Iron compounds catalyze the oxidation of 10-formyl-5,6,7,8-tetrahydrofolic acid to 10-formyl-7,8-dihydrofolic acid

We have previously demonstrated that 10-formyl-7,8-dihydrofolic acid (10-HCO-H(2)folate) is a better substrate for mammalian aminoimidazolecarboxamide ribotide transformylase (EC 2.1.2.3) than is 10-formyl-5,6,7,8-tetrahydrofolic acid (10-HCO-H(4)folate) (J.E. Baggott, G.L. Johanning, K.E. Branham, C.W. Prince, S.L. Morgan, I. Eto, W.H. Vaughn, Biochem. J. 308, 1995, 1031-1036). Therefore, the possible metabolism of 10-HCO-H(4)folate to 10-HCO-H(2)folate was investigated. A spectrophotometric assay for the oxidation of 10-HCO-H(4)folate to 10-HCO-H(2)folate which measures the disappearance of reactant (decrease in absorbance at 356 nm after acidification of aliquots of the reaction solution), is used to demonstrate that iron compounds catalyze the oxidation of 10-HCO-H(4)folate to 10-HCO-H(2)folate in the presence and absence of ascorbate. Chromatographic separation of the 10-HCO-H(2)folate product from the reaction mixture, its UV spectra, a microbiological assay and an enzymatic assay established that the iron-catalyzed oxidation product of 10-HCO-H(4)folate was 10-HCO-H(2)folate; without substantial side reactions. The inhibition of this iron-catalyzed oxidation by deferoxamine, apotransferrin and mannitol and the stimulation by citrate and EDTA indicated a mechanism involving a reaction of 10-HCO-H(4)folate with hydroxyl radicals ((OH)-O-.) generated by Fenton chemistry. The presence of "free iron" (e.g. Fe3+ citrate) in bile, cerebrospinal fluid and intracellularly suggest that this oxidation could occur in vivo and that 10-HCO-H(4)folate may be a (OH)-O-. scavenger. (C) 1998 Elsevier Science Inc. All rights reserved.