A novel preparation of human platelet lipoxygenase. Characteristics and inhibition by a variety of phenyl hydrazones and comparisons with other lipoxygenases.

Acetone-pentane powder preparations of human blood platelets were prepared and the characteristics of the 12L-lipoxygenase were studied using measurements of oxygen consumption. No sharp pH optimum with equal reaction velocities was observed over a range of pH 7.5-8.5 in a variety of buffers. The enzyme could be easily solubilized in 1% deoxycholate, and in this form was moderately stable to heat. Of 10 divalent cations tested at a concentration of 3.7 . 10(-3) M, only zinc and tin were inhibitory. None of he other ions was stimulatory. The products of the oxidation of arachidonate were characterized from both soluble and insoluble enzyme preparations. With the insoluble suspension, 55% of the added arachidonate was recovered as 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) and 27% appeared as trihydroxyeicosatetraenoic acid isomers (THETEs). With the soluble preparation, 84% of the arachidonate appeared as 12-HETE and only 5% as THETEs. The Michaelis constants for dihomo-gamma-linolenic aicd, arachidonic acid, and eicosapentaenoic acid substrates are presented. A series of phenyl hydrazone inhibitors of various structural types were discovered to be potent inhibitors of the human platelet lipoxygenase. The sensitivities of this enzyme to these inhibitors were compared to soybean lipoxygenase and sheep seminal vesicular cyclodioxygenase. In general, the soybean enzyme was most sensitive; the sheep seminal vesicular cyclodioxygenase was the least sensitive and the human platelet lipoxygenase was intermediate between the two. The Ki values for two phenyl hydrazone inhibitors with both soybean and human platelet lipoxygenases is presented.