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

International Journal of Biochemistry

Volume 25, Issue 12, 1993, Pages 1775-1784

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.

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