Acyl-CoA:retinol acyltransferase

The provitamin A, /8-carotene, is cleaved in the enterocyte by a soluble oxygenase w hich requires bile salts for its activity [90]. The product, retinal, is reduced by another soluble enzyme to retinol which is esterified chiefly with palmitic acid by a microsomal enzyme, acyl-CoA retinol acyltransferase which is inhibited by taurocholate in vitro this enzyme is very similar to ACAT [91]. Since j8-carotene is taken up by the intestine and rather efficiently converted to retinyl esters which appear in lymph, it must be inferred that the cytosolic oxygenase is exposed to sufficiently high concentrations of bile salts for the cleavage to occur. 

Harrison, EH Blaner, WS Goodman, DS Ross, AC. Subcellular localization of retinoids, retinoid-binding proteins, and acyl-CoA retinol acyltransferase in rat liver. J LipidRes, 1987 28 973-81. 

As discussed earlier in this chapter, studies by Helgerud et al. (1982, 1983) have demonstrated the formation of retinyl esters in rat intestinal mucosa by way of an analogous reaction, involving the reaction of retinol with a fatty acyl-CoA ester, catalyzed by an acyl CoA retinol acyltransferase (ARAT) enzyme. Similarly, Ross (1982a) demonstrated the formation of retinyl esters by mammary gland microsomes from the lactating rat in a reaction that was greatly stimulated by the formation or addition of fatty acyl-CoA thioesters, and thus had the characteristics of an ARAT-catalyzed reaction. 

Within the enterocyte, retinol is bound to cellular retinol binding protein (CRBP 11) and is esterified by lecithin retinol acyltransferase (LRAT), which uses phosphatidylcholine as the fatty acid donor, mainly yielding retinyl palmitate, although small amounts of stearate and oleate are also formed. At unphysiologically high levels of retinol, when CRBP 11 is saturated, acyl coenzyme A (CoA) retinol acyltransferase (ARAT) esterifies the free retinol that accumulates in intracellular membranes. Then the retinyl esters enter the lymphatic circulation and then the bloodstream (in chylomicrons), together with dietary lipid and carotenoids (Norum et al., 1986 Olson, 1986 Blomhoff et al., 1991 Green et al., 1993 Harrison and Hussain, 2001). 

This enzyme [EC 2.3.1.76], also referred to as retinol fatty-acyltransferase, catalyzes the reaction of an acyl-CoA derivative with retinol to generate coenzyme A and the retinyl ester. The CoA derivative can be palmi-toyl-CoA or other long-chain fatty-acyl derivatives of coenzyme A. 

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