Vectorial accumulation of cathepsin D in retinal pigmented epithelium: effects of age

The extracellular accumulation of n-acetyl-beta-glucosaminidase (NAG) and a cathepsin D-like enzymatic activity were studied in human retinal pigmented epithelium cultured on nitrocellulose filter inserts. These enzymes accumulated in both the apical and basal extracellular fluid. For NAG, the total extracellular activity accumulated in 24 hours was 7.3 +/- 1.75% of the total cell-associated activity, while the cathepsin D-like activity was 10 +/- 0.71% of the total cell-associated activity. The distribution of the extracellular activity was 60/40%, basal/apical, for NAG and was constant regardless of the age of the donor. In contrast, the distribution for the cathepsin D-like activity ranged from 70/30 to 10/90 and showed a marked age dependence in RPE obtained from donors aged 9-93 years. These levels of activity were stable for at least 2 months in culture. The cathepsin D-like activity differed from the NAG in that the presence of mannose-6-phosphate in the medium did not result in increased extracellular accumulation of cathepsin D activity.