Biochemical and Biophysical Research Communications the Self-catalyzed Destruction of Lipoxygenase*

Received October 5, 1970 SUMMARY Lipoxygenase (E.C. 1.99.2.1) from soybean catalyzes its own destruction in the presence of both substrates, oxygen and fatty acid. The apparent first order rate constant for this inactivation was 0.050 min-1 in the presence of 9,12-octadecadienoate and 0.32 min-1 with 5,8,11,14-eicosatetraenoate. Lipoxygenase is a dioxygenase capable of stereospecific attack (1) upon selected fatty acid substrates (1,2). A mechanism of action has been proposed by Tappel et al. (3) and furth er -insight was provided by the isotope enrichment noted by Hamberg and Samuelsson (1). In designing experiments to help elucidate the mechanism by which oxygen adds to unsaturated fatty acids, we noted that lipoxygenase behaved somewhat like a substrate in that it limited the extent, as well as the rate, of the reaction. Such enzyme inactivation may occur during isolation or handling and thus explain losses in lipoxygenase activity such as observed by Dillard et al. -(4) and may also account for the differences in specific activities of the purified enzyme noted by Allen (5). This phenomenon of an enzyme-limited extent of reaction is likely to affect kinetic measurements with soybean lipoxygenase particularly with substrates containing three or more double bonds. EXPERIMENTAL Soybean lipoxygenase was obtained from Sigma Chemical Company. The connnercial enzyme preparation catalyzed the oxygenation of 1.52 umole of