Protein turnover and cathepsin B activity in several individual tissues of foetal and senescent rats.

The fractional rate of protein synthesis has been measured in vivo, and compared in the whole body and 12 major individual tissues of foetal and senescent rats. This synthetic rate was found to decrease in most tissues with increasing animal age. The rate of protein degradation was also determined and compared with cathepsin B activity within each tissue; both protein turnover and the endopeptidase activity decreased with ageing. Age-related changes in each tissue's contributions to the protein mass and synthetic rates of the whole animal are also summarized and related to developmental variations in physiological function.