Formation of 12-[18-O]oxo-cis-10, cis-15-phytodienoic acid from 13[18-O]hydroperoxylinolenic acid by hydroperoxide cyclase.

13-[18-O]Hydroperoxylinolenic acid was permitted to react with an extract of flaxseed acetone powder containing hydroperoxide cyclase activity. The resulting product, 12-oxo-cis-10,cis-15-phytodienoic acid (12-oxo-PDA), contained 18O in the carbonyl oxygen at carbon 12, suggesting that an epoxide was an intermediate in the hyderoperoxide cyclase reaction. A substrate specificity study showed that a cis double bond beta, gamma to the conjugated hydroperoxide group was essential for the substrate to be converted to a cyclic product by hydroperoxide cyclase.