In vivo pools of free and acylated acyl carrier proteins in spinach. Evidence for sites of regulation of fatty acid biosynthesis.

In order to examine potential regulatory steps in plant fatty acid biosynthesis, we have developed procedures for the analysis of the major acyl-acyl carrier protein (ACP) intermediates of this pathway. These techniques have been used to separate and identify acyl-ACPs with chain configurations ranging from 2:0 to 18:1 and to determine the relative in vivo concentrations of acyl-ACPs in spinach leaf and developing seed. In both leaf and seed as much as 60% of the total ACPs were nonesterified (free), with the remaining proportion consisting of acyl-ACP intermediates leading to the formation of palmitate, stearate, and oleate. In spinach leaf the proportions of the various acyl groups esterified to each ACP isoform were indistinguishable, indicating that these isoforms are utilized similarly in de novo fatty acid biosynthesis in vivo. However, the acyl group distribution pattern of seed ACP-II differed significantly from that of leaf ACP-II. The malonyl-ACP levels were less than the 4:0-ACP and 6:0-ACP levels in leaf, and in contrast, the malonyl-ACP-II levels in seed were approximately 3-fold higher than the 4:0-ACP-II and 6:0-ACP-II levels. In addition, the ratio of oleoyl-ACP-II (18:1) to stearoyl-ACP-II (18:0) was higher in seed than in leaf. These data suggest that the differences in acyl-ACP patterns reflect a tissue/organ-specific difference rather than an isoform-specific difference. In extracts prepared from leaf samples collected in the dark, the levels of acetyl-ACPs were approximately 5-fold higher compared to samples collected in the light. The levels of free ACPs showed an inverse response, increasing in the light and decreasing in the dark. Notably there was no concomitant increase in the malonyl-ACP levels. The most likely explanation for the major increase in acetyl-ACP levels in the dark is that light/dark control over the rate of fatty acid biosynthesis occurs at the reaction catalyzed by acetyl-CoA carboxylase.