After not eating all day and feeling absolutely famished, you eat a meal high in vitamin A. Predict and explain what type of transport process (described in class; ie, simple diffusion, facilitated diffusion, or active transport) that you expect the vitamin to use to cross your intestinal membranes.
Dietary retinyl esters are hydrolyzed in the intestine by the pancreatic enzyme: pancreatic triglyceride lipase (PTL), and intestinal brush border enzyme: phospholipase B. Unesterified retinol is taken up by the enterocytes, perhaps involving both simple diffusion (due to solubility of retinol in the lipid bilayer) and protein-mediated facilitated transport. However, until now, no protein has been identified and characterized that might be involved in the uptake of retinol. 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.