Abstract
Phenol sulfotransferase (PST) catalyzes the sulfate conjugation of phenolic monoamines and phenolic drugs. It has been difficult to measure PST activity in tissue homogenates accurately because of the presence of potent endogenous PST inhibitors. Optimal conditions were determined for the assay of rat PST in very dilute tissue homogenates. These conditions negated the effects of endogenous enzyme inhibitors. Apparent Km values for 3-methoxy-4-hydroxyphenylglycol, the sulfate acceptor substrate used, were 0.15, 0.14, and 0.02 mM for liver, kidney, and brain homogenates respectively. Apparent Km values in the same tissues for 3'-phosphoadenosine-5'-pnosphosulfate, the sulfate donor, were 0.11, 0.07, and 0.07 μM respectively. Rat PST activity expressed per mg protein increased 6.3-fold in the liver, 6.6-fold in the brain, and did not change in the kidney between birth and 10 weeks of age. There was a 5-fold increase in kidney PST activity in both adrenalectomized and sham-operated Sprague-Dawley rats after treatment with dexamethasone (7 μmoles/kg daily for 3 days). Brain enzyme activity was unchanged and liver PST activity increased only 41% during 72 hr of daily treatment with dexamethasone. Basal enzyme activities in all three tissues were no different in adrenalectomized and sham-operated animals. The increase in rat kidney PST activity in response to dexamethasone was dose dependent, and treatment of animals with cycloheximide, a protein synthesis inhibitor, blocked the elevation of kidney PST activity after dexamethasone. Treatment of eight inbred and two outbred rat strains with dexamethasone resulted in striking increases in renal PST, smaller increases in liver PST, and no changes in brain enzyme activity in all ten strains.