The extent and nature of protein degradation in the tissues during development.

Protein turnover, defined as the degradation and replacement of proteins, appears to vary between most adult species in the same way as metabolic rate, i.e. as W0.75, although it may be a little lower in man. During development in the rat it also varies as metabolic rate. Thus P Total = 14.7 W0.53kg per day. Most of this turnover occurs in nonmuscle tissues (P = 11.3 W0.50kg per day) with protein turnover in muscle described by P = 3.53 W0.69kg per day. Mechanisms for protein degradation in liver and muscle involve lysosomes although the morphology of the lysosomal system in muscle is different from that in liver. However, heterogeneous turnover is a feature of proteins in both issues including the principal myofibrillar proteins. While the reaction order of protein synthesis can reasonably be described as zero order--a fixed rate per unit of DNA--there is less certainty about degradation. It is postulated that structural and functional characteristics of the cytoplasm of cells determine the accessibility of cellular protein to the degrading system. As a result, a first order rate for a particular cell type is fixed, and this determines the magnitude of the protein-DNA ratio or the functional-cell size. The first order degradation rate of the cytoplasmic protein also determines the specific activity of the degrading enzymes.