Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

---

Query: EC:2.1.1.67 (thiopurine methyltransferase)
551 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Thiopurine S-methyltransferase is a cytosolic enzyme that catalyzes the S-methylation of thiopurine drugs. Although a genetic polymorphism has been recognized for this enzyme in populations of Caucasian descent, there has been scanty information about this polymorphism among Asians. In this study, we measured the erythrocyte thiopurine methyltransferase activity in 119 healthy Chinese subjects by a radiochemical assay. Methyltransferase activity was lower than what might have been expected for a white population. A bimodal frequency distribution was obtained that allowed the identification of four individuals with relatively low methyltransferase activity who may be heterozygotes for thiopurine S-methyltransferase deficiency; if so, the frequency of the mutant allele would be lower in this Chinese population than that observed in a white population (chi 2, p < 0.02). No gender-based differences were observed.
...
PMID:Thiopurine S-methyltransferase activity in a Chinese population. 833 Apr 62

Disulfiram is used in the treatment of alcoholism to inhibit the enzyme aldehyde dehydrogenase. Disulfiram is rapidly reduced in vivo to form diethyldithiocarbamate (DDC), and DDC can undergo methyl conjugation to form S-methyl-DDC. Human tissues contain two separate genetically regulated enzymes that can catalyze thiol S-methylation. Thiol methyltransferase (TMT) is a microsomal enzyme that preferentially catalyzes, the S-methylation of alipathic sulfhydryl compounds, whereas thiopurine methyltransferase (TPMT) is a cytoplasmic enzyme that preferentially catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl compounds. Our experiments were performed to determine whether human liver microsomal and/or cytosolic preparations could catalyze the S-methylation of DDC, and, if so, to determine whether TMT or TPMT might be the enzymes involved. We found that both human liver microsomes and cytosol could catalyze DDC S-methylation. The microsomal activity displayed biphasic substrate kinetics, with apparent Km values for DDC of 7.9 and 1500 microM for the high- and low-affinity activities, respectively. The high-affinity activity had an apparent Km value for S-adenosyl-L-methionine, the methyl donor for the reaction, of 5.8 microM. The thermal inactivation profile and response to methyltransferase inhibitors of the high-affinity microsomal DDC S-methyltransferase activity were similar to those of human liver microsomal TMT. In addition, TMT activity and the activity catalyzing the S-methylation of DDC were highly correlated in 19 individual liver samples (rs = 0.956; P < .0001). Hepatic cytosolic DDC S-methyltransferase activity had an apparent Km value for DDC of 95 microM. The cytosolic enzyme which catalyzed DDC S-methylation and TPMT activity had similar thermal inactivation profiles, similar patterns of response to methyltransferase inhibitors and the two activities coeluted during ion exchange chromatography. Furthermore, the activities of TPMT and cytosolic DDC S-methyltransferase were highly correlated in 20 individual liver samples (rs = 0.963; P < .0001). These results were compatible with the conclusion that both TMT and TPMT could catalyze the S-methylation of DDC in the human liver. Because the activities of both TMT and TPMT are controlled by inheritance, our observations raise the possibility of pharmacogenetic variation in the biotransformation and therapeutic effect of DDC in humans.
...
PMID:Diethyldithiocarbamate S-methylation: evidence for catalysis by human liver thiol methyltransferase and thiopurine methyltransferase. 839 51

Red blood cell (RBC) thiopurine methyltransferase (TPMT), an inactivating pathway of 6-mercaptopurine, is controlled by genetic polymorphism and is subject to ethnic variation. RBC TPMT is a good predictor of clinical outcome in children with acute lymphoblastic leukemia. RBC TPMT activity was determined in 226 patients, 176 of them living in northern Norway (of which 123 were Saami (Lapps)). Demographic variables, use of drugs and presence of chronic diseases were evaluated as possible predictors of RBC TPMT activity by a multiple regression model. Men had higher RBC TPMT activity compared to women. Living in the northernmost county of Norway was associated with increased RBC TPMT activity irrespective of ethnicity. The use of diuretics was associated with increased RBC TPMT activity. The gender difference in RBC TPMT activity may indicate a need to treat male subjects more aggressively with thiopurine drugs compared to female subjects.
...
PMID:Identification of factors regulating thiopurine methyltransferase activity in a Norwegian population. 845 58

Azathioprine can cause severe myelosuppression. The inherited activity of the enzyme thiopurine methyltransferase has been recently recognised as a major factor in the susceptibility to myelosuppression. Thiopurine methyltransferase deficiency occurs at a frequency of one in 300 and is associated with profound myelosuppression after a short course of azathioprine. Very low thiopurine methyltransferase activity represents the TPMTL/TPMTL genotype, and can be detected before therapy with azathioprine is started. We describe the first documented case of azathioprine-induced severe myelosuppression due to thiopurine methyltransferase deficiency in autoimmune liver disease. The azathioprine dose was low (1 mg/kg) and pancytopenia occurred after 56 days therapy. It would be advisable to measure thiopurine methyltransferase activity before patients with autoimmune hepatitis are exposed to azathioprine.
...
PMID:Azathioprine-induced myelosuppression due to thiopurine methyltransferase deficiency in a patient with autoimmune hepatitis. 855 Oct 1

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.
...
PMID:Erythrocyte fraction affects red blood cell thiopurine methyltransferase activity. 858 69

The commonly used immunosuppressive regimen after solid organ transplantation consists of cyclosporine A, azathioprine and steroids. Azathioprine, which is known to carry the risk of severe myelosuppression, is catabolized in vivo by xanthine oxidase and thiopurine methyltransferase, an enzyme which exhibits a common genetic polymorphism; 11% of Caucasians are heterozygous and 0.3% are homozygous with respect to thiopurine methyltransferase deficiency. Toxicity and immunosuppressive effects have been attributed to the 6-thioguanine nucleotides generated from azathioprine. We have studied thiopurine methyltransferase activity and 6-thioguanine nucleotide concentrations in erythrocytes from 39 heart and kidney recipients. Erythrocyte thiopurine methyl-transferase was determined by a radioenzymatic assay and erythrocyte 6-thioguanine nucleotide concentration with HPLC. Thiopurine methyltransferase activity [median (range, 10th-90th percentile)] was significantly (p < 0.05) higher in patients (n = 39) receiving azathioprine [285 (218-362) vs. 262 (160-352) mU/I erythrocytes] than in healthy blood donors as controls (n = 120). When stratified according to thiopurine methyltransferase phenotype, one patient homozygous for the low allele exhibited an excessive erythrocyte 6-thioguanine nucleotide concentration (2210 pmol/0.8 x 10(9) erythrocytes). Heterozygous patients had significantly higher 6-thioguanine nucleotide concentrations median: 435 pmol/0.8 x 10(9) erythrocytes) compared with concentrations in patients homozygous for the high allele (median: 86 pmol/0.8 x 10(9) erythrocytes; p < 0.01), although the azathioprine dosage did not differ (p = 0.66). Erythrocyte thiopurine methyltransferase determination therefore identifies patients at high risk of accumulating 6-thioguanine nucleotides. The monitoring of this enzyme may contribute to the safer management of immunosuppressive therapy with azathioprine. Alternative regimens such as cyclosporin A/mycophenolate mofetil or tacrolimus should also be considered for this patient group.
...
PMID:Azathioprine pharmacogenetics: the relationship between 6-thioguanine nucleotides and thiopurine methyltransferase in patients after heart and kidney transplantation. 872 7

The commonly used immunosuppressive regimen after orthotopic heart transplantation consists of cyclosporine (CsA), azathioprine (AZA), and steroids. Although AZA therapy is generally regarded as unproblematic, its use can be associated with severe side effects, particularly myelosuppression. Since AZA is a prodrug, which must first be metabolized to its active metabolites, AZA therapy, in contrast to CsA therapy, cannot be controlled by measuring blood levels of this drug. Because of the myelosuppressive properties of the AZA metabolites, the 6-thioguanine nucleotides (6-TGN), the white blood cell count is usually monitored in patients on AZA therapy, and AZA is discontinued if neutropenia appears. In a group of 20 consecutive heart recipients, 6-TGN concentrations ranged from < 30 to 2,211 pmol/8 x 10(8) red blood cells (RBCs); levels < or = 450 pmol/8 x 10(8) RBCs were not associated with AZA-induced myelosuppression. Three cases of neutropenia were experienced, two of them with a fatal outcome. One patient died in septicemia owing to total myelosuppression. In this case an excessively high erythrocyte 6-TGN concentration (2,211 pmol/8 x 10(8) RBCs) was associated with a complete deficiency of thiopurine methyltransferase (TPMT), one of the main AZA detoxifying enzymes. The second patient, who had high RBC TPMT activity, developed neutropenia during rehabilitation, and AZA was withdrawn. Coincidentally, in this case the CsA blood level was only 132 g/L, and the RBC 6-TGN level was very low (maximum 46 pmol/8 x 10(8) RBCs). This patient rapidly developed cardiogenic shock with clinical signs of acute rejection and was given a second transplant on an emergency basis, but finally died from rejection of the second graft. Retrospectively, it was determined that neutropenia in this patient was not related to AZA toxicity. A high 6-TGN level (698 pmol/8 x 10(8) RBCs) was also seen in a third patient with mild neutropenia, who required allopurinol, an inhibitor of xanthine oxidase, the other major detoxifying enzyme for AZA. In this patient AZA therapy could be individually adapted by RBC 6-TGN monitoring. Based on our experience, we suggest that RBC 6-TGN monitoring allows for better individualization of treatment with AZA and may help avoid fatal complications.
...
PMID:Should 6-thioguanine nucleotides be monitored in heart transplant recipients given azathioprine? 873 60

We have expressed the human thiopurine methyltransferase cDNA in a baculovirus vector in Sf21 (Spodoptera frugiperda) cells. This system expresses the enzyme at levels such that the thiopurine methyltransferase enzyme may be readily visualised by Coomassie blue stained sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The expressed enzyme catalysed the methylation of 6-mercaptopurine with an apparent Km of 892 microM, similar to that observed in human liver cytosol ie. 657 microM however, the Vmax was 13,500 pmole/mg/min, which is approximately 400 times higher than the Vmax observed in human liver cytosol ie. 33 pmole/mg/min. The thiopurine methyltransferase inhibitors 6-thioxanthine, p-methoxybenzoic acid and 3,5-dimethoxy benzoic acid were found to be potent inhibitors of the expressed enzyme.
...
PMID:Baculovirus-mediated high level expression of a human thiopurine methyl transferase. 885 May 31

The formation of intracellular thionucleotides are a prerequisite for mercaptopurine (MP) cytotoxicity, and interindividual variations in the inherited level of thiopurine methyltransferase (TPMT) activity regulate their formation. Measurement of pretreatment TPMT activities can identify the TPMT "deficient" patient and, conversely, the individual with very high enzyme activities. The former are at higher risk of acute toxicity and potentially fatal bone marrow failure and the latter of suboptimal treatment. Leukaemic children taking MP therapy who form inadequate amounts of thioguanine nucleotides (TGNs) do not experience drug toxicity and are at an increased risk of disease relapse. When low TGNs are due to very high TPMT activities, thioguanine may be a more appropriate thiopurine. Another cause of inadequate TGN concentrations is partial or noncompliance with oral chemotherapy. Compliance problems can be identified by the measurement of both TGNs and methylated drug metabolites.
...
PMID:Individualizing therapy with 6-mercaptopurine and 6-thioguanine related to the thiopurine methyltransferase genetic polymorphism. 885 46

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.
...
PMID:Thiopurine methyltransferase activity in the erythrocytes of adults and children: and HPLC-linked assay. 908 3


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>

---