Substrate specificities and mechanism in the enzymatic processing of vitamin A into 11-cis-retinol

The biosynthesis of 11-cis-retinol in the retinal pigment epithelium requires two consecutive enzymatic reactions. The first involves the esterification of all-trans-retinol by lecithin retinol acyltransferase (LRAT). The second reaction involves the direct conversion of an all-trans-retinyl ester into 11-cis-retinol by an isomerase-like enzyme. This latter reaction couples the free energy of hydrolysis of an ester to the thermodynamically uphill trans to cis conversion, thus providing the energy to drive the latter process. In this paper both enzymes are studied with respect to their substrate specificities to provide information on mechanism. The isomerase is shown to be highly specific with respect to the ionylidene ring system and substitution at C15, whereas sterically bulkier substituents at C9 and C11 are permitted. C5 and C13 demethyl retinoids are isomerized, removing from consideration isomerization mechanisms involving C-H abstraction at the C5 or C13 methyl groups of the retinoid. On the other hand, C9 demethyl retinoids are not isomerized. A C-H abstraction mechanism is unlikely at the C9 methyl group as well, because no kinetic deuterium isotope effect is found with all-trans-19,19,19-trideuterioretinoids and isomerization of unlabeled retinoids occurs without the incorporation of deuterium when the isomerization is performed in D2O. LRAT proved to be broadly specific for retinols but was relatively inert with other hydrophobic alcohols including cholesterol. The enzyme is also highly specific for phosphatidylcholine analogues versus other potential membranous acyl donors such as phosphatidylethanolamine and phosphatidylserine.