UPTAKE IN THE INTESTINE

Retinyl esters (RE) are converted to retinol in the lumen and taken up by the epithelial cells where they are re-esterified and wrapped in chylomicrons.

β-carotene is absorbed by the enterocytes and split into two retinal molecules, which are then further converted to retinol and then esterified. RE have now become part of the chylomicrons in the lymphatics, eventually poured into the blood stream.

Figure Inside the enterocytes the retinol is directed to the enzyme lecithin:retinol acyltransferase (LRAT), which facilitates re-esterification with long chain fatty acids (mainly palmitate) to retinyl ester. In the enterocytes, the retinyl esters and non-modified carotenoids are incorporated into chylomicrons, which are secreted into the lymph and moved into the circulation.

LRAT knockout mice
In mice where LRAT is knocked out, there are only trace levels of retinyl esters, demonstrating the importance of LRAT in this process (Batten et al. 2004).