1. When Na235SO4 is injected intravenously in rats, it is immediately available for sulphate conjugation of the phenolic drug harmol (7-hydroxyl-1-methyl-9H-pyrido[3,4-b]indole) in the liver. This was established by following the time course of the biliary excretion of the sulphate conjugate of harmol, and the incorporation of [35S]sulphate into harmol sulphate. 2. During the 10min immediately after injection of Na235SO4 re-distribution of [35S]sulphate took place, which resulted in a rapid initial decrease in the plasma concentration of [35S]sulphate; a concomitant decrease in the amount of [35S]sulphate incorporated into harmol sulphate was observed, indicating that the co-substrate of sulphation, adenosine 3′-phosphate 5′-sulphatophosphate, equilibrates rapidly with [35S]sulphate in plasma. 3. The results suggest that the pool size of adenosine 3′-phosphate 5′-sulphatophosphate is very small; therefore the specific radioactivity of [35S]sulphate in plasma determines the specific radioactivity incorporated into sulphate esters at any time.

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