Fig. 1.

Model of protein kinetics. A: model of protein synthesis and breakdown. The mass of any protein pool (P) is defined by the rate of protein accretion by synthesis (S) and loss by breakdown processes (B). At tissue protein steady state, P is constant and S and B rates are equal. B: the exponential rise of tracer enrichment over time obtained by incorporation into a protein pool during constant infusion of tracer. The fractional synthesis rate (FSR; k_S) can be assessed by determining the initial rate of tracer enrichment increase in protein (ΔE) during a short period of time (Δt), where the precursor tracer enrichment is steady and equal to E_∞ (enrichment at infinity). Circle shows tracer incorporated into the protein pool (P*). C: time course of disappearance of tracer from the protein pool after input of tracer is stopped at various time points. The fractional breakdown rate (FBR; k_B) of a prelabeled protein pool can be calculated by measuring the rate of tracer disappearance from the protein pool (ΔE) over Δt. It is assumed that no released tracer is reincorporated into the protein.