Cellular Retinol-binding Protein and Cellular Retinoic Acid-binding Protein in the Lacrimal Gland

Abstract

Cells that require or metabolize vitamin A contain cellular retinol-binding proteins (CRBP) and cellular retinoic acid-binding proteins (CRABP) which apparently participate in metabolism and transport of retinoids within the cell. Since the lacrimal gland secretes and metabolizes vitamin A, the cellular retinoid-binding proteins and their mRNAs should also be present in this gland. Total RNA from rat and rabbit lacrimal glands was analysed by RNase protection using 32P-labeled antisense cRNA probes. CRBP-I mRNA is present in rat lacrimal gland at 2% of the level in liver but CRBP-I mRNA could not be identified conclusively in rabbit lacrimal gland using the available rat cRNA probe. CRABP-I mRNA is present in rat lacrimal gland at 5% of the level in the 12-day gestation rat fetus, but was not detectable in the rabbit lacrimal gland. Rabbit or rat lacrimal gland cytosol was incubated with [ 3H]retinol or [ 3H] retinoic acid followed by ion-exchange chromatography using a Tris buffer and NaCl gradient. CRBP is present in rabbit and rat lacrimal gland at 78·5 ± 14·5 and 71·1± 11 pmol g -1 protein, respectively. CRABP is present in rat lacrimal gland at 190·6 ± 13·8 pmol g-1 protein but was not detectable in rabbit lacrimal gland. The presence of CRBP in the lacrimal gland is consistent with its role in secretion of retinol into the lacrimal gland fluid. The apparent lack of CRABP in rabbit lacrimal gland as compared with rat suggests species differences in the role of retinoids in differentiation and maintenance of this organ.