RETINYL ESTER HYDROLYSIS IN THE RABBIT LACRIMAL GLAND

The lacrimal gland stores retinyl esters which are synthesized by the enzyme acyl CoA:retinyl acyl transferase. Retinol is released from retinyl ester reserves by retinyl ester hydrolase (REH). Since the lacrimal gland secretes retinol, this gland should also contain this enzyme. To identify bile salt-dependent REH activity, rabbit lacrimal glands were homogenized in 0.05 M Trismaleate buffer, and enzyme activity was determined in the tissue homogenate, in the membrane fraction and in the cytosolic fraction by measurement of production of retinol from retinyl palmitate (nmol retinol produced/mg protein/h). In the lacrimal gland, production of retinol was optimal in the presence of 200 mM CHAPS at pH 7. The REH activity in the presence of 1000 muM retinyl palmitate was 2.38 +/- 0.18 nmol/mg/h in the homogenate, 1.13 +/- 0.16/nmol/mg/h in membranes and 3.25 +/- 0.26 nmol/mg/h in cytosol. By comparison, REH activity in rabbit liver was 6.58 +/- 0.75 nmol/mg/h. The REH activity in lacrimal gland was not affected by vitamin A deficiency. These data are consistent with the presence of retinyl ester hydrolase activity in the lacrimal gland and provide further evidence that this gland is adapted for metabolism and secretion of retinol.