Neutral and acid retinyl ester hydrolases associated with rat liver microsomes: relationships to microsomal cholesteryl ester hydrolases

Abstract

We recently reported the presence of a neutral, bile salt-independent retinyl ester hydrolase (REH) activity in rat liver microsomes and showed that it was distinct from the previously studied bile salt-dependent REH and from nonspecific carboxylesterases (Harrison, E. H., and M. Z. Gad. 1989. J. Biol. Chem. 264: 17142-17147). We have now further characterized the hydrolysis of retinyl esters by liver microsomes and have compared the observed activities with those catalyzing the hydrolysis of cholesteryl esters. Microsomes and microsomal subfractions enriched in plasma membranes and endosomes catalyze the hydrolysis of retinyl esters at both neutral and acid pH. The acid and neutral REH enzyme activities can be distinguished from one another on the basis of selective inhibition by metal ions and by irreversible, active site-directed serine esterase inhibitors. The same preparations also catalyze the hydrolysis of cholesteryl esters at both acid and neutral pH. However, the enzyme(s) responsible for the neutral REH activity can be clearly responsible for the neutral REH activity can be clearly differentiated from the neutral cholesteryl ester hydrolase(s) on the basis of differential stability, sensitivity to proteolysis, and sensitivity to active site-directed reagents. These results suggest that the neutral, bile salt-independent REH is relatively specific for the hydrolysis of retinyl esters and thus may play an important physiological role in hepatic vitamin A metabolism. In contrast to the neutral hydrolases, the activities responsible for hydrolysis of retinyl esters and cholesterol esters at acid pH are similar in their responses to the treatments mentioned above. Thus, a single microsomal acid hydrolase may catalyze the hydrolysis of both types of ester.(ABSTRACT TRUNCATED AT 250 WORDS)