Methionine and cysteine kinetics at different intakes of cystine in healthy adult men

Open Access
Publisher Website
Google Scholar
Abstract
We investigated plasma methionine and cysteine kinetics in eight healthy adult men receiving for 6 d each of rive 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-l3C]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.