Incorporating turnover in estimates of protein retention efficiency for different body tissues.

Formulated in terms of protein synthesis (PS) and protein retention (PR), a definition of turnover-related protein retention efficiency (kP) allows the expression kP=[1+(PS/PR)/6](-1), 6 representing the ratio of the energy equivalent of protein to the cost of synthesis. By combining plausible hormonal and cellular control mechanisms of protein (P) growth, it is possible to derive (PS/PR)=[Q{(P/alpha)(-(4/9)Y)-1}](-1)+1, allowing the calculation of kP by substitution. The symbol alpha represents the limit value of protein growth, while the term 4/9 derives from the power in the relationship between the concentration of growth factor-related activator in the nucleus and cell volume (cv). Y is the power in the relationship between cv and total tissue protein, and Q represents the proportion of growth factor-activated nuclei in a tissue. The proportion Q can be estimated from simple functions of intake rates or blood growth factor concentrations. Estimates of Y are derived from histological considerations or calculated from experimental observations; Y=1 for multinucleated skeletal muscle fibres and Y=1/3, 1/2, 1/6 on average for mononucleated cell tissues, skin or bone and viscera, respectively. To apply kP to the whole body, an average value of Y=1/2 can be taken. Experimental observations on tissue protein synthesis and breakdown rates yield direct estimates of kP in satisfactory agreement with comparable theoretical predictions.