EFFECT OF CYSTINE INTAKE ON METHIONINE KINETICS AND OXIDATION DETERMINED WITH ORAL TRACERS OF METHIONINE AND CYSTEINE IN HEALTHY-ADULTS

There is evidence based on nitrogen balance that dietary cystine spare s, from similar to 16% to 89%, the total methionine requirement. In a previous study we did not detect, by tracer techniques, a sparing effe ct of cystine when the diet provided methionine at a limiting intake ( requirement level: 13 mg.kg(-1).d(-1)). One reason could be that we us ed an intravenous infusion of the tracer, which may not, therefore, ha ve labeled the carbon dioxide derived from the splanchnic oxidation of dietary methionine. The aim of this study was to compare methionine m etabolism and oxidation in eight healthy adults given for 6 d each of three different diets: 13 mg (87.0 mu mol) methionine.kg(-1).d(-1) and no cystine (diet A); 5 mg (33.5 mu mol) methionine.kg(-1).d(-1) and n o cystine (diet B); and 5 mg (33.5 mu mol) methionine.kg(-1).d(-1) and 6.5 mg (52.4 mu mol) cystine.kg(-1).d(-1) (diet C). On day 7, tracers ([1-C-13, methyl-H-2(3)]methionine and [H-2(2)]cysteine) were adminis tered orally at 30-min intervals for 8 h. Blood and breath samples wer e obtained for analysis during 3-h fasting and consecutive 5-h feeding periods. During fasting, me thionine oxidation and methionine methyl (Q(m)) and carboxyl (Q(c)) fluxes and plasma concentrations were not a ffected by the amount of sulfur amino acids in the three diets. In the fed state methionine oxidation was significantly lower during diets B (3.0 +/- 0.5 mu mol.kg(-1).h(-1)) and C (2.8 +/- 0.6 mu mol.kg(-1).h( -1)) than during diet A (4.1 +/- 0.9 mu mol.kg(-1).h(-1)); there were no significant differences between diets B and C. Q(m) and Q(c) decrea sed with decreased methionine intake but no effect was observed by add ing cystine. Cysteine flux (Q(cys)) was not affected by diet compositi on but it was lower during feeding than during fasting. In conclusion, replacing approximate to 60% of the total requirement for methionine with cystine over a short diet period did not result in a detectable s paring of methionine oxidation.