BIOTRANSFORMATION OF ALL-TRANS-RETINAL, 13-CIS-RETINAL, AND 9-CIS-RETINAL CATALYZED BY CONCEPTAL CYTOSOL AND MICROSOMES

Oxidative conversions of all-trans-retinal (t-RAL), 13-cis-retinal (13 -cRAL), and 9-cis-retinal (9-cRAL) to their corresponding retinoic aci ds (RAs) catalyzed by rat conceptal cytosol (RCC) or microsomes (RCM) were studied. The primary product of RCC-catalyzed oxidations of both t-RAL and 13-cRAL was t-RA, with only trace amounts of 13-cRA and 9-cR A. In the RCC-catalyzed oxidation of 9-cRAL, generated t-RA, 9-cRA, an d 13-cRA constituted approximately 56, 34, and 10%, respectively, of t he total RAs. For all RCC-catalyzed retinal oxidations, NAD was a much more effective cofactor than NADP. And t-RAL and 13-cRAL were much be tter substrates than 9-cRAL. Formaldehyde, acetaldehyde, citral, and d isulfiram were investigated as inhibitors, but only citral and disulfi ram effectively inhibited the RCC-catalyzed conversion of t-RAL or 13- cRAL to t-RA. Methanol and ethanol failed to inhibit either reaction e ven at very high concentrations (greater than or equal to 10 mM). RCM exhibited lower specific enzymatic activities than RCC in catalyzing o xidations of t-RAL, 13-cRAL, and 9-cRAL, indicating that the cytosolic fraction was dominant for converting retinals to RAs. The predominant RA produced from RCM-catalyzed oxidations of t-RAL, 13-cRAL, or 9-cRA L was t-RA for each substrate, and again NAD was a much more effective cofactor than NADP in all cases. For RCM-catalyzed oxidations of RALs , 13-cRAL was a much better substrate than t-RAL or 9-cRAL. Methanol a nd ethanol were not effective inhibitors for RCM-catalyzed oxidations of t-RAL or 13-cRAL. In RCM-catalyzed reactions, citral (10 mM) comple tely inhibited oxidation of t-RAL but showed only a minor effect on ox idation of 13-cRAL. 13-cRA was converted almost completely to t-RA aft er 2 hr of incubation with RCC. (C) 1997 Elsevier Science Inc.