The pattern of protein turnover in the whole animal and the effect of dietary variation.

Total body protein turnover Total body protein turnover is a collective property of the whole organism, in that it is comprised of the sum of the turnover rates of all the individual tissue proteins. Because of this, it has been likened to the basal metabolic rate by Waterlow (1969) who has reviewed the methodology of total turnover in the whole animal. An estimate of total body protein turnover can be obtained by determining the flux rates of an amino acid through the plasma by means of the constant infusion of a labelled amino acid (Waterlow, 1969). This has been done with lysine (Waterlow & Stephen, 1967)) tyrosine (Garlick, unpublished), and glycine (Garlick, 1969). When the flux rates are converted to total body protein turnover, rates of 40 g proteinlkg body-weight (2o%/d) in the young rat and 20 g protein/kg body-weight (Io%/d) in the adult rat are obtained. Waterlow & Stephen (1968) measured the effect of dietary variation on lysine flux in the rat and showed that there was no more than a 1576 variation in the flux rate after a variety of short-term dietary changes. In a previous paper, however, Waterlow & Stephen (1967) showed that longer periods of protein depletion causes a 30% fall in lysine flux. I n children, protein turnover has been measured by means of the constant intragastric infusion of [15"]glycine by Picou & Taylor-Roberts (1969). They showed that a change in protein intake had no effect on total body protein synthesis in recovered children. However, malnourished children exhibited higher synthesis rates than recovered children. When it is considered that the malnourished child has considerably less intracellular protein than the recovered child, these results must mean that the turnover of intracellular protein is even more rapid in the malnourished child. These results therefore are quite different from those obtained with rats.