ABSTRACT
Two enzymes of esterifying retinol have been described: lecithin:retinol acyltransferase (LRAT) and acyl-CoA:retinol acyltransferase (ACAT) (83-85). High activities of both enzymes have been detected in Sertoli cells (86,87). However, Shingleton et al. (86) concluded that only LRAT was involved in the esterification of retinol in the intact Sertoli celL This enzyme uses CRBP-bound retinol as a donor of retinol. As in other tissues, palmitate is the predominant retinyl ester formed, followed retinyl oleate. Similar patterns of retinyl esters were produced by Sertoli cell microsomes and whole Sertoli cells. Furthermore, this pattern was the same whether retinol was administered to the cells in free form or bound to RBP (86). c. Retinyl Ester Hydrolysis Like many other cell types, the Sertoli cells have a
of vitamin A in esterified form, located in lipid droplets in the cytosol. The droplets contain a variety of lipids and have been shown to vary in size and
intracellular localization during the spennatogenic cycle. Mobilization of retinol from retinyl esters involves retinyl ester hydrolase. Both cholate-dependent and cholate-independent enzyme activities have been described (88-90). Studying the latter, Napoli and coworkers (88,89) demonstrated that the release of retinol from endogenous esters was increased fourfold when apo-CRBP was present. They concluded that the cholate-dependent ester hydrolase activity most probably has no physiological importance. d. Retinoic Acid Synthesis Despite the fact that retinol-deficient, retinoic acidmaintained animals suffer loss of spermatogenesis, the expression of retinoic acid receptors and CRABP in the seminiferous tubule indicates that retinoic acid is involved in supporting the vitamin A function in testis. Since present evidence indicates that retinoic acid at physiological levels cannot reach the germ cell on the inside of the blood-testis barrier, local of retinoic acid is called for. The conversion of retinol to retinoic acid is a two-step process. First retinol is oxidized to retinal, which then is further oxidized to retinoic acid. Although alcohol dehydrogenase is capable of producing considerable amounts of retinoic acid in the presence of supraphysiological concentrations of retinol, this pathway is not considered important in vivo. In testis cytosol several retinoid hydrogenases have been demonstrated (91). These enzyme activities are NAD-or NADP-
~v1;vuuv''" and use free retinol as substrate Posch and (92) found that retinol-dehydrogenase and retinal dehydrogenase activities in mouse testis cytosols comigrated in several analytical systems, indicating their possible participation in a functional complex. Recently, a microsomal NADP-depcndent retinol dehydrogenase was described (93,94). This enzyme uses holo-CRBP as a substrate to produce retinal. The specific activity of the enzyme in testis microsomes is one seventh of that found in where the specific
activity has been registered (88,93). In tissues with high levels of CRBP expression, such as testis, the concentration of free retinol is assumed to be too low to result in significant oxidation by the enzymes using free retinol as substrate Therefore, the microsomal NADP-dcpendent retinol dehydrogenase and holoCRBP could be the factors in the regulation of retinol oxidation. A similar view was Shingleton et al. (86).
