Oxidation of dimethylsulphoxide to formaldehyde by oxyhaemoglobin and oxyleghaemoglobin in the presence of hydrogen peroxide is not mediated by "free" hydroxyl radicals.

In the presence of excess hydrogen peroxide, human oxyhaemoglobin and oxyleghaemoglobin from soybean root nodules cause oxidation of dimethylsulphoxide to formaldehyde. This reaction is inhibited by thiourea but not by phenylalanine, HEPES, mannitol or arginine. It is concluded that dimethylsulphoxide oxidation is not mediated by "free" hydroxyl radicals, consistent with previous conclusions that intact haemoglobin, leghaemoglobin or myoglobin molecules do not react with H2O2 to form hydroxyl radicals detectable outside the protein.