Nephrotic proteinuria has no net effect on total body protein synthesis: Measurements with 13C valine

Abstract

In nephrotic syndrome, significant amounts of plasma proteins, mostly of hepatic origin, are lost in urine. Total hepatic protein synthesis increases, suggesting that other protein pools must be conserved to maintain steady state. This can be accomplished either by decreased amino acid oxidation or decreased protein synthesis in other organs to replace lost liver-derived proteins. To determine the effect of nephrotic syndrome on total-body protein metabolism, we compared whole-body valine use in seven nephrotic patients and five controls using a primed continuous infusion of [1-13C]-valine, with additional priming of NaH13CO3 . Plasma [13C]-valine, 13C α ketoisovaleric acid, and the expired 13CO2 enrichments were used to assess whole-body valine flux, valine oxidation, and nonoxidative valine disposal (NOVD). The valine flux into the blood compartment (97.7 ± 3.0 versus 95.3 ± 3.3 μmol/kg/h), oxidation of valine (19.4 ± 1.9 versus 21.2 ± 2.8 μmol/kg/h), and NOVD (78.3 ± 2.5 versus 74.2 ± 2.7 μmol/kg/h) were not statistically different in patients compared with controls. Valine oxidation correlated positively with urinary urea excretion (r = 0.70; P = 0.01) in all subjects. Compared with control subjects who have similar urinary urea excretion, nephrotic subjects do not compensate for urinary loss of protein by decreased amino acid oxidation or decreased nonoxidative valine disposal. Previous studies have shown that synthesis of several hepatic proteins increases when subjects are fed the same dietary regime, whereas the present study shows that total-body protein synthesis does not increase. This would imply reduced synthesis of nonhepatic protein pools.