Kr. Short et al., Whole body protein kinetics using Phe and Tyr tracers: an evaluation of the accuracy of approximated flux values, AM J P-ENDO, 39(6), 1999, pp. E1194-E1200
Citations number
18
Categorie Soggetti
Endocrinology, Nutrition & Metabolism
Journal title
AMERICAN JOURNAL OF PHYSIOLOGY-ENDOCRINOLOGY AND METABOLISM
Phenylalanine (Phe) kinetics are increasingly used in studies of amino acid
kinetics, because the metabolic fate of Phe is limited to incorporation in
to 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 me
asure 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 bo
dy proteins, that eliminate the need for a Tyr tracer. To evaluate the accu
racy of this approach, data were obtained from 12 type I diabetic patients
and 24 nondiabetic control subjects who were studied with the full compleme
nt of tracers both with and without insulin infusion. S-p approximations cl
osely matched measured values in both groups (mean difference <2%, all valu
es <5%), but the agreement was poor for Q(pt) terror range = -8 to +43%) an
d Q(t) terror range -22 to + 41%). Insulin status had no effect on these co
mparisons. The lower approximation error for S-p vs. Q(pt) is due to the sm
all contribution (similar to 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 Ph
e coming from protein breakdown, (Q(t) - Q(pt))/Q(p). Ideally, all fluxes s
hould 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).