The dependence of the rate of sulphate conjugation on the plasma concentration of inorganic sulphate in the rat in vivo.

The dependence of the rate of sulphation of harmol [7-hydroxy-1-methyl-9H-pyrido(3,4-b)indole] on the plasma concn of inorganic sulphate was estimated in the rat in vivo. This was done in rats that were fed a low protein diet for 4 days to deplete the pool of inorganic sulphate, which decreased the plasma concn of sulphate from 0.9 mM normally to about 0.15 mM. Infusion of sodium sulphate at a rate that was stepwise increased, yielded a range of constant plasma sulphate levels below and above the physiological concn in plasma. By a constant infusion of harmol (6 mumoles/min/kg body wt), a steady state in the excretion of harmol sulphate could be reached at various plasma concns of sulphate. The apparent Km for inorganic sulphate was approx 0.3 mM. The relevance of these data in the evaluation of the effects of changes in the availability of inorganic sulphate on sulphate conjugation of xenobiotics is discussed.