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Cysteine kinetics and oxidation at different intakes of methionine and cystine in young adults^1,2,3

¹ From the Laboratory of Human Nutrition, School of Science and Clinical Research Center, Massachusetts Institute of Technology, Cambridge, MA, and Shriners Burns Hospital, Boston.

Background: We previously studied methionine kinetics and oxidation with the tracer L-[1-¹³C, methyl-²H₃]methionine.

Objectives: We sought to explore methionine-cysteine interrelations in adults by using L-[1-¹³C]cysteine under different dietary conditions.

Design: In experiment 1, 12 adults consumed a protein-free diet for 6 d. On day 7, methionine (n = 6) or cysteine (n = 6) oxidation rates were measured during an 8-h continuous infusion of L-[1-¹³C, methyl-²H₃]methionine or L-[1-¹³C]cysteine, respectively. In experiment 2, 6 young men consumed 3 diets for 6 d each before a tracer study on day 7 with L-[1-¹³C]cysteine. The amounts (in mg•kg^-1•d^-1) of methionine and cysteine, respectively, were: high-methionine (HM) diet, 13 and 0; low-methionine (LM) diet, 6.5 and 0; and methionine-plus-cystine (MC) diet, 6.5 and 5.6. Cysteine flux and oxidation rates were determined and sulfur amino acid (SAA, methionine plus cysteine) balances were estimated.

Results: In experiment 1, rates of methionine and cysteine oxidation were similar to losses predicted from obligatory nitrogen losses. In experiment 2, SAA balance was less negative when subjects consumed the HM diet than the LM and MC diets (interaction, P = 0.034), largely because of a difference in fed-state balance (HM compared with LM, P < 0.01; HM compared with MC, P < 0.05). There was no evidence of a sparing effect of dietary cystine on the methionine requirement.

Conclusion: These studies support use of [1-¹³C]cysteine for studying whole-body SAA oxidation and conclusions that maintenance of SAA balance is best achieved by supplying methionine at approximately the FAO/WHO/UNU recommendations for total SAA intake (13 mg•kg^-1•d^-1).

Key Words: Cysteine oxidation • cysteine kinetics • sulfur amino acid metabolism • obligatory oxidative losses • obligatory nitrogen losses • young adults • methionine • cystine

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