Induction period in the lipoxidase-catalyzed oxidation of linoleic acid and its abolition by substrate peroxide.

The non-enzymatic oxidation of linoleate occurs with a definite induction period. Bolland has explained the kinetics of the oxidation in terms of a chain reaction involving free radicals (1). The discovery by Privett et al. (2) that the peroxide formed by the action of lipoxidase is optically active clearly rules out the possibility of a non-enzymatic chain sequence, but it is not impossible that a similar sequence could occur under the control of the enzyme. (For discussions of the mechanism and the possible role of chain reactions, see Bergstrom and Holman (3) ; Tappel et al. (4) ; Lundberg (5) ; and Siddiqi and Tappel (6).) Although the occurrence of an induction period in the course of the enzymatic reaction would not be prima facie evidence for a reaction sequence analogous to that found in autoxidation, its absence would support the opposite view. There appear to be definite indications, both direct and indirect, presently available, to show that an induction period exists in the enzymatic reaction. In Fig. 1 of Sumner and Smith (7) an induction period may be noted in the coupled oxidation of carotene, a mixture of soy bean fatty acids being used as the primary substrate. The data of Kies ((8) Table V) show a pronounced induction period when carotene destruction is employed as a criterion of the rate of linoleic acid oxidation. Holman (9) explicitly cites an induction period in the system, linoleic acidbixin. Recently Tookey et al. (10) have presented additional evidence for an induction period in the linoleic acid-carotene system. The objection can be raised that the kinetics of the oxidation of a secondary substrate such as carotene or bixin may not parallel the primary action of the enzyme on the lipide. However, an induction period has been observed in the Worthington assay procedure in which linoleic acid is employed in the absence of a cosubstrate (11). On the other hand, data shown by Theorell et al. (12) indicate the absence of an induction period, and indeed the assay method proposed by