12-Hydroxyeicosatetraenoic acid (12-HETE) is formed from arachidonic acid either by 12-lipoxygenase or by a cytochrome P450 monooxygenase. 12-Lipoxygenase is generally localized in the soluble cytosolic fraction, and the cytochrome P450 monooxygenase is a microsomal enzyme. In this study, 12-HETE biosynthesis and the regulation of 12-HETE biosynthesis by epidermal growth factor (EGF) in A431 cells were investigated. 12-HETE was biosynthesized from arachidonic acid by the microsomal fraction of A431 cells, but not by the cytosolic fraction. The formation of 12-HETE was inhibited by 5,8,11,14-eicosatetraynoic acid, nordihydroguaiaretic acid, and caffeic acid. Nordihydroguaiaretic acid at 10(-4) M and 5,8,11,14-eicosatetraynoic acid at 10(-5) M almost completely inhibited its formation. However, the formation of 12-HETE was not affected by the presence of an NADPH-generating system, carbon monoxide, or SKF 525A. The biosynthetic 12-HETE was analyzed by chiral stationary phase high performance liquid chromatography and was highly enriched in (12S)-HETE. We therefore concluded that the enzyme responsible for the formation of (12S)-HETE in the microsomes of A431 cells is a 12-lipoxygenase. The microsomal 12-lipoxygenase of A431 cells belongs to the "leukocyte-type" enzyme as determined by substrate specificity and enzyme kinetics studies. The microsomal 12-lipoxygenase oxygenated linoleic acid much faster than the cytosolic platelet 12-lipoxygenase and is a "self-catalyzed inactivation" enzyme. Treatment of cells with 50 ng/ml EGF significantly induced microsomal 12-lipoxygenase activity. The lag period for the expression of the stimulatory effect of EGF on 12-lipoxygenase activity was approximately 10 h. The stimulatory effect of EGF on 12-lipoxygenase activity was completely blocked by treatment with 35 microM cycloheximide, indicating a requirement for de novo protein biosynthesis. Furthermore, the presence of the endogenous inhibitor of 12-lipoxygenase (which masked (12S)-HETE biosynthesis in intact cells) was identified in the cytosolic fraction of A431 cells. The putative inhibitor was enzyme-selective. It inhibited the leukocyte-type 12-lipoxygenase, but not the "platelet-type" enzyme.