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Formation of Methionine from -Amino-n-Butyric Acid and 5'-Methylthioadenosine in the Rat¹

School of Human Ecology, Howard University, Washington, D.C., and Departments of Home Economics and Chemistry, North Carolina Agricultural and Technical State University, Greensboro, North Carolina

Adult rats may utilize two metabolites of methionine for the biosynthesis of this essential amino acid. In separate experiments methionine, labeled with ¹⁴C or with ³⁵S was observed in plasma and urine following the administration of [2-¹⁴C]--amino-n-butyric acid or [³⁵S]-5'-methylthioadenosine by stomach tube. Although -amino-n-butyric acid (ABA) or homoserine, alone or with dietary sodium sulfate, choline, and/or S-methylcysteine, was not utilized for growth, weight loss in weanling rats was decreased by dietary cysteine when fed as an additive to a basal methionine-free, cysteine-free, labile methyl-free, sulfur-free diet. Following the addition of 10 mg ABA and 28 mg 5'-methylthioadenosine/day to the basal diet, growth response was equivalent to that occurring in rats receiving 27 mg of methionine/day with the basal diet. The implications of these findings for adaptation to protein restriction and a discussion of equilibrium and steady state conditions related to the increase in methionine content in the blood are presented.

KEY WORDS: • methionine • methionine biosynthesis • methionine metabolism • -aminobutyric acid • 5'-methylthioadenosine • growth

¹ Supported by Grant AM13804-02 from the National Institutes of Health.

² Present address, Howard University, Washington, D.C.

³ Submitted in partial fulfillment of the requirements for the Master of Science degree in Food and Nutrition, North Carolina Agricultural and Technical State University.

⁴ Present address, Bennett College, Greensboro, North Carolina.

⁵ Present address, North Carolina Agricultural and Technical State University, Greensboro, North Carolina.

⁶ Present address, National Science Foundation, Washington, D.C.

Manuscript received 26 July 1976.