Vitamin B-6 deficiency in rats reduces hepatic serine hydroxymethyltransferase and cystathionine beta-synthase activities and rates of in vivo protein turnover, homocysteine remethylation and transsulfuration.
Vitamin B-6 deficiency causes mild elevation in plasma homocysteine, but the mechanism has not been clearly established. Serine is a substrate in one-carbon metabolism and in the transsulfuration pathway of homocysteine catabolism, and pyridoxal phosphate (PLP) plays a key role as coenzyme for serine hydroxymethyltransferase (SHMT) and enzymes of transsulfuration. In this study we used [(2)H(3)]serine as a primary tracer to examine the remethylation pathway in adequately nourished and vitamin B-6-deficient rats [7 and 0.1 mg pyridoxine (PN)/kg diet]. [(2)H(3)]Leucine and [1-(13)C]methionine were also used to examine turnover of protein and methionine pools, respectively. All tracers were injected intraperitoneally as a bolus dose, and then rats were killed (n = 4/time point) after 30, 60 and 120 min. Rats fed the low-PN diet had significantly lower growth and plasma and liver PLP concentrations, reduced liver SHMT activity, greater plasma and liver total homocysteine concentration, and reduced liver S-adenosylmethionine concentration. Hepatic and whole body protein turnover were reduced in vitamin B-6-deficient rats as evidenced by greater isotopic enrichment of [(2)H(3)]leucine. Hepatic [(2)H(2)]methionine production from [(2)H(3)]serine via cytosolic SHMT and the remethylation pathway was reduced by 80.6% in vitamin B-6 deficiency. The deficiency did not significantly reduce hepatic cystathionine-beta-synthase activity, and in vivo hepatic transsulfuration flux shown by production of [(2)H(3)]cysteine from the [(2)H(3)]serine increased over twofold. In contrast, plasma appearance of [(2)H(3)]cysteine was decreased by 89% in vitamin B-6 deficiency. The rate of hepatic homocysteine production shown by the ratio of [1-(13)C]homocysteine/[1-(13)C]methionine areas under enrichment vs. time curves was not affected by vitamin B-6 deficiency. Overall, these results indicate that vitamin B-6 deficiency substantially affects one-carbon metabolism by impairing both methyl group production for homocysteine remethylation and flux through whole-body transsulfuration.
Martinez
M
M
Food Science and Human Nutrition Department, University of Florida, Gainesville, FL 32611-0370, USA.
Cuskelly
G J
GJ
Williamson
J
J
Toth
J P
JP
Gregory
J F
JF
D000818:Animals; D002851:Chromatography, High Pressure Liquid; D003541:Cystathionine beta-Synthase / Q000378:metabolism*; D004032:Diet; D012696:Glycine Hydroxymethyltransferase / Q000378:metabolism*; D006710:Homocysteine / Q000097:blood / Q000378:metabolism*; D008099:Liver / Q000201:enzymology*; D008297:Male; D008745:Methylation; D011732:Pyridoxal Phosphate / Q000097:blood; D051381:Rats; D017207:Rats, Sprague-Dawley; D012694:Serine / Q000378:metabolism*; D013466:Sulfurtransferases / Q000378:metabolism; D026681:Vitamin B 6 Deficiency / Q000201:enzymology / Q000378:metabolism*
D006710:Homocysteine; D012694:Serine; D011732:Pyridoxal Phosphate; D012696:Glycine Hydroxymethyltransferase; D013466:Sulfurtransferases; D003541:Cystathionine beta-Synthase