Abstract
Membrane-associated and soluble lipoxygenases from green tomato (Lycopersicon esculentum Mill. cv Ailsa Craig) fruit have been identified. Microsomal lipoxygenase was localized partly in the plasma membrane and tonoplast fractions. The possibilities of glycosyl-phosphatidylinositol or transmembrane polypeptide anchors in the membrane were ruled out by differential solubilization and temperature-induced phase separation in Triton X-114. High performance liquid chromatography of reaction products combined with polarography showed that tomato lipoxygenase is capable of specific oxygenation of fatty acids esterified in phospholipids. This possibility of direct action on membrane phospholipids strengthened the hypothesis of a role for lipoxygenase in plant senescence and membrane turnover. Membrane-associated lipoxygenase is polymorphic, with two forms differing by their isoelectric points (pls) (around 4.2 and 5.1). The pl of the soluble lipoxygenase corresponds to the minor microsomal enzyme, with a pl of 5.1. The charge-differing isoforms were separated and analyzed by western blotting using anti-soybean lipoxygenase antibodies. A single polypeptide with an apparent molecular weight of 92,000 was identified in each case for the soluble and microsomal enzymes. It is suggested that a charge modification of the soluble lipoxygenase allows its association with the membrane.