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.
Shih
T W
TW
Southern Research Institute, Birmingham, AL 35255-5305.
Hill
D L
DL
D000818:Animals; D001711:Biotransformation; D002851:Chromatography, High Pressure Liquid; D004790:Enzyme Induction / Q000187:drug effects; D005260:Female; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008670:Metals / Q000494:pharmacology; D008748:Methylcholanthrene / Q000494:pharmacology; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D009249:NADP / Q000378:metabolism; D051381:Rats; D012172:Retinaldehyde / Q000378:metabolism*; D013056:Spectrophotometry, Ultraviolet; D014801:Vitamin A / Q000378:metabolism*
D008670:Metals; D014801:Vitamin A; D009249:NADP; D008748:Methylcholanthrene; D012172:Retinaldehyde