QUANTIFICATION OF THE CARBON FLOW-THROUGH THE FOLATE-DEPENDENT ONE-CARBON POOL USING RADIOLABELED HISTIDINE - EFFECT OF ALTERED THYROID ANDFOLATE STATUS

Hyperthyroidism and folate deficiency are known to alter the concentra tion of hepatic folate pools and the activity of a number of enzymes r elated to one-carbon metabolism. To evaluate the physiological signifi cance of this relationship on a functional basis, we have developed an d utilized an in vivo tracer kinetic technique to quantify the carbon how through the one-carbon pool as a function of thyroid and folate st atus. Control, hyperthyroid, and folate-restricted rats were continuou sly infused with L-[ring-2-C-14]histidine to measure the oxidative flo w of carbon from histidine through the one-carbon pool to CO2. As expe cted, the hepatic activities of a number of enzymes involved in the ca tabolism of histidine to CO2 were markedly decreased in hyperthyroid ( histidase, 69%; urocanase, 30%; 10-formyltetrahydrofolate dehydrogenas e, 65%) and folate-restricted (10-formyltetrahydrofolate dehydrogenase , 44%) rats, In addition, folate-restricted animals exhibited a 63% de crease 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 th reefold 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 e nzyme activities do not reflect the dynamic tracer kinetic assessment of the carbon flux that is actually occurring in vivo. The kinetic dat a also demonstrate that the percentage of total entry into the 10-form yltetrahydrofolate pool originating from histidine catabolism was almo st threefold greater compared to 5-methyltetrahydrofolate pool. (C) 19 96 Academic Press, Inc.