RETINYL ESTER HYDROLYTIC ACTIVITY ASSOCIATED WITH HUMAN INTESTINAL BRUSH-BORDER MEMBRANES

Hydrolysis of retinyl esters in the lumen of the small intestine is re quired before absorption. Previously, rat brush border membranes (BBMs ) were found to contain a pancreatic-derived esterase preferring retin yl esters with short fatty acyl chains and an intrinsic esterase prefe rring esters with long fatty acyl chains. Here, similar activities wer e found for preparations of human BBMs. Long-chain ester hydrolysis wa s stimulated best by deoxycholate, a dihydroxy bile salt, whereas shor t-chain ester hydrolysis was stimulated best by taurocholate, a trihyd roxy bile salt. A 10-fold difference in K-M values for retinyl palmita te (0.53 mu mol/L) and caproate (5.5 mu mol/L) also indicated distingu ishable long-chain and short-chain activities. Differences between ret inyl butyrate and retinyl caproate hydrolysis suggested the possible p resence of two short-chain esterase activities associated with both ra t and human BBMs, in addition to the long-chain activity. Similarities between human BBM retinyl ester hydrolytic activities and chose obser ved for rat BBMs suggest that the rat is a good model for humans in th is step of vitamin A metabolism.