Vitamin B-6 Deficiency in Rats Reduces Hepatic Serine Hydroxymethyltransferase and Cystathionine β-Synthase Activities and Rates of In Vivo Protein Turnover, Homocysteine Remethylation and Transsulfuration

Abstract

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.

Impairments in pyridoxine-dependent sulphur amino acid metabolism are highly sensitive to the degree of vitamin B6 deficiency and repletion in the pig

“…Additionally, decrease in plasma PLP may not have paralleled decrease in hepatic PLP stores, offering an additional potential explanation for the observed delay. The hyperhomocysteinemia induced by vitamin B 6 deficiency in this study is consistent with other reports in pigs (Smolin et al, 1983) and rats (Smolin and Benevenga, 1982;Smolin and Benevenga, 1984;Martinez et al, 2000), but not in healthy humans (Davis et al, 2005) with vitamin B 6 restriction treatment. The discrepancy with the latter observation likely reflects the severity of vitamin B 6 restriction; pigs with depressed plasma PLP concentrations are able to maintain plasma homocysteine concentrations within normal limits, however, this is limited to the first 2 weeks of depletion.…”

“…The patterns of response in plasma PLP to pyridoxine repletion reflect both dose and time dependency. Martinez et al (2000) provided evidence of a dose dependency of plasma PLP to increasing dietary vitamin B 6 from 0 to 2 mg/kg diet in rats. In the current study, the fitting of the plasma PLP data to a logistics function yielded further information with respect to temporal changes in this marker of vitamin B 6 status.…”

“…Further reductions in plasma PLP concentrations were observed during subsequent weeks, with a nadir of 4-6 nmol/l reached by the end of the third week. Reduced plasma PLP concentrations induced by vitamin B 6 restriction have been reported in humans (Davis et al, 2005) and rats (Martinez et al, 2000;Scheer et al, 2005) but data in pigs are limited. In the experiment examining vitamin B 6 repletion, plasma PLP at the beginning of the depletion phase was 24.9 nmol/l and declined to 3.2 nmol/l, consistent with observations from Experiment 1.…”

“…Results from a previous study in pigs (Kirchgessner and Kö sters, 1977) documented the fact that vitamin B 6 -deficient pigs retained less nitrogen than their pair-fed controls. However, to date, little research (Martinez et al, 2000) has been performed to examine the impact of vitamin B 6 deficiency on protein kinetics, despite the recognized importance of this vitamin in protein and amino acid metabolism. An inhibition of transmethylation in vitamin B 6 -deficient animals can also contribute to increased methionine concentrations, although previous researchers did not observe reductions in transmethylation rates in vitamin B 6 -deficient rats (Martinez et al, 2000).…”

“…However, to date, little research (Martinez et al, 2000) has been performed to examine the impact of vitamin B 6 deficiency on protein kinetics, despite the recognized importance of this vitamin in protein and amino acid metabolism. An inhibition of transmethylation in vitamin B 6 -deficient animals can also contribute to increased methionine concentrations, although previous researchers did not observe reductions in transmethylation rates in vitamin B 6 -deficient rats (Martinez et al, 2000). Elevated homocysteine levels are linked to increased S-adenosylhomocysteine concentrations (Yi et al, 2000), an inhibitor of methylation reactions.…”