Microsomal long chain fatty acyl-CoA transacylation: differential effect of sterol carrier protein-2

The recent discovery that sterol carrier protein-2 (SCP-2) binds long chain fatty acyl-CoA (LCFA-CoA) with high affinity (A. Frolov et al., J. Biol. C hem. 271 (1997) 31878-31884) suggests new possible functions of this protei n in LCFA-CoA metabolism. The purpose of the present investigation was to d etermine whether SCP-2 differentially modulated microsomal LCFA-CoA transac ylation to cholesteryl esters, triacylglycerols, and phospholipids in vitro . Microsomal acyl-CoA:cholesterol acyltransferase (ACAT) activity measured with liposomal membrane cholesterol donors depended on substrate LCFA-CoA l evel, mol% cholesterol in the liposomal. membrane, and total amount of lipo somal cholesterol. As compared to basal activity Without liposomes, microso mal ACAT was inhibited 30-50% in the presence of cholesterol poor (1.4 mol% ) liposomes. In contrast, cholesterol rich (> 25 mol%) liposomes stimulated ACAT up to 6.4-fold compared to basal activity without liposomes and nearl y 10-fold as compared to cholesterol poor (1.4 mol%) liposomes. Increasing oleoyl-CoA reversed the inhibition of microsomal ACAT by cholesterol poor ( 1.4 mol%) liposomes, but did not further stimulate ACAT in the presence of cholesterol rich (35 mol%) liposomes. In contrast, high (100 mu M) oleoyl-C oA inhibited ACAT nearly 3-fold. This inhibition was reversed by LCFA-CoA b inding proteins, bovine serum albumin (BSA) and SCP-2. SCP-2 was 10-fold mo re effective (mole for mole) than BSA in reversing LCFA-CoA inhibited micro somal ACAT. Concomitantly, under conditions in which SCP-2 stimulated ACAT it equally enhanced transacylation of oleoyl-CoA into phospholipids, and 5. 2-fold enhanced oleoyl-CoA transacylation to triacylglycerols. In summary, SCP-2 appeared to exert its greatest effects on microsomal transacylation i n vitro by reversing LCFA-CoA inhibition of ACAT and by differentially targ eting LCFA-CoA to triacylglycerols. These data suggest that the high affini ty interaction of SCP-2s with LCFA-CoA may be physiologically important in microsomal transacylation reactions. (C) 1999 Elsevier Science B.V. All rig hts reserved.