Welcome to our bytesize lesson for today.
Vitamin A is obtained as retinol (preformed) from animal sources, or as provitamin A carotenoids from plant sources. The retinol in foods is linked to a fatty acid forming retinyl esters, which are hydrolyzed by an enzyme called lipase (made in the pancreas), the action being taken in the upper part of the small intestine. The retinol is absorbed by the intestinal cells, esterified, passing through the lymphatic system into the blood, incorporated in chylomicrons. Then it is dropped of at various locations, like target tissues and mostly in the stellate cells of the liver for storage.
The provitamin A carotenoids are splitted by an enzyme called dideoxygenase, in two molecules of retinal, but the absorption is not as efficient as in the retinol case. The resulted retinal molecules are converted in retinol, and mixed with the pool of retinol derived from the animal sources, following the same path. When it is released, vitamin A as retinol is binding to a plasma retinol binding protein (RBP), synthetized in the liver. The RBP-retinol complex is small enough to be quickly excreted by the kidneys, so, in order to prevent that, it is coupling with transthyretin, forming RBP-TTR-retinol complex, which in exchange can bind to cell receptors and release the retinol to be used by the cells.
Next post will be about vitamin A functions.
P.S. If you are Romanian, you can also publish your post in the Publish0x Romanian Community group for others to read it.
Disclaimer: This text also can be re-published on my personal blogs, such as this one.