Oxidation of various lipid substrates with unfractionated soybean and wheat lipoxidase

Quantitative determinations of lipoxidase in wheat were made on five milling fractions from four wheat varieties, including two hard red winter and two hard red spring wheats. Lipoxidase content decreased with increasing refinement and was lowest in the flour fraction. In general, the mill fractions from spring wheats exhibited slightly higher lipoxidase content than the corresponding mill fractions from winter wheats. Aqueous extracts from soybeans (variety Hawkeye) and wheat (variety Selkirk, break shorts fraction) were adjusted in concentration so that the lipoxidase activity with respect to linoleic acid was approximately the same. The diluted extracts were then tested for their ability to oxidize other lipid substrates with the necessary 1,4 pentadiene system. Soybean lipoxidase was far more reactive toward methyl linoleate and trilinolein than wheat lipoxidase. Mono‐ and dilinolein were relatively unreactive in both systems although dilinolein was a better substrate in the soybean system than in the wheat. Neither system oxidized highly unsaturated digalactosyl diglycerides alone. However the soybean extract oxidized this substrate to a small extent in the presence of catalytic amounts of linoleic acid while, under identical conditions, the wheat extract remained inactive. Polyacrylamide gel electrophoresis of the aqueous extracts of soybeans and wheat‐milling fractions coupled with a staining procedure specifically for lipoxidase indicated three to four isoenzyme bands in the soybeans and two to four isoenzyme bands in the wheat‐milling fractions. Isoenzymes may be responsible for differences in enzymatic activity on various substrates.