Conversion of retinol to retinal by rat liver microsomes

An NADPH-requiring enzyme present in rat liver microsomes catalyzes the conversion of retinol to retinal; the enzyme activity is induced by 3-methylcholanthrene (MC). The optimum activities for both the constitutive and induced reactions occur within a pH range of 8.2-8.7. For the constitutive enzyme, the KM and Vmax values are 285 microM retinol and 18 nmol of retinal/mg protein/hr, respectively; for the MC-induced activity, the corresponding values are 133 microM retinol and 33 nmol of retinal/mg protein/hr. Neither the constitutive nor the induced activities are detectable in microsomes from seven other tissues. Both hepatic activities are inhibited by citral, ketoconazole, SKF 525-A, and alpha-naphthoflavone; both are stimulated by divalent cations. Neither is inhibited by 3-amino-1,2,4-triazole, pyrazole, or sodium azide or stimulated by monovalent cations. The enzyme appears to be a previously unreported retinol oxidase, distinct from the known cytosolic enzymes, retinal reductase and retinol dehydrogenase.