Thiopurine methyltransferase activity in the erythrocytes of adults and children: and HPLC-linked assay

A non-radioactive method that uses reverse-phase high performance liquid chromatography is described for the determination of thiopurine methyltransferase (E.C. 2.1.1.67) activity in human erythrocytes. The method is based on the direct quantitation of 6-methyl-mercaptopurine produced from 6-mercaptopurine by crude erythrocyte lysates. The method is accurate and reliable and suitable for diagnostic use. Activity values in control adults ranged from 5 to 32 pmol/h/mg haemoglobin. The activity in the erythrocytes of adult males was significantly higher compared to females (21±5 and 15±8 pmol/h/mg haemoglobin, respectively). The activity measured in the erythrocytes of children (22±5 pmol/h/mg haemoglobin) did not show any significant difference compared to adults. Thiopurine methyltransferase activity was measured in a female patient with systemic sclerosis who developed severe bone marrow depression after treatment with azathioprine and allopurinol. Activity (6.3 ± 0.5 pmol/h/mg haemoglobin) was found in the lowest range of controls thus supporting the hypothesis that it could be responsible for increased azathioprine cytotoxicity.