Acetyl-acyl carrier protein is not a major intermediate in fatty acid biosynthesis in spinach

The extent to which acetyl-acyl carrier protein (acetyl-ACP) is an intermediate in fatty acid biosynthesis was examined. Acetyl-ACP was the least effective primer of fatty acid synthesis by spinach extracts when compared to acetyl-CoA, butyryl-ACP or hexanoyl-ACP. Furthermore, the rate of acetyl-ACP-primed fatty acid synthesis was inhibited significantly by cerulenin, indicating that the slow utilization of acetyl-ACP was predominantly by 3-oxoacyl-ACP synthase I. In light-incubated isolated chloroplasts with high rates of fatty acid synthesis (greater than 800 nmol.h-1.mg chlorophyll-1), the rate of acetyl-ACP metabolism was at least 10-30-fold slower than the rate of butyryl-ACP metabolism. The relatively slow metabolism of acetyl-ACP provided in situ evidence that (a) butyryl-ACP was formed principally from condensation of malonyl-ACP with acetyl-CoA and (b) acetyl-ACP was a minor participant in fatty acid biosynthesis.