Phenylalanine (Phe) kinetics are increasingly used in studies of amino acid kinetics, because the metabolic fate of Phe is limited to incorporation into protein (protein synthesis, S_p) and catabolism via hydroxylation (Q_pt) to tyrosine (Tyr). Besides an infusion of labeled Phe to measure Phe flux (Q_p), a priming dose of Tyr and an independent Tyr tracer are used to measure Tyr flux (Q_t) and Q_pt. Alternatively, Q_t, Q_pt, and S_p can be approximated by using equations, based on Phe and Tyr concentrations in body proteins, that eliminate the need for a Tyr tracer. To evaluate the accuracy of this approach, data were obtained from 12 type I diabetic patients and 24 nondiabetic control subjects who were studied with the full complement of tracers both with and without insulin infusion. S_p approximations closely matched measured values in both groups (mean difference <2%, all values <5%), but the agreement was poor for Q_pt (error range = 8 to +43%) and Q_t (error range 22 to +41%). Insulin status had no effect on these comparisons. The lower approximation error for S_p vs. Q_pt is due to the small contribution (~10%) of Q_pt to Q_p. Approximation error for Q_pt (r > 0.99) can be explained by variability in the ratio of Tyr to Phe coming from protein breakdown, (Q_t  Q_pt)/Q_p. Ideally, all fluxes should be directly measured, but these data suggest that whole body S_p can be approximated with an acceptably small margin of error. However, the same equations do not yield reliably accurate values for Q_pt or Q_t.

protein metabolism; stable isotopes

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