Human Platelet Phenol Sulfotransferase: Partial Purification and Detection of Two Forms of the Enzyme

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Abstract
To begin to study the usefulness of platelet phenol sulfotransferase (PST) as a possible measure of the enzyme activity in other organs such as the brain, we purified human platelet PST 36–120-fold. Activity toward 3-methoxy-4-hydroxyphenylglycol (MHPG), dopamine, 5-hydroxytryptamine (5-HT), and phenol eluted in the same Sephadex G-100 and Affi-Gel Blue column fractions. Specific activities of the enzyme with MHPG, dopamine, 5-HT, and phenol as substrates were1198, 1068,401, and 408 units/mg protein, respectively. Optimal assay conditions were established for each substrate. Apparent Km values were 598 μM, 21 μM, 19 μM, and 500 μM for MHPG, dopamine, phenol, and 5-HT, respectively. Apparent Km values for 3′-phosphoadenosine-5′-phosphosulfate (PAPS) with the same four substrates ranged from 0.11 to 0.25 μM. The pH optima were 6.3 for phenol, 6.8 for dopamine, and 7.0 for MHPG and 5-HT. An additional pH optimum at 8.6 was present for 5-HT. A thermolabile form of the enzyme measured with dopamine and 5-HT, as well as a thermostable form measured with phenol, were present. Dichloronitrophenol (10−5M) noncompetitively inhibited the thermostable enzyme activity by 96% but decreased the thermolabile activity by only 36%. These studies provide the basis for a more accurate comparison of human platelet PST with the enzyme in the human brain and in other tissues.