Sulfation of peptides and simple phenols by rat brain phenolsulfotransferase. Inhibition by dichloronitrophenol

Abstract

Brain phenolsulfotransferase (PST) is involved in the sulfation of simple phenols like dopamine and of precursors of biologically active peptides like cholecystokinin octapeptide (CCK-8). Therefore, inhibition of brain PST would provide a new approach to studying the sulfation of CCK-8 and other sulfated compounds. Since 2,6-dichloro-4-nitrophenol (DCNP) produces a prolonged and selective inhibition of the sulfoconjugation of exogenous phenols by the liver, we decided to examine the applicability of DCNP to studies of sulfation of CCK-8 and other compounds by brain. DCNP was capable of completely inhibiting PST activity in rat brain homogenates incubated with p-nitrophenol, phenol or dopamine as substrates. The IC50 values for p-nitrophenol and dopamine were 12 and 14 microM respectively. The concentrations of DCNP in brain cortex and plasma were measured by high pressure liquid chromatography (HPLC) after a dose of 100 mumoles/kg, i.p. Peak concentrations of 380 microM in plasma and 25 mumoles/kg in brain were achieved 30 min after injection. Subsequently, DCNP concentrations decreased with half-lives of 8 and 6 hr in plasma and brain cortex, respectively. To establish if DCNP can inhibit CCK sulfation in vivo, rats were injected with 100 micromoles/kg, i.p., of the drug 30 min before injection of 35SO4(2-) into the cerebral cortex and were killed 4.5 hr later. DCNP caused a 55% inhibition of [35S]CCK-8-SO4 formation as measured by HPLC. No change in the content of endogenous CCK-8-SO4 was detectable, however, in the brain cortex of rats treated with DCNP for up to 4 days, indicating that the PST which remained active was capable of maintaining CCK-8 content at steady state.