Specificities of the Acyl-Acyl Carrier Protein (ACP) Thioesterase and Clycerol-3-Phosphate Acyltransferase for Octadecenoyl-ACP lsomers' ldentification of a Petroselinoyl-ACP Thioesterase in Umbelliferae

This study was designed to address the question: How specific for double bond position and conformation are plant enzymes that act on oleoyl-acyl carrier protein (ACP)? Octadecenoyl-ACPs with cis double bonds at positions A6, A', A', A', A'", A", or A'' and elaidyl ( 18:lAWr""')-ACP were synthesized and used to characterize the substrate specificity of the acyl-ACP thioesterase and acylACPsn-glycerol-3-phosphate acyltransferase. The two enzymes were found to be specific for the A' position of the double bond. The thioesterase was highly specific for the A9 cis conformation, but the transferase was almost equally active with the ris and the trans isomer of 181A9-ACP. In plants such as the Umbelliferae species coriander (Coriandrum sativum 1.) that accumulate petroselinic acid (18:lAkiS) in their seed triacylglycerols, a high petroselinoyl-ACP thioesterase activity was found in addition to the oleoyl-ACP thioesterase. The two activities could be separated by anion-exchange chromatography, indicating that the petroselinoylACP thioesterase is represented by a distinct polypeptide.