Protein and nucleic acid metabolism in organs from mice selected for larger and smaller body size

Abstract

Studies of the growth and composition of Q-strain mice selected over 20 generations for high and low body weight at 6 weeks of age, and their unselected controls, were made on livers and kidneys of males from the five selection replicates A, B, C, D and F. Differences in growth rate between Large and Small QD mice were confirmed from 2 to 9 weeks of age, but were greatest in the third, fourth, sixth and seventh weeks. Total amounts of dry matter, protein, free amino acids, bulk RNA and ribosomes were increased or decreased from control values in proportion to organ weight. A less-perfect relationship between DNA content and organ weight suggested that some small changes in average cell mass had accompanied the main change in cell number in organs from the selected lines. Absorbance profiles of polyribosomes from both organs were identical in selected and control mice: selection had not operated on the proportion of single (currently inactive) ribosomes. Attempts to relate the observed differences in growth rate in QD mice to differences in the rate of protein synthesis produced an unexpected result: incorporation of radioactively labelled amino acids was consistently higher in the organs of the Small mice. Measurements of rates of protein turnover, and calculated rates of protein degradation, suggested that protein might also be degraded more rapidly in the small mice.

Protein turnover, physiological energetics and heterozygosity in the blue mussel, Mytilus edulis : the basis of variable age-specific growth

“…It may seem counter-intuitive th a t whole-body rates of both protein synthesis and catabolism were greatest among slow (S) rather th an faster growing (L) mussels of similar age (table 3). Nevertheless, comparable findings have been reported in the isolated tissues of rats ) and mice (Priestley & Robertson 1973) artificially selected and bred for greater body mass. F urther, we have shown th a t elevated efficiencies of protein synthesis ((net protein balance/gross protein synthesis) x 100) compensated for reduced intensities of protein turnover to help mediate increased protein deposition in more heterozygous (L) mussels.…”

Human Growth

“…Priestley and Robertson (1973) found no difference in the proportions of free ribosomes (inactive) between the strains and that, contrary to expectation, small mice show a higher rate of amino acid incorporation, but also a greater rate of protein degradation. Roberts (1981) showed that the larger mice both eat more and convert the food more efficiently.…”

Aspects of temperature regulation in mice selected for large and small size

“…In fact, direct selection for differences in body weight has indeed altered other traits contributing to thermoregulation. The situation is undoubtedly more complex, since there is also evidence for differential rates of protein turnover in large and small mice (Priestly & Robertson, 1973) which may be related to differences in appetite control (Radcliffe & Webster, 1976;Roberts, 1981). On a per gramme basis, small mice eat more than large mice, and their correspondingly larger amount of brown fat suggests that they may be burning off this excess food, rather than assimilating it as body mass.…”