Mechanisms involved in the intestinal digestion and absorption of dietary vitamin A.

Dietary retinyl esters are hydrolyzed in the intestine by the pancreatic enzyme, pancreatic triglyceride lipase (PTL), and intestinal brush border enzyme, phospholipase B. Recent work on the carboxylester lipase (CEL) knockout mouse suggests that CEL may not be involved in dietary retinyl ester digestion. The possible roles of the pancreatic lipase-related proteins (PLRP) 1 and 2 and other enzymes require further investigation. Unesterified retinol is taken up by the enterocytes, perhaps involving both diffusion and protein-mediated facilitated transport. Once in the cell, retinol is complexed with cellular retinol-binding protein type 2 (CRBP2) and the complex serves as a substrate for reesterification of the retinol by the enzyme lecithin:retinol acyltransferase (LRAT). Retinol not bound to CRBP2 is esterified by acyl-CoA acyltransferase (ARAT). The retinyl esters are incorporated into chylomicrons, intestinal lipoproteins that transport other dietary lipids such as triglycerides, phospholipids, and cholesterol. Chylomicrons containing newly absorbed retinyl esters are then secreted into the lymph.