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++ (LCFA-CoA) with high affinity (A. Frolov et al., J. Biol. Chem. 271 (1997) 31878-31884) suggests new possible functions of this protein in LCFA-CoA metabolism. The purpose of the present investigation was to determine whether SCP-2 differentially modulated microsomal LCFA-CoA transacylation 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 level, mol% cholesterol in the liposomal membrane, and total amount of liposomal cholesterol. As compared to basal activity without liposomes, microsomal 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 nearly 10-fold as compared to cholesterol pool (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 microM) oleoyl-CoA inhibited ACAT nearly 3-fold. This inhibition was reversed by LCFA-CoA binding proteins, bovine serum albumin (BSA) and SCP-2. SCP-2 was 10-fold more effective (mole for mole) than BSA in reversing LCFA-CoA inhibited microsomal 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 in vitro by reversing LCFA-CoA inhibition of ACAT and by differentially targeting LCFA-CoA to triacylglycerols. These data suggest that the high affinity interaction of SCP-2s with LCFA-CoA may be physiologically important in microsomal transacylation reactions.
Chao
H
H
Department of Pathobiology, Texas A&M University, TVMC, College Station, TX 77843-4467, USA.
Billheimer
J T
JT
Kier
A B
AB
Schroeder
F
F
D000214:Acyl Coenzyme A / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism* / Q000494:pharmacology; D002784:Cholesterol / Q000032:analysis; D002788:Cholesterol Esters / Q000378:metabolism; D004789:Enzyme Activation; D008081:Liposomes / Q000737:chemistry; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D010743:Phospholipids / Q000378:metabolism; D010940:Plant Proteins; D051381:Rats; D017207:Rats, Sprague-Dawley; D002785:Sterol O-Acyltransferase / Q000037:antagonists & inhibitors / Q000378:metabolism; D014280:Triglycerides / Q000378:metabolism
D000214:Acyl Coenzyme A; D002352:Carrier Proteins; D002788:Cholesterol Esters; D008081:Liposomes; D010743:Phospholipids; D010940:Plant Proteins; D014280:Triglycerides; C072087:sterol carrier proteins; C017585:oleoyl-coenzyme A; D002784:Cholesterol; D002785:Sterol O-Acyltransferase