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Effects of Cystine Deficiency and Trypsin Inhibitor on the Metabolism of Methionine^1,2,

Department of Biochemistry, Vanderbilt University Medical School, Nashville, Tennessee

While seeking methods capable of determining the cystine nutriture of animals, it was found that the addition of L-cystine to diets made with purified soybean protein resulted in an increased production of ¹⁴CO₂ from labeled methionine when compared with animals not receiving cystine. A similar enhancement of CO₂ production occurred if unheated soybean meal was fed, rather than heated meal, or if purified soybean trypsin inhibitor was given orally to rats maintained with either the heated or unheated meal. It is proposed that both cystine and trypsin inhibitor diminish the conversion of methionine to cysteine through the cystathionine pathway. Two mechanisms of soybean trypsin inhibitor action are proposed, one an interference with the incorporation of cystine into protein which increases the body pool of cystine which represses cystathionine synthase and the other, an interference of soybean trypsin inhibitor directly with cystathionine synthase.

² A portion of this work was presented by A. B. Frost as partial fulfillment of the requirements for the degree of Doctor of Philosophy at Vanderbilt University.

³ Career Investigator, National Heart Institute, National Institutes of Health, K56-HE8288.

Manuscript received 15 November 1965.