1‐Acyl‐ sn ‐glycerol‐3‐phosphate acyltransferase in maturing safflower seeds and its contribution to the non‐random fatty acid distribution of triacylglycerol

A 20000 × g particulate preparation isolated from maturing safflower seeds catalyzed the acylation of 1‐acyl‐ sn ‐glycerol 3‐phosphate with acyl‐CoA to form phosphatidate. The specific activity of the reaction exceeded 200 nmol min −1 mg protein −1 . Although this preparation was also capable of catalyzing the acylation of sn ‐glycerol 3‐phosphate with acyl‐CoA, the hydrolysis of phosphatidate, and the acylation of 1,2‐diacylglycerol, phosphatidate was the only major product when the preparation was incubated with 1‐acyl‐glycerol‐3‐ P and acyl‐CoA. The enzyme responsible for this phosphatidate synthesis, 1‐acyl‐glycerol‐3‐ P acyltransferase, showed a strict acyl‐CoA specificity. The relative order of specificity for acyl‐CoA was linoleoyl = oleoyl > palmitoleoyl > elaidoyl > cis ‐vaccenoyl > stearoyl = palmitoyl. This observation strongly suggests that the fatty acid composition of position 2 in phosphatidate synthesized in vivo primarily depends on both the acyl‐CoA specificity of the 1‐acyl‐glycerol‐3‐ P acyltransferase and the fatty acid composition of the acyl‐CoA pool in the cell. Thus, the absence of saturated fatty acids at position 2 of safflower triacylglycerol may be explained in terms of the acyl‐CoA specificity of the 1‐acyl‐glycerol‐3‐ P acyltransferase. The fatty acid moiety esterified at position 1 of glycerol‐3‐ P also affected the effectiveness of the reaction. The 1‐acyl‐glycerol‐3‐ P acyltransferase utilized 1‐acyl‐glycerol‐3‐ P molecular species in the following order of effectiveness: linoleoyl = oleoyl > palmitoyl. With a rise in incubation temperature, the initial rates of acylation with unsaturated acyl‐CoA species increased more rapidly than those for saturated acyl‐CoA species. A similar tendency was observed for saturated and unsaturated acyl acceptors. These data suggest that affinity of the acyltransferase for substrates may vary in response to changes in temperature, and that 1‐acyl‐glycerol‐3‐ P acyltransferase may be involved in the alteration of the individual fatty acid compositions at positions 1 and 2 of glycerolipids in tissues grown at different temperatures. Based on these findings, further metabolism of 1‐acyl‐glycerol‐3‐ P acyltransferase products could be the major factor determining the non‐random distribution of fatty acids in safflower triacylglycerol.