5-Methyltetrahydrofolate is reabsorbed and metabolized in isolated perfused rat kidney

The renal regulation of folate excretion is an important component in maintaining the body burden of folate. The tubular processes for folate disposition have been examined by a variety of methods to elucidate the mechanism by which renal folate excretion is regulated. Accordingly, the isolated perfused rat kidney technique was evaluated by investigating the clearance and metabolic patterns of 5-methyltetrahydrofolate (5-CH3-H4PteGlu). Kidneys from male Sprague-Dawley rats were perfused in vitro with [3H]5-CH3-H4PteGlu (1-2000 nmol/L). Linear regression analysis of 5-CH3-H4PteGlu excretion vs. filtered load revealed a tubular transport maximum of 7.5 pmol x min-1.g-1. A dual component system for tubular transport of 5-CH3-H4PteGlu was found: a high capacity, nonsaturable system and a low capacity, saturable system represented by the transport maximum. Furthermore, HPLC analysis of urine demonstrated renal uptake and metabolism of the labeled tracer. Tetrahydrofolate was identified as one metabolic product that indicated secretion of this compound. Additional metabolites were identified from kidney samples. Results suggest that 5-CH3-H4PteGlu undergoes net reabsorption by a dual component transport system; some of the reabsorbed 5-CH3-H4PteGlu is metabolized to other products that may be secreted in the urine.