Thiouric acids as substrates and inhibitors of mammalian urate oxidase

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Abstract
Of the thiouric acids, only the 6-thio analogue is oxidized by liver urate oxidase and at a rate about 1% of the breakdown of uric acid. The pH-activity curve of 6-thiouric acid is similar to that of uric acid, but is shifted towards the acid side. It appears that the bisanion of the 6-thio analogue is refractory towards urate oxidase, in contrast with the bisanion of uric acid. Thiouric acids inhibit urate oxidase in the descending order 6,8-dithio-, 8-thio-, 6-thio-, 2-thio-uric acid. The inhibitory effect of 6-thiouric acid is influenced less by pH changes than is the rate of its enzymic oxidation. The differences between uric acid and its 6-thio analogue are tentatively explained by the assumption that dissociation in the latter involves first N-3, then N-1, where-as the ionization of uric acid follows the sequence N-9 and N-3.

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