1. The fractional synthesis rate of protein is commonly measured by either the constant infusion method or the flooding dose method. The two methods often give different results.

2. An underlying assumption of the traditional flooding dose formula is that the protein synthesis rate is not stimulated by the flooding dose. A new formula for calculation of the fractional synthesis rate is derived with the alternative assumption that the protein synthesis rate is stimulated by an amount proportional to the change in the intracellular concentration of the infused amino acid. The alternative formula is:

where EB and EF are the enrichments of bound and free amino acid, respectively (atom per cent excess), and C=1-(EF/EI), where EI is the enrichment of the infusate. This approach defines the lowest possible value for the fractional synthesis rate. The traditional equation gives a maximal value for the fractional synthesis rate.

3. When data from the literature are considered, the fractional synthesis rate of muscle protein as calculated by the constant infusion technique falls between the values of fractional synthesis rate calculated by the two flooding dose formulae when leucine is the tracer, suggesting that a flooding dose of leucine exerts a stimulatory effect on the rate of protein synthesis, but that the increase is not as great as the increase in the intracellular concentration of leucine.

4. The precision of the formula for the calculation of fractional synthesis rate is limited by the accuracy of the underlying assumptions regarding the effect of the flooding dose on the fractional synthesis rate. At present, the best approach would appear to be the use of both equations to calculate the upper and lower bounds of the true fractional synthesis rate.

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