MEMBRANE-ASSOCIATED AND SOLUBLE LIPOXYGENASE ISOFORMS IN TOMATO PERICARP - CHARACTERIZATION AND INVOLVEMENT IN MEMBRANE-ALTERATIONS

Membrane-associated and soluble lipoxygenases from green tomato (Lycop ersicon esculentum Mill. cv Ailsa Craig) fruit have been identified. M icrosomal lipoxygenase was localized partly in the plasma membrane and tonoplast fractions. The possibilities of glycosyl-phosphatidylinosit ol or transmembrane polypeptide anchors in the membrane were ruled out by differential solubilization and temperature-induced phase separati on in Triton X-114. High performance liquid chromatography of reaction products combined with polarography showed that tomato lipoxygenase i s capable of specific oxygenation of fatty acids esterified in phospho lipids. This possibility of direct action on membrane phospholipids st rengthened the hypothesis of a role for lipoxygenase in plant senescen ce and membrane turnover. Membrane-associated lipoxygenase is polymorp hic, with two forms differing by their isoelectric points (pIs) (aroun d 4.2 and 5.1). The pI of the soluble lipoxygenase corresponds to the minor microsomal enzyme, with a pI of 5.1. The charge-differing isofor ms were separated and analyzed by western blotting using anti-soybean lipoxygenase antibodies. A single polypeptide with an apparent molecul ar weight of 92,000 was identified in each case for the soluble and mi crosomal enzymes. It is suggested that a charge modification of the so luble lipoxygenase allows its association with the membrane.