Activity measurement of lysosomal cysteine proteinases, cathepsins B, H and L, in crude tissue extracts, and their relation to the fractional rate of protein degradation.

1. The fractional rate of protein degradation was measured in vivo and compared with the lysosomal enzyme activities of several rat tissues. Good agreement was observed between them. 2. A specific method for determining lysosomal cysteine-dependent cathepsins, B, L and H, was demonstrated.