Characteristics of protein degradation in liver and mammary gland.

There is no correlation of protein degradation rate (measured by double isotope methods) with protein subunit molecular weight or isoelectric point in liver subcellular and suborganelle fractions and mammary cytosolic fractions. Most isotope ratios in each fraction fall in exponential distributions corresponding to populations of protein degradation rates; there is no statistically significant difference between protein degradation rates within several populations. The prime determinant for the degradation of proteins in these populations may be the morphological localization of the proteins. Pulse-chase labelling profiles indicate that a considerable proportion of newly-synthesised casein is rapidly degraded in mammary explants from mid-pregnant rabbits. This casein degradation is not co-translational; it appears to be a lysosomally-mediated event. In mammary tissue, the intracellular calcium concentration may acutely regulate synthesis and degradation of intracellular proteins, and the synthesis of casein.