Vitamin A metabolism in chick liver: some properties of the cytosolic lipid-protein aggregate.

Incubation of hepatic microsomes with retinol resulted in formation of retinyl esters and glucuronides. The presence of the cytosolic lipid-protein aggregate (LPA) in the system in addition induced release of holo-retinol-binding protein from the microsomes. The extent of these reactions was influenced by the addition of coenzyme A and ATP, or uridine diphosphate glucuronic acid. Incubation of hepatic microsomes containing labelled retinyl esters with the LPA resulted in the appearance of the labelled retinyl esters in the LPA. Small amounts of retinoic acid were formed on incubation of retinol with microsomes (approximately 1% of added retinol); this was found to be associated with a protein of approximately 14 500 molecular weight, and less than 10% was associated with the LPA. This is in contrast to retinol, which was found to be almost completely associated with the LPA. The cytosolic LPA was associated both with carotene-cleavage activity and alcohol dehydrogenase (NAD(P)+) (EC 1.1.1.71) activity. These findings lend some support to the concept of a specific role for hepatic LPA.