Studies on the in vivo absorption of micellar solutions of tocopherol and tocopheryl acetate in the rat: demonstration and partial characterization of a mucosal esterase localized to the endoplasmic reticulum of the enterocyte.

The in vivo absorption of alpha-tocopherol from micellar solubilized solutions of free alpha-tocopherol and alpha-tocopheryl acetate was investigated using isolated loops of rat jejunum and found to be similar. Although analysis of the fluid remaining in the loop following the absorptive period with tocopheryl acetate showed that esterase activity and free tocopherol were present, calculations suggested that luminal hydrolysis of the ester could not have accounted for the similar rates of absorption of the free and esterified tocopherols. A mucosal source of the esterase activity was postulated and subsequently identified and characterized with respect to pH optimum, enzyme kinetics, and activation by bile salts, and shown to be distinct from pancreatic esterase. Analytical subcellular fractionation studies on homogenates of isolated jejunal enterocytes demonstrated that esterase activity against both p-nitrophenyl acetate and tocopheryl acetate was localized to the endoplasmic reticulum. These studies suggest that although the bulk of tocopheryl esters are normally hydrolyzed in the intestinal lumen by pancreatic esterase prior to uptake, some of the esters may be hydrolyzed intraluminally by the mucosal enzyme and some taken up intact by the jejunal mucosa and hydrolyzed intracellularly. This mucosal esterase may be of particular importance in conditions of pancreatic insufficiency.