Determination of ITPase activity in erythrocyte lysates obtained for determination of TPMT activity

The indication for the determination of both thiopurine methyltransferase (TPMT) and inosine triphosphate pyrophosphohydrolase is identical (i.e., adverse drug reactions toward mercaptopurines). Therefore, we tested whether or not our standard procedure to prepare erythrocyte lysates for measurement of TPMT activity, which includes treatment with Chelex 100 (a chelating resin), was suitable for the measurement of ITPase activity. It also was tested to see if ITPase activity differs in EDTA and Heparin anti-coagulated blood samples. We found that there was no difference between the ITPase activity in erythrocyte lysates prepared from EDTA or Heparin anti-coagulated blood. Treatment with a chelating resin or omission of magnesium from the assay procedure resulted in decreased and nearly absent ITPase activity, respectively. We conclude that untreated erythrocyte lysates obtained for determination of TPMT activity are suitable for determination of ITPase activity. However, after treatment with Chelex 100 the erythrocyte lysates become unsuitable for determination of ITPase activity.