Retinyl palmitate hydrolase activity in normal rat liver.

Retinyl esters are hydrolyzed in liver both during uptake and mobilization of vitamin A. Studies were conducted to explore the enzymatic hydrolysis of retinyl palmitate in normal rat liver. Retinyl palmitate hydrolase activity was assayed with a sensitive and accurate microassay, employing retinyl [~-‘~C]palmitate as substrate. The products of the reaction were identified as retinol and free fatty acid. Hydrolase activity was stimulated by sodium cholate and inhibited by a-tocopherol. Fractionation of liver homogenates showed greatest enrichment of retinyl palmitate hydrolase activity in the washed “nuclear” fraction, and least activity in the microsomal fraction. About one-third of the total activity was found in the soluble supernatant fraction. This unusual distribution was similar to that previously seen with retinol-depleted rats. Hydrolase activity was found to vary over 50-fold in individual rat livers. The variation was not related to the age, time of day, or order of death of the animals. Hydrolytic activities against cholesteryl oleate and triolein showed a similar variation and correlated strongly with retinyl palmitate hydrolase activity, whereas hydrolytic activities against retinyl acetate and p-nitrophenyl acetate did not. Hydrolase activity eluted as one major peak on gel filtration in the presence of 2.5% cholate; with a lower concentration of cholate (0.05%) a diffuse elution pattern was seen. Retinyl palmitate hydrolase activity was partially purified by hydrophobic interaction chromatography. Co-purification, with a comparable enrichment, of the hydrolytic activities against cholesteryl oleate and triolein was observed. The chromatographic and other data indicate that retinyl palmitate hydrolase activity has distinct hydrophobic physical properties. These properties may be involved in its role of catalyzing lipid ester hydrolysis.