Oxidation of esterified arachidonate by rat liver microsomes

Phospholipase A(2)s and lipid peroxidation

Lipid peroxidation of membrane phospholipids can proceed both enzymatically via the mammalian 15-lipoxygenase-1 or the NADPH-cytochrome P-450 reductase system and non-enzymatically. In some cells, such as reticulocytes, this process is biologically programmed, whereas in the majority of biological systems lipid peroxidation is a deleterious process that has to be repaired via a deacylation-reacylation cycle of phospholipid metabolism. Several reports in the literature pinpoint a stimulation by lipid peroxidation of the activity of secretory phospholipase A(2)s (mainly pancreatic and snake venom enzymes) which was originally interpreted as a repair function. However, recent experiments from our laboratory have demonstrated that in mixtures of lipoxygenated and native phospholipids the former are not preferably cleaved by either secretory or cytosolic phospholipase A(2)s. We propose that the platelet activating factor (PAF) acetylhydrolases of type II, which cleave preferentially peroxidised or lipoxygenated phospholipids, are competent for the phospholipid repair, irrespective of their role in PAF metabolism. A corresponding role of Ca(2+)-independent phospholipase A(2), which has been proposed to be involved in phospholipid remodelling in biomembranes, has not been addressed so far. Direct and indirect 15-lipoxygenation of phospholipids in biomembranes modulates cell signalling by several ways. The stimulation of phospholipase A(2)-mediated arachidonic acid release may constitute an alternative route of the arachidonic acid cascade. Thus, 15-lipoxygenase-mediated oxygenation of membrane phospholipids and its interaction with phospholipase A(2)s may play a crucial role in the pathogenesis of diseases, such as bronchial asthma and atherosclerosis.

Fluvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase, scavenges free radicals and inhibits lipid peroxidation in rat liver microsomes

We investigated the effect of fluvastatin sodium (fluvastatin) and pravastatin, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, on the formation of thiobarbituric acid reactive substances both in vivo and in vitro in rat liver microsomes and on active oxygen species. Oral administration of fluvastatin at low doses (3.13 and 6.25 mg/kg) inhibited the formation of thiobarbituric acid reactive substances in rat liver microsomes, but high doses (12.5 and 25 mg/kg) did not change the formation of thiobarbituric acid reactive substances. Fluvastatin at any dose used had no effect on the content of cytochrome P-450 and the activity of NADPH-cytochrome P-450 reductase. In in vitro experiments, concentrations of fluvastatin ranging from 1 x 10(-6) - 1 x 10(-4) M markedly inhibited NADPH-dependent lipid peroxidation in liver microsomes, but pravastatin weakly inhibited lipid peroxidation. The order of magnitude of inhibition of each drug on in vitro lipid peroxidation was butylated hydroxytoluene > probucol > or = fluvastatin > pravastatin. Moreover, fluvastatin chemically scavenged active oxygen species such as hydroxyl radicals and superoxide anion generated by the Fenton reaction and by the xanthine-xanthine oxidase system, respectively, but pravastatin showed no scavenging of superoxide anion. These results indicate that the suppression of in vivo and in vitro lipid peroxidation in liver microsomes may be, at least in part, due to the scavenging by fluvastatin of free radicals.

A microsomal membrane component associated with iron reduction in NADPH-supported lipid peroxidation

This study was conducted to determine whether a factor responsible for reduced nicotinamide adenine dinucleotide phosphate (NADPH)-supported lipid peroxidation in rat liver microsomes is involved in iron reduction by cooperation with NADPH-cytochrome P450 reductase. Under anaerobic conditions, NADPH-dependent reduction of ferric pyrophosphate in microsomes was not dependent on cytochrome P450 levels and was not inhibited by carbon monoxide (CO). All of the iron complexes with chelators such as adenosine 5'-diphosphate, pyrophosphate, nitrilotriacetate, oxalate or citrate were reduced in microsomes, although in the reconstituted system containing purified NADPH-cytochrome P450 reductase little or no iron reduction was found. A cytochrome P450-free fraction from a cholate-solubilized preparation of microsomes after passage through a laurate sepharose column was required for reduction of iron pyrophosphate in the reconstituted system leading to lipid peroxidation. The iron reduction was not inhibited by CO and was destroyed by heat treatment or trypsin digestion of the fraction. All iron complexes were reduced in the presence of the fraction, using a reducing equivalent of NADPH via NADPH-cytochrome P450 reductase. The results indicate that a heat-labile component, which is probably a protein distinct from cytochrome P450, is associated with iron reduction responsible for lipid peroxidation in microsomes.

Cholate solubilization of liver microsomal membrane components which promote NADPH-supported lipid peroxidation

NADPH-supported lipid peroxidation monitored by malondialdehyde (MDA) production in the presence of ferric pyrophosphate in liver microsomes was inactivated by heat treatment or by trypsin and the activity was not restored by the addition of purified NADPH-cytochrome P450 reductase (FPT). The activity was differentially solubilized by sodium cholate from microsomes, and the fraction solubilized between 0.4 and 1.2% sodium cholate was applied to a Sephadex G-150 column and subfractionated into three pools, A, B, and C. MDA production was reconstituted by the addition of microsomal lipids and FPT to specific fractions from the column, in the presence of ferric pyrophosphate and NADPH. Pool B, after removal of endogenous FPT, was highly active in catalyzing MDA production and the disappearance of arachidonate and docosahexaenoate, and this activity was abolished by heat treatment and trypsin digestion, but not by carbon monoxide. The rate of NADPH-supported lipid peroxidation in the reconstituted system containing fractions pooled from Sephadex G-150 columns was not related to the content of cytochrome P450. p-Bromophenylacylbromide, a phospholipase A2 inhibitor, inhibited NADPH-supported lipid peroxidation in both liver microsomes and the reconstituted system, but did not block the peroxidation of microsomal lipid promoted by iron-ascorbate or ABAP systems. Another phospholipase A2 inhibitor, mepacrine, poorly inhibited both microsomal and pool-B'-promoted lipid peroxidation, but did block both iron-ascorbate-driven and ABAP-promoted lipid peroxidation. The phospholipase A2 inhibitor chlorpromazine, which can serve as a free radical quencher, blocked lipid peroxidation in all systems. The data presented are consistent with the existence of a heat-labile protein-containing factor in liver microsomes which promotes lipid peroxidation and is not FPT, cytochrome P450, or phospholipase A2.

Antioxidant mechanism of Mn(II) in phospholipid peroxidation

Antioxidant action of Mn2+ on radical-mediated lipid peroxidation without added iron in microsomal lipid liposomes and on iron-supported lipid peroxidation in phospholipid liposomes or in microsomes was investigated. High concentrations of Mn2+ above 50 microM inhibited 2,2'-azobis (2-amidinopropane) (ABAP)-supported lipid peroxidation without added iron at the early stage, while upon prolonged incubation, malondialdehyde production was rather enhanced as compared with the control in the absence of Mn2+. However, in a lipid-soluble radical initiator, 2,2'-azobis (2,4-dimethyl-valeronitrile) (AMVN)-supported lipid peroxidation of methyl linoleate in methanol Mn2+ apparently did not scavenge lipid radicals and lipid peroxyl radicals, contrary to a previous report. At concentrations lower than 5 microM, Mn2+ competitively inhibited Fe(2+)-pyrophosphate-supported lipid peroxidation in liposomes consisting of phosphatidylcholine with arachidonic acid at the beta-position and phosphatidylserine dipalmitoyl, and reduced nicotinamide adenine dinucleotide phosphate (NADPH)-supported lipid peroxidation in the presence of iron complex in microsomes. Iron reduction responsible for lipid peroxidation in microsomes was not influenced by Mn2+.

Mechanism of cobalt (II) ion inhibition of iron-supported phospholipid peroxidation

Co2+ inhibited nonenzymatic iron chelate-dependent lipid peroxidation in dispersed lipids, such as ascorbate-supported lipid peroxidation, but not iron-independent lipid peroxidation. Histidine partially abolished the Co2+ inhibition of the iron-dependent lipid peroxidation. The affinity of iron for phosphatidylcholine liposomes in Fe(2+)-PPi-supported systems was enhanced by the addition of an anionic lipid, phosphatidylserine, and Co2+ competitively inhibited the peroxidation, while the inhibiting ability of Co2+ as well as the peroxidizing ability of Fe(2+)-PPi on liposomes to which other phospholipids, phosphatidylethanolamine, or phosphatidylinositol had been added was reduced. Co2+ inhibited microsomal NADPH-supported lipid peroxidation monitored in terms of malondialdehyde production and the peroxidation monitored in terms of oxygen consumption. The inhibitory action of Co2+ was not associated with iron reduction or NADPH oxidation in microsomes, suggesting that Co2+ does not affect the microsomal electron transport system responsible for lipid peroxidation. Fe(2+)-PPi-supported peroxidation of microsomal lipid liposomes was markedly inhibited by Co2+.

4-Hydroxy-2,3-trans-nonenal stimulates microsomal lipid peroxidation by reducing the glutathione-dependent protection

Glutathione (GSH) protects liver microsomes against lipid peroxidation. This is probably due to the reduction of vitamin E radicals by GSH, a reaction catalyzed by a membrane-bound protein. Pretreatment of liver microsomes with 0.1 or 1mM 4-hydroxy-2,3-trans-nonenal (HNE), a major product of lipid peroxidation, reduces the GSH-dependent protection. GSH and vitamin E concentrations are not affected by this pretreatment. Pretreatment with 0.1 mM N-ethyl maleimide (NEM), a synthetic sulfhydryl reagent, resulted in a reduction similar to that with HNE of the GSH-dependent protection against lipid peroxidation. The reduction of the GSH-dependent protection by HNE and NEM is probably the result of inactivation of the membrane-bound protein by covalent binding to an essential SH group on the protein. If the GSH-dependent protection would proceed via the microsomal GSH transferase, pretreatment with NEM, which activates the microsomal GSH transferase, should enhance the GSH-dependent protection. Actually a decrease in the GSH-dependent protection is found. Apparently the GSH-dependent protection does not proceed via the microsomal GSH transferase. Also the microsomal phospholipase A2 is not involved, since addition of 0.1 mM mepacrine, an inhibitor of phospholipase A2, did not preclude the GSH-dependent protection. Once the process of lipid peroxidation, either in vivo or in vitro, has started, the protection of liver microsomes by GSH is less effective. This might be the result of formed HNE. In this way an endproduct of lipid peroxidation stimulates the process that generates this product.