Sulphate conjugation of biologically active monoamines and their metabolites by human platelet phenolsulphotransferase

Abstract

The substrate specificity of phenolsulphotransferase in human platelets has been studied using a wide range of biogenic amines and their metabolites. Substantially differing activities were observed at 30 μmol/l; the enzyme was more active towards the catecholamines and their alcoholic metabolites than the corresponding acids (with the exception of 3,4-dihydroxyphenylacetic acid which did not appear to be a substrate) and the relative order was not changed by dialysis to remove possible low molecular mass inhibitors. However, most of the V values were similar to each other, reflecting a large variation in K m values, ranging from 0.3 μmol/l for 3-methoxytyramine to 3700 μmol/l for 4-hydroxy-3-methoxymandelic acid. All substrates showed substrate inhibition. Dopamine, noradrenaline and adrenaline all had a high affinity for the enzyme, with K m values of 3.0, 5.0 and 2.7 μmol/l respectively. These values are considerably lower than those for monoamine oxidase and the relative importance of oxidation and sulphoconjugation of these amines in vivo may be concentration dependent. Human platelet phenolsulphotransferase appears different from the rat enzyme, but similar to that described by others in human brain. The platelet should be a useful source of enzyme for clinical studies.