1-Acylglycerol 3-phosphate acyltransferase from rat liver.

The substrate specificity of 1-acylglycerol 3-phosphate acyltransferase from rat liver microsomes for both acylcoenzyme A and I-acylglycerol 3-phosphate has been determined at levels below the critical micelle concentrations of the substrates. The fatty acid of the acylglycerol 3-phosphate is not important, but for the acyl-CoAs the specificity is oleyl > palmitoleyl N linoleyl N palmityl > myristyl > stearyl > lauryl. I-Acylglycerol 3-phosphate acyltransferase appears to work only with monomeric substrates. The K, for palmitylCoA is less than 0.1 PM, while the K, values for the l-acylglycerol 3-phosphates are in the low micromolar range (5 to 25 w), but are dithcult to measure accurately because micelles form at 4 to 8 pM I-acylglycerol-3-P and the velocity ceases to increase. By contrast, acyl-CoA hydrolase appears to prefer micelles as substrate. This enzyme shows little activity at 3 @ acyl-CoA, and shows 8 times more activity with palmityl-CoA than with any other acyl-CoA tested. In order to plan the specificity studies, the critical micelle concentrations of the acyl-CoAs and 1-acylglycerol 3-phosphates used were determined by the dye adsorption technique with pinacyanol chloride. For acyl-CoAs in 6.7 mu phosphate, 10 mu K+, pH 6.9, the values were: lauryl, 9.5 BM; myristyl, 4 PM; pahnityl, 3.6 (4 @ in 50 IIIM Tris-HCI, pH 7.4; 6 PM in deionized water); pahnitoleyl, 2 pM; stearyl, 2 PM; oleyl, 4.5 @I; linoleyl, 5.5 m. For 1-acylglycerol 3phosphates in 50 mM Tris-HCI, pH 7.4, the values were: myristyl, 120 PM; pahnityl, 35 /JM; stearyl, 7 pM; oleyl, 23 @; bnoleyl, 34 PM; 1-acylglycerol 3-phosphate prepared from egg lecithin (80% palmitic), 37 pM.