COMPARTMENTATION OF FOLATE METABOLISM IN RAT PANCREAS - NITROUS-OXIDEINACTIVATION OF METHIONINE SYNTHASE LEADS TO ACCUMULATION OF 5-METHYLTETRAHYDROFOLATE IN CYTOSOL

Folate-dependent one-carbon metabolism and methylation reactions have been implicated in the secretory function of the pancreas. Because vit amin B-12 deficiency perturbs folate metabolism, we determined the eff ects of nitrous oxide inactivation of methionine synthase on the compa rtmentation of folate metabolism in rat pancreas. Rats were exposed to an atmosphere of nitrous oxide and oxygen (80 and 20%, respectively) for 18 h; control rats breathed air. Folate coenzyme concentrations we re determined by HPLC and Lactobacillus casei microbiological assay of the cytosolic and mitochondrial fractions of pancreas, which containe d 62 and 46%, respectively, of the total folate. In pancreas of contro l rats, cytosolic folates were 5-methyltetrahydrofolate (31% of total folates), tetrahydrofolate (54%) and 5- and 10-formyltetrahydrofolate (6 and 8%, respectively), In the rats exposed to nitrous oxide, cytoso lic 5-methyltetrahydrofolate concentrations were significantly greater (59% of total folates) and tetrahydrofolate concentrations were signi ficantly lower (32%) than in controls; however, total cytosolic folate levels were unaffected by nitrous oxide exposure. In controls, mitoch ondrial folates were composed of 5-methyltetrahydrofolate (9% of total folates), tetrahydrofolate (60%) and 5- and 10-formyltetrahydrofolate (22 and 10%, respectively). Exposure to nitrous oxide led to signific antly lower total mitochondrial folates (1.49 +/- 0.18 vs. 0.75 +/- 0. 29 nmol/g, control vs. nitrous oxide, P < 0.05). This was due to a sig nificantly lower concentration of tetrahydrofolate and 5-formyltetrahy drofolate, but not of 5-methyl- or 10-formyltetrahydrofolate. The acti vity of methionine synthase was 85% lower (P < 0.001) in pancreatic ex tracts of rats exposed to nitrous oxide than in controls. These result s show that cytosolic folates accumulate in pancreas as the 5-methyl d erivative at the expense of other reduced folates, as happens in liver . However, in contrast to results in liver, the mitochondrial folate c oncentration was lower in the pancreas of rats exposed to nitrous oxid e, and this decline was limited to the 5-formyl- and tetrahydrofolate derivatives.