Tyrosine turnover and protein synthesis during parenteral nutrition

Three patients receiving total parenteral nutrition and one healthy subject receiving 5% glucose were infused for 10 hours with L-(U14C)tyrosine (1.5 microCi/hr). Two alternative assumptions were made regarding the fate of labeled carbon. Total and oxidative fluxes of tyrosine were calculated and these were used to determine rates of protein synthesis and breakdown. Patients showed positive net protein synthesis and oxidation of infused phenylalanine via tyrosine was shown not to be excessive.