Theoretical analysis of the importance of recycling in measurements of protein turnover by constant infusion of a labelled amino acid.

In studies of whole body protein turnover, recycling of tracer from the breakdown of labelled protein is usually neglected; this neglect may introduce a significant error. A three-pool model with fast and slowly turning over protein pools has been used to calculate recycling rates over a range of sizes and turnover rates of the protein pools. Complete and approximate solutions of the equations are given. The recycling rate of 1% per hour would fit the available data on the turnover rates of human tissue proteins.