Estimation of whole body protein turnover in growing steers

Abstract

Lysine and threonine turnover was measured in two growing Angus steers by infusion of L-4,5-('3)H-lysine and L-G-('3)H-threonine for 12 hours. Deuterium oxide (75 ml) was injected at the beginning of the infusion period to determine empty body water, protein and mass, while measuring water flux from the plasma pool. Measurements were made of irreversible losses of amino acids from the whole body, and calculations were made to determine protein flux. Protein synthesis rates, calculated from lysine and threonine plasma levels, averaged 353 g/day. Mean daily irreversible losses of amino acids catabolized for energy were 24.5 m moles of lysin and 4.7 m moles of threonine. Mean empty body water volume was 165.9 liters, and mean empty body protein and lean body weights were 50.0 kg and 227.4 kg, respectively. Turnover rate of total body protein was estimated to be (.353/50) x 100, or .71%/day. Nitrogen balance was determined in four growing Angus steers to determine net crude protein retention in whole animal bodies. Urinary 3-methylhistidine and creatinine were analyzed to quantitate protein breakdown. Total protein synthesis was calculated from net synthesis and protein breakdown. ...