The effect of pH on yields of hydroxyl radicals produced from superoxide by potential biological iron chelators

The efficiency of conversion of superoxide to hydroxyl radicals was measured by determining the yields of fluorescent hydroxybenzoates. A variety of iron-containing catalysts were tested. Citrate was the only organic salt which showed catalytic activity at neutral pH. Adenine nucleotides had little or no activity under similar conditions. Heme proteins were inactive and any catalytic activity measured with transferrin, lactoferrin, and conalbumin could be explained by free Fe³⁺ released by the former two at acid pH. Many of the potential catalysts tested showed maximum activity near pH 4.8, where the rate of dismutation of O^{{minus sign, dot below}} ₂ is highest. This suggests that in most systems the ratecontrolling step in the superoxide-driven Fenton process was the formation of H₂O₂. It was concluded that, with the exception of citrate, none of the biological compounds tested were able to assist the conversion of O^{{minus sign, dot below}} ₂ to HO. with significant efficiency at neutral pH in homogeneous solutions.