Quantification of the carbon flow through the folate-dependent one-carbon pool using radiolabeled histidine: effect of altered thyroid and folate status.

Hyperthyroidism and folate deficiency are known to alter the concentration of hepatic folate pools and the activity of a number of enzymes related to one-carbon metabolism. To evaluate the physiological significance of this relationship on a functional basis, we have developed and utilized an in vivo tracer kinetic technique to quantify the carbon flow through the one-carbon pool as a function of thyroid and folate status. Control, hyperthyroid, and folate-restricted rats were continuously infused with L-[ring-2-14C]histidine to measure the oxidative flow of carbon from histidine through the one-carbon pool to CO2. As expected, the hepatic activities of a number of enzymes involved in the catabolism of histidine to CO2 were markedly decreased in hyperthyroid (histidase, 69%; urocanase, 30%; 10-formyltetrahydrofolate dehydrogenase, 65%) and folate-restricted (10-formyltetrahydrofolate dehydrogenase, 44%) rats. In addition, folate-restricted animals exhibited a 63% decrease in the hepatic concentration of total reduced folates. However, tracer kinetic analysis indicated an enhanced catabolism of histidine: the carbon flux from histidine to CO2 was increased approximately threefold in hyperthyroid rats and twofold in folate-restricted animals. Thus, in the case of hyperthyroidism and dietary folate restriction, changes in static measurements such as metabolite concentrations and enzyme activities do not reflect the dynamic tracer kinetic assessment of the carbon flux that is actually occurring in vivo. The kinetic data also demonstrate that the percentage of total entry into the 10-formyltetrahydrofolate pool originating from histidine catabolism was almost threefold greater compared to 5-methyltetrahydrofolate pool.