Pigment bleaching by soybean lipoxygenase type-2 and the effect of specific chemical modifications

Abstract

In contrast to lipoxygenase-1 of the soybean, the lipoxygenase-2 isomer had very little anaerobic activity as determined by diene formation, carbonyl production and pigment bleaching. Chlorophyll and carotene inhibited the aerobic activity of this isomer. The inhibition at low concentrations appeared to be noncompetitive and at higher concentrations, competitive. Increasing pigment concentrations over 20 μg/ml for carotene and 3.75 μg/ml for chlorophyll decreased the rate of bleaching. When chlorophyll was added after the lipoxygenase had begun to react, diene formation was inhibited but pigment bleaching was even greater than when the chlorophyll was added together with the lipoxygenase linoleic acid mixture. This increased bleaching is most probably due to the presence of free radicals. Antioxidants reduced diene and carbonyl formation as well as chlorophyll and carotene bleaching through their action as free radical scavengers. Bromosuccinimide and iodoacetic acid caused a pronounced decrease in activity. The former indicates the involvement of tryptophan in the active center as shown as well by the change in spectrum caused by this reagent. The latter indicates that methionine is most probably involved in the lipoxygenase action. Methyl mercuric iodide and tetranitromethane had very little effect on the lipoxygenase activity, indicating for the former that no sulphydryl groups are involved and for the latter that tyrosine is not in the active center. This is in contrast to the effects of these reagents on lipoxygenase-1. A scheme is proposed to show the pathways of lipoxygenase-2 activities.