Stimulation by unsaturated fatty acid of uptake in rat liver microsomes squalene

Supernatant protein factor (SPF) and anionic phospholipids such as phosphatidylglycerol (PG) stimulate squalene epoxidase activity in rat liver microsomes by promoting ['Hlsqualene uptake as well as substrate translocation (Chin, J., and K. Bloch. 1984. J Biol. C h . 259: 11735-11738). This process is postulated to be membrane-mediated and not carriermediated. Here we show that treatment of PG with phospholipase A2 in the presence of bovine serum albumin abolishes the stimulatory effect of SPF on epoxidase activity. Disaturated fatty acyl-PGs are not as effective as egg yolk lecithin PG in the SPF effect. @I These findings suggest an important role for the unsaturated fatty acid moiety of PG. We also show that at submicellar concentrations, cis-unsaturated fatty acids stimulate microsomal epoxidase activity whereas saturated fatty acids do not. This effect is due to an increase in substrate uptake which in turn may facilitate substrate availability to the enzyme. -Chin, J., and K. Bloch. Stimulation by unsaturated fatty acid of squalene uptake in rat liver microsomes. J. Lipid Res. 1985. 26: 819-823. Supplementary key w o r d s oleate squalene epoxidase supernatant protein factor * anionic phospholipid The conversion of squalene to lanosterol in liver is catalyzed by two cholesterogenic enzymes located in the endoplasmic reticulum, squalene epoxidase and squalene epoxide-lanosterol cyclase. Stimulation of these enzyme activities in preparations of liver microsomes by supernatant protein factor (SPF) and anionic phospholipid such as phosphatidylglycerol (PG) or phosphatidylserine (1-5) has been shown to be due to effects on substrate uptake (6) and the postulated substrate translocation from an inactive to an active membrane pool (7, 8). The postulated mechanism does not involve a conventional protein carrier function (9) since SPF has not been shown to bind squalene or squalene epoxide under any experimental conditions (2, 5). PG, however, which is required for SPF to be effective, binds both to SPF and to the microsomal membrane. As previously reported, lysophosphatidylserine does not substitute for phosphatidylserine in the SPF stimulation of epoxidase (2). Here we show that PG treated with phospholipase A2 in the presence of bovine serum albumin is completely ineffective and that disaturated fatty acyl-PGs only partially substitute for normal PG in the SPF effect. We also describe stimulation of microsomal squalene uptake and conversion by submicellar concentrations of unsaturated fatty acids. The present findings suggest an important role for the unsaturated fatty acid moiety of anionic phospholipids. MATERIALS AND METHODS