Identification of a novel arachidonate 12-lipoxygenase in bovine tracheal epithelial cells distinct from leukocyte and platelet forms of the enzyme.

Abstract

We examined the characteristics of an arachidonate 12-lipoxygenase in bovine tracheal epithelial cells in relation to the enzyme expressed in leukocytes and platelets. Homogenous preparations of intact or disrupted tracheal epithelial cells metabolized arachidonic acid predominantly to (12S)-hydroxyeicosatetraenoic acid, and subcellular fractionation by differential centrifugation demonstrated that the 12-lipoxygenase activity was localized predominantly to the 100,000 x g supernatant (cytosol fraction). Analysis of cytosolic enzymatic activity for pH dependence (maximum activity at pH 7.4-8.0), divalent cation effects (no dependence on cations), and kinetic characteristics (lag phase elimination by addition of hydroperoxide) exhibited similarity to leukocyte and platelet 12-lipoxygenases. Immunoprecipitation experiments demonstrated that the epithelial 12-lipoxygenase reacted with a monoclonal antibody (lox-2) directed against leukocyte 12-lipoxygenase but not with an antibody (HPLO-3) against the platelet enzyme. Immunoaffinity chromatography of the epithelial 100,000 x g supernatant fraction using lox-2 linked to Affi-Prep 10 yielded a single predominant protein band (Mr = 72,000) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis identical in apparent mass to the bovine leukocyte lipoxygenase. Western blotting using a polyclonal antibody to leukocyte 12-lipoxygenase showed peroxidase staining of the same 72-kDa protein band. Activity assays of the purified enzymes demonstrated that substrate specificity for the epithelial 12-lipoxygenase was similar to that of the leukocyte enzyme, but the epithelial enzyme more efficiently converted 18-carbon fatty acids to the corresponding monohydroxylated conjugated dienes. We conclude that bovine tracheal epithelial cells express a 12-lipoxygenase that has immunological reactivity similar to leukocyte and distinct from platelet 12-lipoxygenase and possesses substrate specificity distinct from both enzymes. We further suggest that lipoxygenase heterogeneity may provide a basis for different functional roles for the enzyme in different cell types.