Methionine Transamination and Catabolism in Vitamin B-6 Deficient Rats

Abstract

G. BELI VEAU EVERETT,2 A. D. MITCHELL 3ANDN. J. BENEVENGADepartments of Nutritional Sciences and Meat andAnimal Science, University of Wisconsin,Madison, Wisconsin 53706ABSTRACT Methionine (Met) transamination and oxidation of theMet or a-keto-y-methiolbutyrate (aKyMB) carboxyl and methyl carbonswere studied in weanling rats fed control or vitamin B8 deficient diets containing 18 or 60% casein for a period of 21 days. In homogenates preparedfrom the livers of Be deficient rats, transamination of Met was reduced to8 to 16% of controls. Transamination was partially restored by addition ofpyridoxal phosphate (PALP) to the homogenates. Vitamin B6 deficiencyresulted in a 61 to 76% reduction in oxidation of the Met carboxyl carbonwhich was also partially restored by the addition or PALP. The high proteindiet stimulated transamination of Met and oxidation of the carboxyl carbonsof both Met and aKyMB. Oxidation of the methyl carbon was relativelyunaffected by either B6 deficiency or high protein diet. In vivo, the oxidation of the Met carboxyl carbon was not influenced by either B8 deficiencyor high protein diet, however, oxidation of the methyl carbon increased 32to 53% in rats fed the B6 deficient diets. It appears that transamination wasnot rate limiting in vivo, even with a B6 deficiency, however, in vitro withgreater substrate loading transamination may become rate limiting foroxidation of the carboxyl carbon of Met. In vivo the B8 deficiency may haveresulted in a shift of Met metabolism from the transsulfuration pathway,which conserves the methyl carbon and partially conserves the carboxylcarbon depending on the fraction of homocysteine remethylated, to thetransaminatA¬vepathway which results in extensive oxidation of both carbons. J. Nutr. 109: 597-605, 1979.INDEXING KEY WORDS methionine •transamination •pyri-doxine •o-keto-y-methiolbutyrate •ratPyridoxal-5'-phosphate (PALP) functions in rats fed an 18% casein diet deficient inas a coenzyme for a number of enzymes in- pyridoxine. This was overcome by additionvolved in L-methionine (Met) and L-cys- of 0.6 mg of pyridoxine •HC1 per kg of diet.teine metabolism. These include cystathio- Much attention has centered on the trans-nine /3-synthase (EC 4.2.1.22), cystathionase sulfuration pathway (fig. 1) of methionine(EC 4.4.1.1), cysteine sulfinic acid trans-