Human red blood cell (RBC) membranes have been reported to contain both high and low affinity 'forms' of the drug metabolizing enzyme thiol methyltransferase (TMT). The biochemical characteristics of the two 'forms' of human RBC TMT were compared. Apparent Km constants of the high affinity activity for 2-mercaptoethanol and S'-adenosyl-l-methionine, cosubstrates for the TMT reaction, were 0.38 μmol/l and 2.6 μmol/l, respectively. These constants may be compared with values of 20 mmol/l and 43 μmol/l, respectively, previously reported for the low affinity form of RBC TMT activity. The properties and regulation of the two forms of TMT were then compared with each other and also with those of two 'control' enzymes, phenol methyltransferase (PMT) and β-glucuronidase. When high and low affinity TMT, PMT and β-glucuronidase activities were measured in RBC membranes from 22 individual subjects, there were highly significant correlations among all three methyltransferase activities (all r values < (0.95), but β-glucuronidase activity did not correlate significantly with any of the methyltransferase activities (all r values < 0.40). The thermal stabilities of the three methyltransferases were very similar. They were all inactivated approximately 50% by incubation at 48°C for 15 min. β-Glucuronidase activity was approximately 50% inactivated by incubation at 76°C for 15 min. PMT and both TMT activities had similar subcellular distributions and similar responses to ions and to enzyme inhibitors. These results suggested that high and low affinity TMT and PMT activities might be catalyzed by the same enzyme. Alternatively, these three RBC membrane methyl-transferase activities might be regulated in a parallel fashion.
ASJC Scopus subject areas
- Clinical Biochemistry