Hydroperoxide isomerase: a new enzyme of lipid metabolism.

An enzyme has been isolated from flaxseed (Linum usitatissimum) which utilizes the product of lipoxidase for its substrate. The enzyme, termed hydroperoxide isomerase, converts the conjugated diene hydroperoxide of linoleic acid to the corresponding monoenoic ketohydroxy fatty acid. The structure of the latter has been determined by ultraviolet, infrared, and nuclear magnetic resonance spectroscopy; periodate and permangate oxidation; gas chromatography; and thin layer chromatography. Hydroperoxide isomerase activity has also been demonstrated in crude extracts from barley (Hordeum vulgare), wheat germ (Triticum aestivum), mung beans (Phaseolus aureus), and corn (Zea mays) and from partially purified extracts of soybean (Glycine max).The hydroperoxide isomerase enzyme from flaxseed has a pH optimum of 7.0. The enzyme was not inhibited by nordihydroguairetic acid, p-chloromercuribenzoic acid, or cyanide, but it was inhibited by cupric ion. One hundred per cent of the activity was lost by heating the enzyme for 1 minute at 68 C. Both linoleic and linolenic acid hydroperoxides can serve as substrates for the enzyme. The pH optimum for the hydroperoxide isomerase enzyme from barley is 6.2; from wheat germ, 6.1; and from soybean, 6.1.The identification of the hydroperoxide isomerase enzyme clarifies the role of lipoxidase in plant tissue and suggests a participation of lipid in the electron transport system.