Effect of phosphatase inhibition of in vitro dopamine sulfation and 3′-phosphoadenosine-5′-phosphosulfate catabolism in human brain

The effects of inhibition of phosphatase activity in 100,000 g supernatant solution from human frontal cortex on dopamine (DA) conjugation were examined using the phosphatase substrate p-nitrophenyl phosphate (pNPO₄). The increases in DA sulfation seen in the presence of pNPO₄ suggested that inhibition of phosphatase activity in high speed supernatant solutions of brain may substantially alter the pattern of 3′-phosphoadenosine-5′-phosphosulfate (PAPS) metabolism and subsequently the rate of DA sulfation. Accordingly, the effects of the pyrophosphate analog phosphonoacetic acid (PAA) on the extent of DA sulfation and PAPS metabolism were examined in 100,000 g supernatant solution from human frontal cortex. At concentrations up to 10 mM, PAA markedly reduced PAPS hydrolysis to inorganic sulfate and 3′-phosphoadenosine-5′-phosphate (PAP) and significantly extended the linear time period for the sulfation of DA. These findings suggest that the phosphatase enzymes that degrade PAPS to produce the end product inhibitor, PAP, and possibly other break-down products of PAP, play an important role in determining the observed levels of phenol sulfotransferase activity in tissue from human brain in vitro.