SPECIFICITIES OF THE ACYL-ACYL CARRIER PROTEIN (ACP) THIOESTERASE AND GLYCEROL-3-PHOSPHATE ACYLTRANSFERASE FOR OCTADECENOYL-ACP ISOMERS - IDENTIFICATION 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 Delta(6), Delta(7), Delta(8), Delta(9), Delta(10), Delta(11), or Delta(12) and elaidyl (18:1(Delta 9trans))-AcP were synthesized and used to characterize the substrate specificity of the acyl-ACP thioesterase and acyl-ACP:sn-glycerol-3-phosphate acyltransferase. The two enzymes were found to be specific for the Delta(9) position of the double bond. The thioesterase was highly specific for the Delta(9) cis conformation, but the transferase was almost equally active with the cis and the trans isomer of 18:1(Delta 9)-ACP. In plants such as the Umbelliferae species coriander (Coriandrum sativum L.) that accumulate petroselinic acid (18:1(Delta 6cis)) 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 petroselinoyl-ACP thioesterase is represented by a distinct polypeptide.