Bile salt-independent retinyl ester hydrolase in the rat lacrimal gland

The lacrimal gland secretes retinol and synthesizes and stores retinyl esters. In this study, presence of retinyl ester hydrolase in rat lacrimal gland was investigated. A bile salt-independent retinyl ester hydrolase was identified in lacrimal gland homogenates and microsomes which were incubated in a tris-maleate buffer with retinyl palmitate as substrate. Retinol produced was measured by HPLC and activity expressed as pmoles retinol produced/mg protein/hr. In homogenates, the enzyme activity had a pH optimum at pH 6 and maximum specific activity of 57.7 pmoles/mg/hr. In microsomes, the activity was optimal at pH 7 and maximum specific activity was 1073 pmoles/mg/hr. In contrast, rat liver retinyl ester hydrolase had a pH optimum at pH 8 and a specific activity of 24.7 pmoles/mg/hr. These observations demonstrate that a complete cycle of retinyl ester metabolism is present in the lacrimal gland.