The reaction of hemoglobin with paraquat radicals in the presence and absence of O2.

Paraquat radicals generated with xanthine oxidase in N2 reduced methemoglobin to deoxyhemoglobin. In air, reduction to oxyhemoglobin occurred but at one third the rate. In air the reaction was 10 times faster and gave greater overall reduction to oxyhemoglobin than with O2 generated at a similar rate. Aerobic methemoglobin reduction in the presence of paraquat was inhibited by superoxide dismutase. It is concluded that paraquat radicals reduce methemoglobin directly in air and that this reaction must be very fast with a rate constant of the order of 10(9) M-1 s-1. This is an example where paraquat radicals in air do not all react with O2. It cannot therefore be assumed that the radical reactions of paraquat in air are due solely to O2 and its products.