Protein degradation in skeletal muscle: implications of a first order reaction for the degradative process

The rate of protein degradation is usually thought to be first order, i.e. determined by the nature of the protein as a substrate. It is not immediately apparent if this is the case for the overall process in the cell since rates of turnover of individual proteins may vary between tissues. In muscle the characteristics of protein turnover in relation to DNA-unit size have led to the development of a model for protein turnover in which degradation rates are determined by the rate of dissociation of protein subunits from the myofibrillar matrix. This is a necessary step if heterogeneous turnover occurs and if degradation and resynthesis of myofibrillar proteins occurs peripherally to the myofibril. As a result a first order rate can be envisaged so that during muscle growth the protein mass per unit DNA increases to a characteristic amount thus determining the specific activity of the degrading system. Such a mechanism may apply to all cells.