A study of L-(14C)cystathionine metabolism in the brain, kidney, and liver of pyridoxine-deficient rats

Plasma glutathione and cystathionine concentrations are elevated but cysteine flux is unchanged by dietary vitamin B-6 restriction in young men and women

Cysteine synthesis from homocysteine is catalyzed by two pyridoxal 5'-phosphate (PLP)-dependent enzymes. This suggests that vitamin B-6 status might affect cysteine and glutathione homeostasis, but it is unclear whether this occurs in humans. We assessed the effects of vitamin B-6 status on static and kinetic parameters of cysteine and glutathione metabolism in healthy female (n=5) and male (n=4) volunteers (20-30 y) before and after 4 wk of dietary vitamin B-6 restriction (<0.5 mg vitamin B-6/d). Rates of reactions related to cysteine metabolism were measured from blood sampled during primed, constant infusions of [(13)C(5)]methionine, [3-(13)C]serine, and [(2)H(2)]cysteine that were conducted after an overnight fast at baseline and after the dietary protocol. Vitamin B-6 restriction reduced the concentration of PLP (55.1+/- 8.3 vs. 22.6+/-1.3 nmol/L; P=0.004) and increased concentrations of cystathionine (124%; P<0.001) and total glutathione (38%; P<0.008) in plasma. Concentrations of plasma homocysteine, cysteine, cysteinylglycine, and C-reactive protein (an indicator of systemic inflammation) were not affected by dietary vitamin B-6 restriction. The rate of cysteine synthesis via transsulfuration was below detection limits in this protocol. Neither the fractional synthesis rate of cystathionine nor whole-body cysteine flux was affected by vitamin B-6 restriction. These data indicate that glutathione homeostasis is altered by dietary vitamin B-6 deficiency and appears to be unrelated to cysteine flux under conditions of minimal amino acid intake as evaluated in this study.

Metabolism of NG,NG-and NG,N'G-dimethylarginine in rats

The metabolic fates of NG,NG-and NG,N'G-dimethylarginines in rats were investigated isotopically and novel metabolites, alpha-keto-delta-(N,N-dimethylguanidino)-and alpha-keto-delta-(N,N'-dimethylguanidino)valeric acids and gamma-(N,N-dimethylguanidino)-and gamma-(N,N'-dimethylguanidino)butyric acids were identified. In the case of the rats injected with NG,NG-dimethyl-L-[1,2,3,4,5-14C]arginine, about 13% of the radioactivity was recovered in the first 12-h urine and was distributed in the following metabolites (relative ratios): unchanged NG,NG-dimethyl-L-arginine (35.2%), gamma-(N,N-dimethylguanidino)butyric acid (18.4%), alpha-keto-delta-(N,N-dimethylguanidino)valeric acid (16.4%), and N alpha-acetyl-NG,NG-dimethyl-L-arginine (8.5%). The radioactivity retained in the tissues was found mainly in citrulline and was further distributed in ornithine, arginine, and glutamic acid and even in protein-bound arginine. In the case of NG,N'G-dimethyl-L-[1,2,3,4,5-14C]arginine-injected rats, about 75% of the radioactivity was excreted in the first 12-h urine and was recovered in the following metabolites (relative ratios): N alpha-acetyl-NG,N'G-dimethyl-L-arginine (48.4%), unchanged NG,N'G-dimethyl-L-arginine (23.7%), alpha-keto-delta-(N,N'-dimethylguanidino)valeric acid (20.2%), and gamma-(N,N'-dimethylguanidino)butyric acid (9.6%). In the tissues, most of the radioactivity was associated with unchanged NG,N'G-dimethyl-L-arginine. These findings show that both dimethylarginines are metabolized by a pathway forming the corresponding alpha-ketoacid analogs and the oxidatively decarboxylated products of the alpha-ketoacids in addition to the N alpha-acetyl conjugates identified previously (K. Sasaoka, T. Ogawa, and M. Kimoto (1982) Arch. Biochem. Biophys. 219, 454-458), and NG,NG-dimethyl-L-arginine is catabolized by an additional pathway leading to the formation of citrulline and its metabolically related amino acids. By considering their catabolism, an attempt to use urinary dimethylarginines as an index of in vivo breakdown of tissue proteins is invalid at least in rats.

Interrelationship between dietary pyridoxine and free plasma amino acids in chicks

The interrelationships between dietary pyridoxine and free plasma amino acids in young chicks were studied with two different diets. Chicks fed isolated soybean protein supplemented with less than 1.0 mg pyridoxine-HCl/kg diet had reduced body weight gain and feed consumption and a higher mortality rate (19%) than those fed 1 or 5 mg pyridoxine-HCl/kg. A casein diet required greater supplementation (2.0 mg pyridoxine/kg) to prevent reduced body weight gain and depressed feed intake. The concentrations of free plasma taurine, threonine, and serine were significantly lower in chicks fed the isolated soybean protein diet supplemented with less than 1.0 mg pyridoxine-HCl/kg, while those of isoleucine, valine, glycine, and ornithine were significantly higher than in chicks fed greater levels of pyridoxine. Supplementation of casein diets with less than 2.0 mg resulted in a significant accumulation of threonine, glycine, cystathionine, and ornithine. Valine was significant higher in chicks fed casein supplemented with only .6 mg pyridoxine-HCl/kg. Feeding isolated soybean protein or casein diets without any pyridoxine supplementation resulted in the dealth of 95% and 100% of the chicks, respectively. Pair feeding a group fed the isolated soybean protein diet supplemented with 1.0 mg to a group receiving .5 mg pyridoxine-HCl/kg resulted in a similar body weight gain and concentration of amino acids in the plasma.

Cysteine metabolism in vivo of vitamin B6-deficient rats

The expirations of 14CO2 from DL-[1-14C]-, DL-[3-14C]- and L-[U-14C] cysteine used as isotopic tracers were estimated in order to determine the in vivo metabolic distribution of L-cysteine in pyridoxine deficient rats. The expired 14CO2 from L-[U-14C] cysteine was increased by pyridoxine deficiency. The loading of non-physiological dose of L-cysteine resulted in remarkable increase in the expiration of 14CO2 from each tracer in deficient rats as well as in controls. The in vivo metabolic distributions of L-cysteine were calculated from the expired 14CO2 from these isotopic tracers. The in vivo metabolic distribution of L-cysteine calculated showed that the remarkable lesion in taurine pathway occurred in pyridoxine deficient rats, and when non-physiological dose of L-cysteine was loaded the catabolism of L-cysteine of controls was markedly increased in either pyruvate or taurine pathway, whereas the L-cysteine catabolism in deficient rats was increased only in pyruvate but not in taurine pathway. The urinary excretions of 35S-labeled metabolites such as sulfate or taurine were also examined in deficient and control rats.