Methionine Transamination and Catabolism in Vitamin B-6 Deficient Rats

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Abstract
Methionine (Met) transamination and oxidation of the Met or α-keto-γ-methiolbutyrate (αKγMB) carboxyl and methyl carbons were studied in weanling rats fed control or vitamin B6 deficient diets containing 18 or 60% casein for a period of 21 days. In homogenates prepared from the livers of B6 deficient rats, transamination of Met was reduced to 8 to 16% of controls. Transamination was partially restored by addition of pyridoxal phosphate (PALP) to the homogenates. Vitamin B6 deficiency resulted in a 61 to 76% reduction in oxidation of the Met carboxyl carbon which was also partially restored by the addition or PALP. The high protein diet stimulated transamination of Met and oxidation of the carboxyl carbons of both Met and αKγMB. Oxidation of the methyl carbon was relatively unaffected by either B6 deficiency or high protein diet. In vivo, the oxidation of the Met carboxyl carbon was not influenced by either B6 deficiency or high protein diet, however, oxidation of the methyl carbon increased 32 to 53% in rats fed the B6 deficient diets. It appears that transamination was not rate limiting in vivo, even with a B6 deficiency, however, in vitro with greater substrate loading transamination may become rate limiting for oxidation of the carboxyl carbon of Met. In vivo the B6 deficiency may have resulted in a shift of Met metabolism from the transsulfuration pathway, which conserves the methyl carbon and partially conserves the carboxyl carbon depending on the fraction of homocysteine remethylated, to the transaminative pathway which results in extensive oxidation of both carbons.