Occurrence and Properties of Enzymes Associated with Mevalonic Acid Synthesis in the Orange

SUMMARY —Active enzyme preparations were obtained from mature orange fruit in a quest for the various enzymes associated with terpenoid synthesis. These preparations were examined systematically for the presence of enzymes involved in the biosynthetic pathway to mevalonic acid. Orange fruit was shown to contain acetyl-CoA synthetase (EC.6.2.1.1) forming acetyl coenzyme A from acetate. It also contained the iodoacetamide-sensitive acetoacetyl-CoA thlolase (EC.2.3.1.9) the enzyme responsible for the synthesis of acetoacetyl coenzyme A. Hydroxymethylglutaryl-CoA synthase (EC.4.1.3.5) and the corresponding reductase (EC.1.1.1.34), however, were not found in preparations from orange fruit. Nonetheless, acetoacetyl coenzyme A, formed by the thiolase, was further metabolized in the mature fruit via 3-hydroxybutyryl-CoA dehydrogenase (EC.1.1.1.35), a constituent enzyme in lipid biosynthesis. These results suggest that mature orange fruit derives mevalonic acid or other necessary basic intermediates for terpenoid biosynthesis from other sources.