BACKGROUND

Excessive accumulation of lipofuscin in the retinal pigment epithelium with age and in various hereditary and degenerative retinal diseases, is of pathogenetic significance. We have shown that the major lipofuscin fluorophor A2-E (N-retinylidene-N-retinylethanolamine) affects the lysosomal degradation of human RPE cells and damages the cellular metabolism by phototoxic properties. Herein we sought to determine mechanisms for the inhibitory effect on lysosomal function apart from pH elevation.

METHODS

Potter-Elvejem homogenates of RPE cells were used to measure the activity of 24 lysosomal enzymes before and after incubation with A2-E.

RESULTS

This is the first time that RPE cells have been screened for a large spectrum of lysosomal hydrolases including proteases, lipidases, glycosidases, nucleases, sulfatases and phosphatases. The activities of these hydrolases were readily detectable in cultured RPE cells. Incubation of RPE cell homogenates even with high A2-E concentrations (up to 10 microM) did not affect the activity of isolated lysosomal enzymes.

CONCLUSIONS

The results suggest that a direct inhibition of lysosomal enzyme activity would not explain the inhibitory effect on lysosomal degradation. A2-E increases the acidic intralysosomal pH thereby probably hindering pH-dependent lysosomal enzymatic activities. The understanding of the inhibitory effects of A2-E on RPE cell metabolism may contribute to new approaches for treatment of retinal diseases with excessive lipofuscin accumulation such as ARMD or M. Stargardt.