IDentification of a microsomal retinoic acid synthase as a microsomal cytochrome P-450-linked monooxygenase system

• 1.1. To characterize an enzyme which metabolizes retinal in liver microsomes, several properties of the enzymatic reaction from retinal to retinoic acid were investigated using rabbit liver microsomes.
• 2.2. The maximum pH of the reaction in the liver microsomes was 7.6.
• 3.3. The K_m and V_max values for all-trans, 9-cis and 13-cis-retinals were determined.
• 4.4. The reaction proceeded in the presence of NADPH and molecular oxygen.
• 5.5. The incorporation of one atom of molecular oxygen into retinal was confirmed by using oxygen-18, showing that the reaction comprised monooxygenation, not dehydrogenation.
• 6.6. The monooxygenase activity was inhibited by carbon monoxide, phenylisocyanide and antiNADPH-cytochrome P-450 reductase IgG, but not by anti-cytochrome b₅ IgG.
• 7.7. The enzymatic activity inhibited by carbon monoxide was photoreversibly restored by light of a wavelength of around 450 nm.
• 8.8. The retinal-induced spectra of liver microsomes with three isomeric retinals were type I spectra.
• 9.9. The microsomal monooxygenase activity induced by phenobarbital or ethanol were more effective than that by 3-methylcholanthrene, clotrimazole or β-naphthoflavone.
• 10.10. These results showed that the monooxygenase reaction from retinal to retinoic acid in liver microsomes is catalyzed by a cytochrome P-450-linked monooxygenase system.