The reaction mechanism of soy bean lipoxidase.

Based on their observations that the lipoxidase-catalyzed oxidation of linoleate, like autoxidation, gave primarily conjugated diene peroxides (1,2) and caused a coupled oxidation of some easily oxidizable substances, such as carotenoids (3) and antioxidants (4), Bergstrom and Holman proposed a reaction mechanism for lipoxidase catalysis. They suggested (4) that lipoxidase could cause initiation of a free radical chain reaction similar to that operative in the autoxidation of methyl linoleate (5). Because the lipoxidase reaction yields peroxides whose molecular extinction coefficient is considerably greater than the molecular extinction coefficient of peroxides produced in autoxidation (4) under the same conditions, Lundberg (6) suggested that lipoxidase may not function merely to initiate a chain reaction like that prevailing in autoxidation but may form a complex with each linoleate molecule and thus influence the structure of the resulting peroxides. More recently Kunkel (7) reported results that were interpreted as being in accord with a chain reaction mechanism for lipoxidase catalysis. He found that the rate of oxidation of methyl linoleate measured by bixin decolorization was proportional to the square root of the chain initiator (lipoxidase) concentration and, therefore, conformed to the same kinetic relationships that had previously been reported for the autoxidation of ethyl linoleate (5). Direct proportionality between reaction velocity and the square root of the enzyme concentration would in general be expected if a chain mechanism is operative (8). Kunkel also reported data on the inhibition of lipoxidase by CYand r-tocopherols, from which he calculated that the minimum chain length in his system was 12 molecules. An understanding of lipoxidase catalysis is of interest in relation to proposals that some enzyme reactions may proceed by chain reactions (g-12). Other than for lipoxidase, speculations on possible chain mechanisms are largely without experimental support. It was the purpose of