Autoxidation of ferrous ion complexes: a method for the generation of hydroxyl radicals

The kinetics of the production of hydroxyl radicals during the autoxidation of ferrous ion complexes at pH 7.4 was investigated using the fluorescent probe coumarin-3-carboxylic acid. Polyphosphates (tri- and tetrapolyphosphate and their adenosine derivatives), citrate, and acetic derivatives of ethyleneamine ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), triethylenetetraminehexaacetic acid (TTHA), ethylenediamine-(N,N')-diacetic acid (EDDA) and nitrilotriacetic acid (NTA) were used as iron chelators. Production of hydroxyl radical in these chemical systems was compared with that by radiation to determine the equivalent doses of radiation that produced equal amounts of .OH. The amount of .OH formed during ferrous ion autoxidation is determined by the concentration of the complex, its structure and the radical scavenging by the chelator molecule. Production of .OH for homologous ethylenamine acetates increases with increased complex stability: NTA < EDDA < TTHA < EDTA < DTPA. The radiation dose equivalence for 0.1 mM complexes increased from 5 Gy for NTA to 25 Gy for DTPA. The radiation dose equivalence for polyphosphates was 15 Gy for tripolyphosphate and 32 Gy for tetrapolyphosphate. The dose equivalences for adenosine phosphates are lower, 5 Gy for ATP and 10 Gy for adenosine tetraphosphate, due to intramolecular .OH scavenging. The rate of generation of .OH shows an inverse correlation with the charge of the ferrous ion complex, varying from 2 cGy/s for DTPA to 1.2 Gy/s for EDTA. The data presented indicate the usefulness of autoxidation of ferrous ion complexes for generation of .OH in chemical systems. The ability to control the amount and the rate of production of .OH may prove useful for examining the cytotoxic effects of .OH generated in biological systems.