Characterization of Two Isoenzymes of Lipoxygenase from Bush Beans

Two isoenzymes of lipoxygenase, a and b, have been obtained from bush beans (Phaseolus vulgaris) as electrophoretically homogeneous proteins. Both proteins have a molecular weight of 100,000, contain 1 atom of iron, and appear to be composed of a single peptide chain. However, these enzymes appear to differ in some other respects. Thus, lipoxygenase a has an isoelectric point of 6.03 while lipoxygenase b has a value of 5.57. Their pH optima are 5 to 7 and 6.5 to 7, respectively. Both lipoxygenase a and b, when acting on linoleic acid plus the product hydroperoxide, generate what are presumably keto-dienes with an absorption maximum at 280 nm. Whereas lipoxygenase a can catalyze this secondary reaction in the presence of O(2), lipoxygenase b does so only under anaerobic conditions. Lipoxygenase a is stimulated by Ca(2+) while lipoxygenase is not. An unexpected finding is the strong inhibition of lipoxygenase a by Mn(2+) (50% inhibition at 12.5 muM under standard reaction conditions). Lipoxygenase b is inhibited by Mn(2+) but only at concentrations about 250 times greater.