A cryptic, microsomal-type arachidonate 12-lipoxygenase is tonically inactivated by oxidation-reduction conditions in cultured epithelial cells

Cultured ovine tracheal epithelial cells converted arachidonic acid to prostaglandin E₂ (PGE₂), but microsome-containing subcellular fractions prepared from these cells under calcium-free conditions converted arachidonic acid to PGE₂ and to 12-hydroxyeicosatetraenoic acid (12-HETE) at a high rate (2-4 nmol/mg of protein/15 min). Identification of the membrane-bound 12-HETE-forming activity as a 12-Iipoxygenase included 12S-stereospecificity of product formation and trapping of 12-hydroperoxyeicosatetraenoic acid as a reaction product. The 12-lipoxygenase activity was extracted from cell membranes only with detergent (1% Triton X-100), and the activity (membrane-bound or detergent-solubilized) was completely inactivated by mixing with the cytosol-containing subcellular fraction. The inhibitory effect of the cytosolic fraction was reversed by treating the cytosol with GSH-depleting agents (2-cyclohexene-l-one or N-ethylmaleimide) or by mixing it with lipid hydroperoxide (13-hydroperoxyoctadecadienoic acid) at a concentration that had little direct effect on enzyme activity. Inhibition of 12-lipoxygenase activity could also be achieved by treatment of enzyme preparations with GSH at levels (0.1-10 mM) found in epithelial cell cytosol. In addition, treatment of cultured epithelial cells with a GSH-depleting agent (buthionine sulfoximine) and lipid hydroperoxide restored cellular 12-lipoxygenase activity. Little or no detectable 12-lipoxygenase activity was found in freshly isolated ovine tracheal epithelial cells, but the cytosolic 12-lipoxygenase found in freshly isolated bovine tracheal epithelial cells was relatively insensitive to regulation by GSH or lipid hydroperoxide. These observations indicate that a 12-lipoxygenase is expressed in a cryptic, microsomal-type form in primary-culture epithelial cells and that this form of the enzyme may be selectively regulated by changes in cellular oxidation-reduction conditions dependent on cytosolic levels of GSH versus lipid hydroperoxide.