Activity of soybean lipoxygenase isoforms against esterified fatty acids indicates functional specificity

In soybean (Glycine max L.) vegetative tissue at least five lipxoygenase is ozymes are present. Four of these proteins have been localized to the parav einal mesophyll, a layer of cells that is thought to function in assimilate partitioning In order to determine the role of the lipoxygenase isozymes w ithin the soybean plant, the leaf lipoxygenases were cloned into bacterial expression vectors and expressed in Escherichia coti. The recombinant lipox ygenases were then characterized as to substrate preference, pH profiles fo r the most common plant lipoxygenase substrates, linoleic acid, and alpha - linolenic acid, and the reaction products with the substrates linoleic acid , alpha -linolenic acid, arachidonic acid, gamma -linolenic acid, and the t riacylglycerol trilinolein. All five enzymes were shown to be (ISS)-lipoxyg enases against linoleic acid. The results of these assays also indicate tha t two of these isozymes are highly active against esterified fatty acid gro ups, such as those found in triacylglycerols. Lipid analysis of leaves from plants subjected to sink limitation conditions indicates that the soybean leaf lipoxygenases are active in vivo against both free fatty acids and est erified lipids, and that the quantities of lipoxygenase products found in l eaf tissue show a positive correlation with the level of lipoxygenase in th e leaf. Implications for the putative role of these enzymes in the paravein al mesophyll are discussed. (C) 2001 Academic Press.