The relative proportions of lysosomal enzyme activities in bovine retinal pigment epithelium.

The lysosome fractions from bovine retina, liver and retinal pigment epithelium were isolated by subcellular fractionation and compared with regard to the relative proportions of several hydrolytic enzyme activities. It was found that the lysosome fraction of the retinal pigment epithelium is more than three times as active as the lysosome fractions from other tissues in degrading the rhodopsin of photoreceptor (rod) cell outer segments. This proteolytic activity is attributable to a cathepsin D-like proteinase, and the possible biochemical bases for its increased activity in the pigment epithelium are discussed, including interaction with phospholipase A. It is suggested that the lysosomes of the retinal pigment epithelium are specialized in their content of hydrolytic enzymes for the degradation of photoreceptor cell outer segments.