Cobalamin Deficiency Results in Increased Production of Formate Secondary to Decreased Mitochondrial Oxidation of One-Carbon Units in Rats

Background: Formate is produced in mitochondria via the catabolism of serine, glycine, dimethylglycine, and sarcosine. Formate produced by mitochondria may be incorporated into the cytosolic folate pool where it can be used for important biosynthetic reactions. Previous studies from our lab have shown that cobalamin deficiency results in increased plasma formate concentrations. Objective: Our goal was to determine the basis for elevated formate in vitamin B-12 deficiency. Methods: Male Sprague Dawley rats were randomly assigned to consume either a cobalamin-replete (50 mu g cobalamin/kg diet) or -deficient (no added cobalamin) diet for 6 wk. Formate production was measured in vivo and in isolated liver mitochondria from a variety of one-carbon precursors. We also measured the oxidation of [3-C-14]-l-serine to (CO2)-C-14 in isolated rat liver mitochondria and the expression of hepatic genes involved in one-carbon unit and formate metabolism. Results: Cobalamin-deficient rats produce formate at a rate 55% higher than that of replete rats. Formate production from serine was increased by 60% and from dimethylglycine and sarcosine by similar to 200% in liver mitochondria isolated from cobalamin-deficient rats compared with cobalamin-replete rats. There was a 26% decrease in the (CO2)-C-14 produced by mitochondria from cobalamin-deficient rats. Gene expression analysis showed that 10-formyltetrahydrofolate dehydrogenase-cytosolic (Aldh1/1) and mitochondrial (Aldh1/2) expression were decreased by 40% and 60%, respectively, compared to control, while 10-formyltetrahydrofolate synthetase, mitochondrial, monofunctional (Mthfd1l) expression was unchanged. Conclusion: We propose that a bifurcation in mitochondrial one-carbon metabolism is a key control mechanism in determining the fate of one-carbon units, to formate or CO2. During cobalamin deficiency in rats the disposition of 10-formyltetrahydrofolate carbon is shifted in favor of formate production. This may represent a mechanism to generate more one-carbon units for the replenishment of the S-adenosylmethionine pool which is depleted in this condition.