Background The aim of this review is to assess the current understanding of the regulation of protein synthesis and degradation in striated muscle, a tissue which constitutes the largest single element of the protein-bound nitrogen pool in the mammalian organism. Nu-merous experimental interventions alter protein turnover rates in vitro and we will discuss whether or not these might fulfil regulatory functions in vivo. We will also consider the molecular mechanisms that may be involved in the regulation of protein turnover.

Protein turnover is a cyclical process with synthesis of protein being opposed by its concomitant degradation. Because of this, rates of protein synthesis and degradation are considerably greater than the net flux (kg) through the protein turnover cycle. Amino acids derived from intracellular protein degradation are reutilized for protein synthesis to a considerable extent. Thus the rate of whole body protein synthesis can be much greater than the rate of dietary influx of amino acids or the rate of nitrogen excretion. Because of the cyclical nature of protein turnover, any imbalance between ks and kd will lead to a change in the size of the tissue protein pool. Thus a loss of tissue protein does not necessarily entail an increase in kd nor does a gain necessarily entail an increase in ks. k. is expressed as the rate of amino acid incorporation into protein relative to tissue or protein weight, or as a fractional synthesis rate. Most measurements ofk. cited in this review refer to the overall synthesis rates of total cell protein, although the