Retinyl esters in liver

Hydrolysis of retinyl esters occurs in the liver both during the hepatic uptake of dietary vitamin A and during the mobilization of retinol from its stores in the liver. The hydrolysis of chylomicron retinyl esters that occurs during hepatic uptake has been discussed above. In addition, retinyl ester hydrolysis must precede the mobilization of retinol from hepatic stores of retinyl ester since retinol is mobilized in the form of the unesterified alcohol (retinol) bound to RBP. Accordingly, it is clear that the enzymatic hydrolysis of retinyl esters in liver represents an important process in the overall metabolism of retinol in the body. 

Retinyl ester stored in the liver must be hydrolyzed before the retinol can be mobilized from the liver as the retinol-RBP complex. Information is needed as to whether or not the processes of retinyl ester hydrolysis and of RBP production and secretion might, under some circumstances, be coordinated in some way. The question to be asked is whether or not the hydrolysis of retinyl esters might at times serve as one of the regulatory steps in the overall process of retinol mobilization from the liver. Information available about the enzymatic hydrolysis of retinyl esters in liver is reviewed in Chapter 7. 

The enzymatic hydrolysis of retinyl esters in liver has been examined in retinol-depleted, retinoic acid-supplemented rats (Harrison et al., 1979) as well as in normal rats (Prystowsky et al., 1981). In both studies, the enzyme retinyl palmitate hydrolase showed an unusual subcellular distribution, being found in the nuclear, mitochondrial, and supernatant fractions and displayed a large variability in activity from rat to rat. For additional information regarding the esterification and hydrolysis of retinol the reader is again referred to Chapters 7 and 10. 

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