Branched-Chain Amino Acids: Metabolism, Physiological Function, and Application Transamination of Leucine and Nitrogen Accretion in Human Pregnancy and the Newborn Infant

Kinetics of leucine and its oxidation were determined in human pregnancy and in the newborn infant, using stable isotopic tracers, to quantify the dynamic aspects of protein metabolism. These data show that in human pregnancy there is a decrease in whole-body rate of leucine turnover compared with nonpregnant women. In addition, data in newborn infants show that leucine turnover expressed as per kg body weight is higher compared with adults. The administering of nutrients resulted in a suppression of the whole-body rate of proteolysis. Because nonessential amino nitrogen is an important component of nutritional nitrogen and can be limiting for growth under certain circumstances, and because BCAA are an important source of nonessential amino nitrogen, we have examined the relations among the transamination of leucine, leucine N kinetics, and urea synthesis and glutamine kinetics in human pregnancy and newborn infants. In human pregnancy, early in gestation, there is a significant decrease in urea synthesis in association with a decrease in the rate of transamination of leucine. A linear correlation was evident between the rate of leucine reamination and urea synthesis during fasting in pregnant and nonpregnant women. In healthy-term newborn and growing infants, although the reamination of leucine was positively related to glutamine flux, leucine reamination was negatively related to urea synthesis, suggesting a redirection of amino N toward protein accretion. The regulatory mechanism involved in this redirection of nitrogen from irreversible loss to accretion remains under investigation. J. Nutr. 136: 281S–287S, 2006.