title: Effects of [15N]leucine infused at low rates on leucine metabolism in humans.
abstract: The present studies were carried out to determine whether infusions of [15N]leucine at low rates affect estimates of leucine oxidation and of proteolysis and protein synthesis in humans. Three groups of normal subjects were infused for 3 h with either [15N]leucine at a rate of 0.16 or 0.26 mumol X kg-1 X min-1 or saline using [2H3]leucine and alpha-[14C]ketoisocaproate as isotopic tracers of leucine metabolism. Data were analyzed at steady state using both single- and dual-isotope models. Preliminary studies were carried out to characterize the dual-isotope model in humans using infusions of [3H]leucine and alpha-[14C]ketoisocaproate. In the postabsorptive state estimates of leucine appearance, disappearance, and oxidation derived from the two isotope models were in good agreement. Infusion of stable isotope up to approximately 10% of the leucine carbon flux do not have a significant effect on leucine metabolism, but the data derived from such studies must be properly controlled and interpreted with care because these tracers are not massless.
mesh_terms: D001944:Breath Tests; D002250:Carbon Radioisotopes; D003903:Deuterium; D005260:Female; D006801:Humans; D007651:Keto Acids / Q000097:blood; D007930:Leucine / Q000097:blood / Q000378:metabolism / Q000494:pharmacology*; D008297:Male; D008954:Models, Biological; D009587:Nitrogen Isotopes; D010084:Oxidation-Reduction; D011506:Proteins / Q000378:metabolism*; D014316:Tritium
chemical_list: D002250:Carbon Radioisotopes; D007651:Keto Acids; D009587:Nitrogen Isotopes; D011506:Proteins; D014316:Tritium; C013082:alpha-ketoisocaproic acid; D003903:Deuterium; D007930:Leucine