Substrate Specificity of Soybean Lipoxidase

Purified soybean lipoxidase was used to test the substrate specificity of all cis,cis-methylene-interrupted isomers of linoleic acid. The natural 9,12-isomer was found to be the best substrate, and the 13,16-isomer 50% as effective. The presence of calcium ions broadened the pattern of specificity. The rates of reaction of a series of polyunsaturated acids in which chain length, number of double bonds, and positions of double bonds varied were tested with crude lipoxidase in the presence of calcium ion. The substrates which exhibit high rates of reaction all possessed double bonds at the 6th and 9th carbon atoms counting from the methyl group. Thus the terminal structure is critical to the specificity of the enzyme. For enzyme reaction, the carboxyl group must not be sterically hindered, and the unsaturation at the 6th carbon atom from the methyl end of the chain must be a double bond.