Comparative sulfite metabolism in the rat, rabbit, and rhesus monkey

The clearance of injected sulfite, which is due principally to its metabolism to sulfate, was measured in rats, rabbits, and rhesus monkeys for the purpose of estimating the in vivo function of sulfite oxidase. The order of clearance was rat > rabbit > rhesus monkey, and the relative rates were 1.00:0.34:0.20. If it is assumed that man clears sulfite similarly to the rhesus monkey, it would appear that the rat is not a good model for prediction of human toxicity. In vitro measurements of the sulfite oxidase activity of the major metabolizing tissues of these three species produced a change in the above rank order to rat > rhesus monkey > rabbit, with relative magnitudes of 1.00:0.12:0.05, respectively. The transposing of the relative order of the rabbit and rhesus monkey with respect to in vitro enzyme activity as compared with in vivo function demonstrates that factors other than the absolute amount of sulfite oxidase play a significant role in the functioning of the enzyme.