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 aminoimidazolec arboxamide 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. Johan ning, K.E. Branham, C.W. Prince, S.L. Morgan, I. Eto, W.H. Vaughn, Bio chem. J. 308, 1995, 1031-1036). Therefore, the possible metabolism of 10-HCO-H(4)folate to 10-HCO-H(2)folate was investigated. A spectrophot ometric assay for the oxidation of 10-HCO-H(4)folate to 10-HCO-H(2)fol ate which measures the disappearance of reactant (decrease in absorban ce 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 produc t from the reaction mixture, its UV spectra, a microbiological assay a nd an enzymatic assay established that the iron-catalyzed oxidation pr oduct of 10-HCO-H(4)folate was 10-HCO-H(2)folate; without substantial side reactions. The inhibition of this iron-catalyzed oxidation by def eroxamine, apotransferrin and mannitol and the stimulation by citrate and EDTA indicated a mechanism involving a reaction of 10-HCO-H(4)fola te with hydroxyl radicals ((OH)-O-.) generated by Fenton chemistry. Th e 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.