Lipoxygenase activity in microsomal fraction of mouse epidermal homogenate was characterized in comparison with cytosol lipoxygenase activity. The major activity was identified as 12-lipoxygenase in microsomal fraction as well as in cytosol fraction by the analyses with high-performance liquid chromatography and gas chromatography-mass spectrometry. Apparent Km values of cytosol and microsomal 12-lipoxygenase for arachidonic acid were 5.0 μM and 6.2 μM respectively. Apparent Vmax values were 14 pmol/min/mg protein for the cytosol enzyme and 32 pmol/min/mg protein for the microsomal enzyme. Net activities of cytosol and microsomal 12-lipoxygenase were 214 and 109 pmol/min/g wet tissue, respectively. Both cytosol and microsomal lipoxygenase activities were neither dependent on calcium nor ATP. Carbon monoxide failed to affect these enzyme activities. There were considerable differences either in the effect of glutathione or in the sensitivities toward several lipoxygenase inhibitors, indicating that the cytosol and the microsomal 12-lipoxygenase activities are derived from two different enzymes. Alternatively, the differences could be attributable to the different microenvironments of these enzymes.
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