Enzymatic properties of the lipoxygenase from pea seeds

A lipoxygenase preparation was obtained from dried green pea seeds. Disc electrophoresis with enzyme staining indicated the presence of only one main isoenzyme corresponding to the isoenzyme PL I according to Yoon and Klein (J. Agric. Food Chem. 24, 955 (1979)), whereas PL II was absent. The assay for pea lipoxygenase has been optimized by using a final concentration of 0.53 mM potassium linoleate in the presence or absence of 0.2% sodium cholate. Without detergent the rho H optimum was 5.9, in its presence 6.8. The formation of conjugated dienes absorbing at 234 nm accounted for 75% of the oxygen uptake. The difference is mainly due to the aerobic formation of oxodienoic acids absorbing at 285 nm via a lipohydroperoxidase activity concomitant with the dioxygenase reaction. Other lipohydroperoxidase products were formed only to a minor extent under aerobic conditions, whereas in the anaerobic lipohydroperoxidase reaction of pea lipoxygenase in the system 13L8-hydroperoxylinoleic acid/linoleic acid fatty acid dimers containing conjugated double bonds were formed additionally. The pea enzyme showed self-inactivation at 37 degrees C, but in contrast to the lipoxygenase from rabbit reticulocytes the self-inactivation appeared only syncatalytically during the aerobic reaction. The antioxidant 2,6-di-t-butyl-4-hydroxytoluene (BHT, 1 mM) did not protect from self-inactivation. In contrast to the lipoxygenase from soybeans, wheat and rabbit reticulocytes the pea lipoxygenase caused a co-oxidation of Cu-chlorophyllin in the presence of linoleate at 5 degrees C. The co-oxidation was completely inhibited by 1 mM BHT which did not inhibit the dioxygenation of linoleate at this temperature. Unlike the reticulocyte enzyme the pea lipoxygenase failed to attack mitochondrial membranes or to produce inhibition of the respiratory chain. The results lead to the conclusion that a simple classification of lipoxygenases in type I and type II enzymes is not justified. A reaction scheme is proposed to explain both the co-oxidative activity and the aerobic formation of oxodienoic acids by pea lipoxygenase, presuming the dissociation of a linoleic acid radical from the ferrous lipoxygenase as a side reaction of the catalytic cycle.