PST (E.C. 2.8.2.1) plays an important role in the metabolism of many drugs, catecholamine metabolites, and catecholamines. PST activity was detected in each of 178 human RBC lysates. When MHPG was used as a substrate, the activity ranged from 28 to 1385 U/ml of RBC, with an average value of 217.7 +/- 13.1 (mean +/- S.E.M.). However, there was not a direct relationship between quantity of RBC lysate and enzyme activity, an observation that raised the possibility of endogenous enzyme inhibitors. Therefore human RBC PST was partially purified (415-fold) to use in the study of tissue enzyme inhibitors. The pH optimum of the partially purified enzyme was 7.5, with an apparent Km value for [35S]PAPS of 0.46 microM and an apparent Km value for MHPG of 260 microM. When the partially purified enzyme was added to each of 20 human RBC lysates, its activity was inhibited an average of 91% +/- 0.6 (mean +/- S.E.M.). Endogenous inhibitors were also present in homogenates of human renal cortex and in homogenates of a variety of rat tissues. RBC lysates contained at least two PST inhibitors: a low-molecular-weight inhibitor (less than 2000) that was heat-, acid-, and base-stable, dialyzable, and resistant to digestion by chymotrypsin; and a high-molecular-weight inhibitor (greater than 65,000) that was heat-labile and nondialyzable. Whether the RBC enzyme activity may serve as an indicator of PST activity in less accessible tissues remains to be determined. However, the first step toward testing this hypothesis will require the accurate measurement of PST activity in tissue preparations by a procedure that removes or inactivates enzyme inhibitors.