelin oil (HCO), partially hydrogenated herring oil (HHO), and rapeseed oil (RSO) for 10 weeks. The partially hydrogenated oils were supplemented with linoleic acid corresponding to 4.6 cal % in the diets. Accumulation of linoleic acid and reduced amount of total linoleic acid metabolites were observed in liver microsomal phospholipids from rats fed partially hydrogenated oils as compared to PO feeding. The most striking effects on the distribution of omega 6-polyunsaturated fatty acids was obtained after feeding HHO, a marine oil with a moderate content of trans fatty acids in comparison with HPO but rich in isomers of eicosenoic and docosenoic acids. Liver microsomal delta 6- as well as delta 5-desaturase activities as measured in vitro were reduced in rats kept on HHO as compared to PO dietary treatment. The results obtained suggest that the dietary influence of partially hydrogenated marine oils on the metabolism of linoleic acid might be better related to the intake of isomeric eicosenoic and docosenoic acids than to the total intake of trans fatty acids.

Studies on the Interactions between Phospholipids and Membrane-Bound Enzymes in Microsomes. Effects of Phospholipases C on the Glucose-6-Phosphatase System of Rat Liver Microsomes1

The role of phospholipids in the glucose-6-phosphatase system, including glucose-6-P phosphohydrolase and glucose-6-P translocase, was studied in rat liver microsomes by using phospholipases C and detergents. In the time course experiments on detergent exposure, the maximal activation of glucose-6-P phosphohydrolase varied according to the nature of the detergent used. On treatment of microsomes with phospholipase C of C. perfringens, the activity of glucose-6-P phosphohydrolase without detergent (i.e. without rupture of translocase activity) was gradually decreased with the progressive hydrolysis of phosphatidylcholine and phosphatidylethanolamine on the microsomal membrane, and was restored by incubation of these microsomes with egg yolk phospholipids. The extent of decrease in this phosphohydrolase activity in the detergent-exposed microsomes (with rupture of translocase activity) also varied depending on the detergent used (Triton X-114 or taurocholate). When 66% of the phosphatidylinositol on the membrane was hydrolyzed by phosphatidylinositol-specific phospholipase C of B. thuringiensis, the inhibition of glucose-6-P phosphohydrolase activity without detergent was very small. Although the inhibition of enzyme activity with detergent was apparently greater than that without detergent, the enzyme activity was stimulated by the breakdown of phosphatidylinositol when the enzyme activity was measured at lower concentration (0.5 mM) of substrate, glucose-6-P. The latency of mannose-6-P phosphohydrolase, a plausible index of microsomal integrity, remained above 70% after the hydrolysis of phosphatidylcholine, phosphatidylethanolamine, or phosphatidylinositol. The results show that the glucose-6-phosphatase system requires microsomal phospholipids for its integrity, suggesting that there exists a close relation between phosphatidylinositol and glucose-6-P translocase.

Inhibitory effect of glucose on the maturation of rat liver mitochondria at birth. Phospholipid and oxidative metabolism

(1) The rate of ATP synthesis coupled with succinate oxidation in rat liver mitochondria is low at birth and increases rapidly during the first postnatal hours (Nakazawa, T., Asami, K., Suzuki, H. and Yakawa, O. (1973) J. Biochem. 73, 397–406). A glucose injection given to newborn rats immediately after birth seemed to delay this maturation process. (2) Glucose administration specifically diminished the rate of 32P i incorporation into phosphatidylcholine both in microsomes and in mitochondria while other phospholipids remained unaffected. (3) In newborn rat liver, 32P i incorporation into phospholipids can be explained by de novo synthesis of phospholipids in microsomes followed by transfer to mitochondria with two exceptions: phosphatidylserine and sphingomyelin. Indeed, after a 20-min incorporation of 32P i into phospholipids, the specific radioactivity of phosphatidylserine and sphingomyelin was higher in mitochondria than in microsomes. (4) As far as phospholipid synthesis is concerned, no precursor-product relationship could be observed between light and heavy mitochondria.

51] Nonspecific lipid transfer protein from rat and beef liver: Purification and properties

This chapter discusses the purification and properties of nonspecific lipid transfer protein from rat and beef liver. The nonspecific lipid transfer proteins accelerate the intermembrane transfer of phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, phosphatidic acid, phosphatidylglycerol, sphingomyelin, cholesterol, neutral glycosphingolipids, and gangliosides. The transfer proteins are useful tools for the study of the distribution and movement of phospholipids in artificial vesicles, microsomes, intact erythrocytes, and Semliki Forest virus and of cholesterol in the brush-border plasma membranes of the rabbit intestine. The assay of transfer activity is based on the ability of nonspecific lipid transfer proteins to accelerate the transfer of labeled phosphatidylethanolamine from phosphatidylethanolamine–phosphatidylcholinc small unilamellar vesicles to beef heart mitochondria. A trace of labeled triolein is included in the small unilamellar vesicles during preparation to correct for vesicle recovery. After the incubations, the mitochondria are pelleted and aliquots of the supernatants, containing the vesicles, are counted. A measure of the transfer activity is given by the decrease in the phosphatidylethanolamine/triolein ratio of the supernatants.

Liver microsome phospholipids and cytochrome P.450 conceptration in phenobarbital treated rats fed on different diets

Liver microsomal concentration of cytochrome P.450 is increased in animals which are fed diets rich in polyunsaturated fatty acids. On the other hand, the effects of phenobarbital are more important when the dietary fat is more unsaturated. The unsaturation index in liver microsomal phosphatidylcholines depends on the unsaturation of the dietary fats. The treatment with phenobarbital constantly results in a decrease of the unsaturation index of fatty acids both in lecithins and cephalins. The importance of the liver microsomal cytochrome P.450 increase and the importance of the unsaturation index decrease in liver microsomal lecithins, both promoted by phenobarbital, are in good agreement.

Reconstitution studies on the involvement of radiation-induced lipid peroxidation in damage to membrane enzymes.

The effect of radiation on the drug-metabolizing enzyme system of microsomes, reconstituted with liposomes of microsomal phospholipids, NADPH-cytochrome P-450 reductase and cytochrome P-450, was examined to elucidate the role of lipid peroxidation of membranes in radiation-induced damage to membrane-bound enzymes. The reconstituted system of non-irradiated enzymes with irradiated liposomes showed a low activity of hexobarbital hydroxylation, whereas irradiated enzymes combined with non-irradiated liposomes exhibited an activity equal to that of unirradiated controls. Irradiation of liposomes caused a decrease in cytochrome P-450 content by destruction of the haem of cytochrome P-450 and also inhibited the binding capacity of cytochrome P-450 for hexobarbital. The relationship between radiation-induced lipid peroxidation and membrane-bound enzymes is discussed.

Role of cholesterol in the structure and function of gastric microsomal vesicles

Digitonin was used as a tool to investigate the organization and function of cholesterol in gastric microsomes. Microsomal vesicles were treated with digitonin for different time at 0-4 degrees C under isotonic conditions. The effects of digitonin treatment of the vesicles on removal of cholesterol, ultrastructural changes, (H+ + K+)-ATPase activity, and gastric ATPase-dependent H+ uptake ability were investigated. Microsomal cholesterol was extracted in an exponential manner with a t1/2 of 32 min. There was no release of microsomal phospholipids by digitonin treatment during the same period. Digitonin treatment (30 min) produced visible "holes" in the vesicles; at the same time (H+ + K+)-ATPase-dependent H+ uptake was abolished. Under the same conditions the K+-stimulated ATPase activity, however, was moderately (about 35%) reduced, although the response of K+ stimulation to valinomycin was obliterated. Longer digitonin treatment resulted in gradual diffusion and eventual disappearance of the "holes" with the generation of distorted cup-shaped microsomes. The data strongly suggest that membrane lipids are freely mobile and that there is a certain degree of specialization in the organization of gastric microsomal cholesterol for the proper maintenance of the membrane structure and function.

Source of lung surfactant phospholipids: comparison of palmitate and acetate as precursors.

The phospholipids and the fatty acid compositions of major phospholipids in rat lung parenchyma, microsomes, lamellar bodies and alveolar wash were quantified. Adult rats were injected simultaneously with [3H]palmitate and [14C]acetate into the femoral vein. The appearance of labeled phosphatidylcholine (PC), desaturated phosphatidylcholine (DSPC) and phosphatidylglycerol (PG) in each lung fraction was measured during short periods of time (5 min to 2 hr) after isotope administration. Relatively more PC, DSPC and PG labeled with acetate radioactivity in lung microsomes entered lamellar body and alveolar wash fractions than those labeled with palmitate radioactivity. However, there was no difference between palmitate and acetate labeled phospholipids in the transport from microsomes to lamellar bodies by phospholipid exchange proteins. On the other hand, prior injection of colchicine resulted in decrease in the transport of PC from microsomes to alveolar space to a relatively greater extent in the acetate radioactivity than in the palmitate radioactivity.

Asymmetric synthesis of ethanolamine phospholipids in chicken brain microsomes, through the cytidine pathway

Asymmetric synthesis of ethanolamine phospholipids in chicken brain microsomes, through the cytidine pathway

Separation and characterization of the aldehydic products of lipid peroxidation stimulated by ADP-Fe2+ in rat liver microsomes.

1. Methods using t.l.c. and high-pressure liquid chromatography (h.p.l.c.) have been used to separate the complex variety of substances possessing a carbonyl function that are produced during lipid peroxidation. 2. The major type of lipid peroxidation studied was the ADP-Fe2+-stimulated peroxidation of rat liver microsomal phospholipids. Preliminary separation of the polar and non-polar products was achieved by t.l.c.: further separation and identification of individual components was performed by h.p.l.c. Estimations were performed on microsomal pellets and the supernatant mixture after incubation of microsomes for 30 min at 37 degrees C. 3. The polar fraction was larger than the non-polar fraction when expressed as nmol of carbonyl groups/g of liver. In the non-polar supernatant fraction the major contributors were n-alkanals (31% of the total), alpha-dicarbonyl compounds (22%) and 4-hydroxyalkenals (37%) with the extraction method used. 4. Major individual contributors to the non-polar fraction were found to be propanal, 4-hydroxynonenal, hexanal and oct-2-enal. Other components identified include butanal, pent-2-enal, hex-2-enal, hept-2-enal, 4-hydroxyoctenal and 4-hydroxyundecenal. The polar carbonyl fraction was less complex than the non-polar fraction, although the identities of the individual components have not yet been established. 5. Since these carbonyl compounds do not react significantly in the thiobarbituric acid reaction, which largely demonstrates the presence of malonaldehyde, it is concluded that considerable amounts of biologically reactive carbonyl derivatives are released in lipid peroxidation and yet may not be picked up by the thiobarbituric acid reaction.

The multiple effects of ethylenediaminetetraacetate in several model lipid peroxidation systems

Experiments were performed which illustrate the various ways EDTA can influence lipid peroxidation. Either detergent-dispersed linoleate, or liposomes made from extracted microsomal phospholipids were utilized as substrates for peroxidation. Peroxidation was accomplished using Fe 2+ or Fe 3+. In systems utilizing Fe 2+, EDTA chelation facilitated Fe 2+ autoxidation which in turn caused peroxidation of detergent-dispersed linoleate. Peroxidation was not initiated during EDTA-Fe 2+ autoxidation when the substrate lipids were in a liposomal configuration. Systems utilizing Fe 3+ required an enzyme (either xanthine oxidase or NADPH-cytochrome P 450 reductase) to reduce the iron for peroxidative activity. EDTA chelation of Fe 3+ enhanced the xanthine oxidase and NADPH-cytochrome P 450 reductase-catalyzed peroxidation of detergent-dispersed linoleate, presumably by facilitating the reduction of Fe 3+. Catalase and mannitol inhibited both EDTA-Fe 2+- and EDTA-Fe 3+-dependent lipid peroxidation. EDTA-Fe 3+ was not capable of initiating peroxidation of phospholipid liposomes following enzymatic reduction by either enzyme, but ADP-chelated iron effectively initiated liposomal peroxidation in similar systems. With xanthine oxidase-catalyzed peroxidation of liposomes with ADP-Fe 3+, the inclusion of EDTA-Fe 3+ caused a modest enhancement of activity. EDTA-Fe 3+ greatly stimulated NADPH-cytochrome P 450 reductase-catalyzed peroxidation of liposomes with ADP-Fe 3+. In contrast, the addition of EDTA, rather than EDTA-Fe 3+ inhibited the liposomal peroxidation catalyzed by either enzyme with ADP-Fe 3+ when the EDTA concentration exceeded the concentration of Fe 3+.