Conversion of Retinyl Methyl Ether into Retinol in the Rat in Vitro

Abstract

In rats retinyl methyl ether (RME) is converted into retinol by everted intestinal sacs, liver slices and liver homogenates. The RME cleavage enzyme of liver is localized in the microsomal fraction, and can be solubilized and stabilized by the preparation of an acetone powder. Mg 2+ and EDTA have an additive stimulatory effect on the fresh microsomal enzyme but not on the acetone powder preparation. The Km for RME was found to be 4 X 10 -4 m from kinetic studies. Tetrahydropteridine is a required cofactor, also with a Km of 4 X 10 -4 M. The pteridine analog, tetrahydroquinazoline, inhibits the reaction by competing with tetrahydropteridine, and has a K i of 4.25 X 10 -4 m. Molecular oxygen is also required, and NADPH enhances the enzyme activity, presumably by reducing dihydropteridine. Thus, the microsomal enzyme which catalyzes the cleavage of RME to retinol appears to be a typical pteridine requiring monooxygenase. © 1972, American Chemical Society. All rights reserved.