TPMT
Thiopurine S-methyltransferase (EC 2.1.1.67) (Thiopurine methyltransferase)
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Low thiopurine S-methyltransferase (TPMT) enzyme activity is associated with increased thiopurine drug toxicity, particularly myelotoxicity. Pre-analytic and analytic variables for TPMT genotype and phenotype (enzyme activity) testing were reviewed. A systematic literature review was performed, and diagnostic laboratories were surveyed. Thirty-five studies reported relevant data for pre-analytic variables (patient age, gender, race, hematocrit, co-morbidity, co-administered drugs and specimen stability) and thirty-three for analytic variables (accuracy, reproducibility). TPMT is stable in blood when stored for up to 7 days at room temperature, and 3 months at -30°C. Pre-analytic patient variables do not affect TPMT activity. Fifteen drugs studied to date exerted no clinically significant effects in vivo. Enzymatic assay is the preferred technique. Radiochemical and HPLC techniques had intra- and inter-assay coefficients of variation (CVs) below 10%. TPMT is a stable enzyme, and its assay is not affected by age, gender, race or co-morbidity.
MeSH Terms
- Aging
- Azathioprine
- Enzyme Stability
- Health Care Surveys
- Hematocrit
- Humans
- Mercaptopurine
- Methyltransferases
- Prescription Drugs
- Purines
- Sex Characteristics
- Thioguanine
Red blood cell (RBC) thiopurine methyltransferase (TPMT) metabolizes the cytotoxic drugs 6-mercaptopurine and azathioprine. RBC TPMT activity has been reported to predict clinical outcome in children with acute lymphoblastic leukaemia and in kidney transplant patients. We first suspected that the erythrocyte fraction affected the calculated TPMT activity when we examined intraindividual TPMT activities in kidney transplant recipients. We demonstrated that the erythrocyte fraction affected the calculated TPMT activity, thus causing a methodological inaccuracy. A low erythrocyte fraction gave an erroneously low TPMT activity. Mean variation of 7.0% was observed within the normal limits of the haematocrit level in healthy subjects. The slopes of the TPMT activity between erythrocyte fraction 0.1 and 0.5 were all significantly different from zero, and the activity displayed good linearity from erythrocyte fraction 0.2. There was a strong association between TPMT activity and erythrocyte fraction in a population sample of children, but not in two other population samples. We propose that the TPMT assay should be performed in lysates at a standardized erythrocyte fraction to avoid variation in activity due to the range of the haematocrit in a population.
MeSH Terms
- Adult
- Aged
- Aging
- Child
- Erythrocytes
- Humans
- In Vitro Techniques
- Methyltransferases
- Subcellular Fractions
Thiopurine S-methyltransferase (TPMT) catalyses the S-methylation of aromatic and heterocyclic sulfhydryl compounds, including thiopurine antimetabolites (i.e. mercaptopurine and thioguanine). The activity of TPMT in erythrocytes and other tissues exhibits genetic polymorphism, which is inherited as an autosomal codominant trait. Although inheritance is the principal determinant of TPMT activity, other factors (e.g. renal function, race and thiopurine therapy) have been shown to influence erythrocyte TPMT activity. Because the TPMT polymorphism has not been established in early erythrocyte populations, and the activity of many enzymes differs in neonates, we determined the activity of TPMT in erythrocytes obtained from 60 full-term newborns. Median peripheral blood TPMT activity was 25.3 U per ml pRBC (range 9-52.8 U per ml pRBC), which was > 50% higher than race matched healthy adults (p < 0.001). Western blot analysis demonstrated higher TPMT protein content in erythrocytes from newborns compared to adults, and revealed a significant correlation between TPMT protein and TPMT activity in erythrocytes (rs = 0.63, p = 0.03). Although erythrocyte TPMT activity was significantly higher in newborns, the distribution of activity in newborns was consistent with the genetic polymorphism previously observed in adults.
MeSH Terms
- Adult
- African Continental Ancestry Group
- Aging
- Blotting, Western
- Erythrocytes
- European Continental Ancestry Group
- Fetal Blood
- Humans
- Infant, Newborn
- Methyltransferases
- Polymorphism, Genetic
- Reference Values
- United States
1. Thiopurine methyltransferase (TPMT) catalyses the S-methylation of thiopurine drugs. TPMT activity in the kidneys of male Sprague-Dawley (S-D) rats is approximately twice that present in the kidneys of female S-D rats, and this difference is testosterone-dependent. Renal TPMT activities in these animals also increase dramatically during growth and development. 2. Our studies were conducted to determine whether variations in TMPT activity in the S-D rat kidney were due to differences in the quantity of TPMT protein. Rabbit polyclonal antibodies to partially purified rat kidney TPMT were used to develop an immunoprecipitation assay for immunoreactive TPMT protein. 3. Gender-related differences in renal TPMT activities in S-D rats were due to a lower content of immunoreactive TPMT protein in kidneys of female animals. TPMT enzyme activities and immunoreactive protein levels were also directly correlated in renal preparations from castrated and sham-operated male rats, from testosterone-treated castrated and sham-operated male rats, and from testosterone-treated and control female rats. 4. There was also a significant positive correlation between TPMT enzymic activities and immunoreactive TPMT protein levels in renal tissue from different aged male S-D rats (rs = 0.955, n = 15, P less than 0.001.) 5. These results demonstrate that changes in S-D kidney TPMT activity during growth and development, in the two sexes and in response to testosterone, were due to variations in the quantity of immunoreactive TPMT protein.
MeSH Terms
- Aging
- Animals
- Animals, Newborn
- Female
- Kidney
- Male
- Methyltransferases
- Orchiectomy
- Precipitin Tests
- Rats
- Sex Characteristics
- Testosterone