METHIONINE AND CYSTEINE KINETICS AT DIFFERENT INTAKES OF CYSTINE IN HEALTHY ADULT MEN

We investigated plasma methionine and cys cysteine kinetics in eight h ealthy adult men receiving for 6 d each of five L-amino acid diets sup plying 13 mg methionine.kg(-1).d(-1) without cystine or 6.5 mg methion ine.kg(-1).d(-1) plus 0, 5.2, 10.5, or 20.9 mg cystine.kg(-1).d(-1). O n the morning of day 7, primed, constant intravenous infusions of L-[H -2(3)-methyl, 1-C-13]methionine and L-[3,3-H-2]cysteine were given for 8 h (for the first 3 h subjects remained in a fasted state and for th e next 5 h received small, equal meals at hourly intervals to achieve a fed state). Methionine and cysteine fluxes and rate of methionine ox idation were estimated from plasma methionine and cysteine labeling an d C-13 in expired air. Methionine oxidation declined (P < 0.05) with l owered methionine intake. Cysteine flux was similar across diets and d ietary 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 t hese amino acids within the splanchnic region.