Methionine and cysteine kinetics at different intakes of cystine in healthy adult men.
We investigated plasma methionine and cysteine kinetics in eight healthy adult men receiving for 6 d each of five L-amino acid diets supplying 13 mg methionine.kg-1.d-1 without cystine or 6.5 mg methionine.kg-1.d-1 plus 0, 5.2, 10.5, or 20.9 mg cystine.kg-1.d-1. On the morning of day 7, primed, constant intravenous infusions of L-[2H3-methyl, 1-13C]methionine and L-[3,3-2H]cysteine were given for 8 h (for the first 3 h subjects remained in a fasted state and for the next 5 h received small, equal meals at hourly intervals to achieve a fed state). Methionine and cysteine fluxes and rate of methionine oxidation were estimated from plasma methionine and cysteine labeling and 13C in expired air. Methionine oxidation declined (P < 0.05) with lowered methionine intake. Cysteine flux was similar across diets and dietary cystine did not affect tracer methionine oxidation. If there is a sparing effect of dietary cystine on the methionine requirement in adults, it probably takes place during the "first-pass" removal of these amino acids within the splanchnic region.
Hiramatsu
T
T
Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge 02139.
Fukagawa
N K
NK
Marchini
J S
JS
Cortiella
J
J
Yu
Y M
YM
Chapman
T E
TE
Young
V R
VR
D000328:Adult; D002247:Carbon Isotopes; D003545:Cysteine / Q000097:blood*; D003553:Cystine / Q000008:administration & dosage*; D003903:Deuterium; D004044:Dietary Proteins / Q000008:administration & dosage*; D006801:Humans; D007262:Infusions, Intravenous; D007700:Kinetics; D008297:Male; D008715:Methionine / Q000008:administration & dosage / Q000097:blood*; D010084:Oxidation-Reduction
D002247:Carbon Isotopes; D004044:Dietary Proteins; D003553:Cystine; D008715:Methionine; D003903:Deuterium; D003545:Cysteine