Characterization of sn-glycerol 3-phosphate acyltransferase from guinea pig harderian gland microsomes

Microsomal sn-glycerol 3-phosphate acyltransferase from the guinea pig Harderian gland was studied. Its specific activity (1.0 nmol/min X mg, with palmitoyl-CoA as a substrate) was almost the same as that of the rat liver microsomal enzyme. The enzyme acted on various types of acyl-CoA, the relative reaction rates being as follows: palmitoyl-CoA, 100(%); stearoyl-CoA, 30; oleoyl-CoA, 50; linoleoyl-CoA, 40; and arachidonoyl-CoA, 20. When assayed in the presence of 1 mM 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), the activity on palmitoyl-CoA was inhibited by only 20-30%, whereas those for other acyl-CoAs were completely abolished. The DTNB-resistant activity was inhibited by 0.1 mM dihydroxyacetonephosphate and 0.5 mM dithiothreitol, whereas the DTNB-sensitive activity was not affected. Furthermore, heat treatment at 50 degrees C for 15 min abolished most of the DTNB-sensitive activity, but not the DTNB-resistant activity. These results, taken together, suggested that the microsomal fraction of the guinea pig Harderian gland contained at least two types of sn-glycerol 3-phosphate acyltransferase, and that, in contrast to in the case of rat liver microsomes, a DTNB-resistant enzyme that utilized exclusively palmitoyl-CoA was predominant.