Isolation and identification of inosine triphosphate from human erythrocytes

ITPA (inosine triphosphate pyrophosphatase): from surveillance of nucleotide pools to human disease and pharmacogenetics

Cellular nucleotide pools are often contaminated by base analog nucleotides which interfere with a plethora of biological reactions, from DNA and RNA synthesis to cellular signaling. An evolutionarily conserved inosine triphosphate pyrophosphatase (ITPA) removes the non-canonical purine (d)NTPs inosine triphosphate and xanthosine triphosphate by hydrolyzing them into their monophosphate form and pyrophosphate. Mutations in the ITPA orthologs in model organisms lead to genetic instability and, in mice, to severe developmental anomalies. In humans there is genetic polymorphism in ITPA. One allele leads to a proline to threonine substitution at amino acid 32 and causes varying degrees of ITPA deficiency in tissues and plays a role in patients' response to drugs. Structural analysis of this mutant protein reveals that the protein is destabilized by the formation of a cavity in its hydrophobic core. The Pro32Thr allele is thought to cause the observed dominant negative effect because the resulting active enzyme monomer targets both homo- and heterodimers to degradation.

Beyond TPMT: genetic influences on thiopurine drug responses in inflammatory bowel disease

Azathioprine and 6-mercaptopurine are widely used in the management of inflammatory bowel disease (IBD). However, approximately 25% of IBD patients experience toxicity, and up to 10% show resistance to these thiopurine drugs. The importance of genetic variability in determining thiopurine toxicity was first recognized over 25 years ago with the discovery of the thiopurine S-methyltransferase (TPMT) polymorphism and the occurrence of azathioprine-induced myelosuppression in TPMT-deficient patients. In the intervening period, TPMT has become the foremost example of pharmacogenetics, and TPMT deficiency represents one of the few pharmacogenetic phenomena that have successfully made the transition from the research laboratory to diagnostics. While TPMT activity predicts some cases of myelosuppression, deficiency in this enzyme is neither predictive of other adverse drug reactions, nor resistance to thiopurine therapy. As myelosuppression only accounts for approximately 2.5% of adverse reactions in IBD patients, researchers are increasingly turning their attention to other enzymes involved in thiopurine metabolism to find molecular explanations for intolerance and resistance to azathioprine and 6-mercaptopurine. In this review, we summarize the current state of knowledge with regards to TPMT, and also explore genetic variability, beyond TPMT, that may contribute to thiopurine response in IBD patients.

IMPDH1 promoter mutations in a patient exhibiting azathioprine resistance

Around 9% of inflammatory bowel disease (IBD) patients are resistant to azathioprine. We hypothesized that these patients may carry mutations within inosine-5'-monophosphate dehydrogenase (IMPDH). To test this hypothesis, we screened 20 azathioprine-resistant patients for variations in the two IMPDH genes (IMPDH1 and IMPDH2) using dHPLC and DNA sequencing. A 9 bp insertion within the IMPDH1 P3 promoter was found in a patient exhibiting severe azathioprine resistance. The insertion is predicted to abolish a cAMP-response element (CRE) and was found to significantly reduce IMPDH1 P3 promoter activity in a luciferase reporter gene assay (P-value <0.001). This in vitro assay suggests the variant promoter has altered function in vivo and consequently may have contributed to the thiopurine resistance observed in this patient. The absence of functional variants within the other patients indicates that if IMPDH genetic variability contributes to azathioprine resistance it does so infrequently.

Association of inosine triphosphatase 94C>A and thiopurine S-methyltransferase deficiency with adverse events and study drop-outs under azathioprine therapy in a prospective Crohn disease study

BACKGROUND

Azathioprine (aza) therapy is beneficial in the treatment of inflammatory bowel disease, but 10%-30% of patients cannot tolerate aza therapy because of adverse drug reactions. Thiopurine S-methyltransferase (TPMT) deficiency predisposes to myelotoxicity, but its association with other side effects is less clear. Inosine triphosphatase (ITPA) mutations are other pharmacogenetic polymorphisms possibly involved in thiopurine metabolism and tolerance.

METHODS

We analyzed data from a 6-month prospective study including 71 patients with Crohn disease undergoing first-time aza treatment with respect to aza intolerance. Patients were genotyped for common TPMT and ITPA mutations and had pretherapy TPMT activity measured.

RESULTS

Early drop-out (within 2 weeks) from aza therapy was associated with ITPA 94C > A [P = 0.020; odds ratio (OR), 4.6; 95% confidence interval (95% CI), 1.2-17.4] and low TPMT activity [<10 nmol/(mL erythrocytes . h); P = 0.007; OR = 5.5; 95% CI, 1.6-19.2]. A high-risk group defined by ITPA 94C > A or TPMT <10 nmol/(mL erythrocytes . h) showed significant association with early drop-out (P = 0.001; OR = 11.3; 95% CI, 2.5-50.0) and all drop-outs (P = 0.002; OR = 4.8; 95% CI, 1.8-13.3). For only drop-outs attributable to aza-related side effects (n = 16), there was a significant association with ITPA 94C > A (P = 0.002; OR = 7.8; 95% CI, 2.1-29.1). Time-to-event analysis over the 24-week study period revealed a significant association (P = 0.031) between the time to drop-out and ITPA 94C > A mutant allele carrier status.

CONCLUSIONS

Patients with ITPA 94C > A mutations or low TPMT activity constitute a pharmacogenetic high-risk group for drop-out from aza therapy. ITPA 94C>A appears to be a promising marker indicating predisposition to aza intolerance.

Cloning, expression, and characterization of a human inosine triphosphate pyrophosphatase encoded by the itpa gene

ITP and dITP exist in all cells. dITP is potentially mutagenic, and the levels of these nucleotides are controlled by inosine triphosphate pyrophosphatase (EC ). Here we report the cloning, expression, and characterization of a 21.5-kDa human inosine triphosphate pyrophosphatase (hITPase), an enzyme whose activity has been reported in many animal tissues and studied in populations but whose protein sequence has not been determined before. At the optimal pH of 10.0, recombinant hITPase hydrolyzed ITP, dITP, and xanthosine 5'-triphosphate to their respective monophosphates whereas activity with other nucleoside triphosphates was low. K(m) values for ITP, dITP, and xanthosine 5'-triphosphate were 0.51, 0.31, and 0.57 mm, respectively, and k(cat) values were 580, 360, and 640 s(-1), respectively. A divalent cation was absolutely required for activity. The gene encoding the hITPase cDNA sequence was localized by radiation hybrid mapping to chromosome 20p in the interval D20S113-D20S97, the same interval in which the ITPA inosine triphosphatase gene was previously localized. A BLAST search revealed the existence of many similar sequences in organisms ranging from bacteria to mammals. The function of this ubiquitous protein family is proposed to be the elimination of minor potentially mutagenic or clastogenic purine nucleoside triphosphates from the cell.

As to the clastogenic-, sister-chromatid exchange inducing-and cytotoxic activity of inosine triphosphate in cultures of human peripheral lymphocytes

The influence of commercial inosine triphosphate (ITP) on the chromosome aberration rate, the mitotic rate, sister-chromatid exchange (SCE) frequency, and the proportion of first (X1), second (X2) and third (X3) division metaphases was investigated in 72h cultures of human peripheral lymphocytes. The blood donors had mild inactive arthrosis and a normal health check-up. All cultures of each volunteer were set-up simultaneously. In contrast to a previous report [Arch. Biochem. Biophys. 278 (1990) 238-244], it was demonstrated in two preliminary studies (number of subjects, n=5 each) that ITP at a final concentration of 100 microM does not induce chromosomal aberrations and, furthermore, that not ITP concentrations higher than 100 microM but ITP doses higher than 3.8mM prohibit culture growth. Based on these results, cultures with a final ITP concentration of 3.6mM (max.) and 1.8mM (max./2) were compared with control cultures (number of subjects n=10; three males and seven females, mean age x=57.6 years). Whereas no increase in the chromosomal breakage rate was observed in cultures with an ITP concentration of 1.8mM and only a marginally significant one (P=0.048) for 3.6mM ITP cultures, a highly significant induction of SCEs, not only at an ITP concentration of 3.6mM (P<0.0001) but also at 1.8mM (P<0.0001) was seen. The increase in the SCE frequency was not linear, but steeper from 0 to 1.8mM than from 1.8 to 3.6mM. Nevertheless, the difference between 1.8 and 3.6mM cultures was significant (P=0.027). The distribution of the number of SCEs per metaphase as well as the distribution of SCEs per chromosome correspond to the expected Poisson values. The investigation of the cytotoxic effect of the studied ITP concentrations revealed a highly significant reduction of the mitotic rate from 0 to 1.8mM as well as from 1.8 to 3.6mM in the aberration studies (all P values are equal to smallest possible one for a sample size of 10, namely, 0.002), and in the SCE studies there is a significant decrease in the X3 frequency when ITP is increased (0-1.8mM: P=0.0061 and 1.8-3.6mM: P<0.0001). The proportion of X1 within all X1 and X2 metaphases changes significantly only at the second dose step (0-1.8mM ITP: P=0.22 and 1.8-3.6mM ITP: P<0.0001). The results are discussed.

Clastogenic inosine nucleotide as components of the chromosome breakage factor in scleroderma patients

In the present study, we attempted to identify the chemical nature of the clastogenic factor (CF) from patients with progressive systemic sclerosis (scleroderma). Computerized mass spectrometry of clastogenic fractions obtained by HPLC of plasma ultrafiltrates detected molecular peaks compatible with inosine triphosphate and inosine diphosphate (ITP and IDP). The concomitant detection of IDP, together with ITP, and the absence of these peaks in nonclastogenic fractions and corresponding control fractions are arguments in favor of a biological relevance of these observations. The most important confirmation came from the clastogenic effect of commercial ITP and IDP added to the culture medium of the test cultures. The induction of chromatid type damage by these substances in lymphocytes exposed in the G0 phase of their cell cycle and the prevention of this damage by superoxide dismutase are analogous to the observations with CF.

Inosine/pyruvate/phosphate medium but not adenosine/pyruvate/phosphate medium introduces millimolar amounts of 5-phosphoribosyl 1-pyrophosphate in human erythrocytes. A 31P-n.m.r. study

Incubation of human erythrocytes in medium containing inosine (10 mM), pyruvate (10 mM), phosphate (50 mM) and NaCl (75 mM) at pH 6.6 leads to a more than 1000-fold increase in the concentration of 5-phosphoribosyl 1-pyrophosphate (PRPP), as identified and quantified by 31P-n.m.r. spectroscopy. The accumulation is highly pH-dependent, with a maximum at extracellular pH 6.60, and the maximum value of 1.3-1.6 mmol/l of erythrocytes is attained within 1 h at 37 degrees C. PRPP was accumulated despite high concentrations of 2,3-bisphosphoglycerate (2,3-BPG), an inhibitor of PRPP synthetase. The concentration of PRPP correlated with the intracellular concentration of inorganic phosphate (Pi). Substitution of either adenosine or adenosine plus inosine for inosine in the medium did not lead to 31P-n.m.r.-detectable accumulation of PRPP. These results show that neither 2,3-BPG nor PRPP itself inhibits the synthesis of PRPP in the human erythrocyte. Adenosine, however, prevents the inosine-stimulated accumulation of PRPP.

Incorporation of adenosine and adenine into hypoxanthine nucleotides of fresh red blood cells

Incorporation of adenosine and adenine into hypoxanthine nucleotides of fresh red blood cells was monitored using 8-14C-adenosine and 8-14C-adenine added to the incubation medium containing adenosine, pyruvate and inorganic phosphate (APP medium). Using 8-14C-adenosine it was shown that 21.7% of the isotope contained in the incubation medium penetrated red blood cells. Of that quantity about 50% becomes incorporated into nucleotides. Of the isotope 5.3% was found in hypoxanthine nucleotides (1.3% in ITP and 4.0% in IMP). During incubation of red blood cells in APP medium fortified with the 8-14C-adenine about 95% of isotope penetrated into cells and 60% of that quantity became incorporated into nucleotides. In hypoxanthine nucleotides only trace amounts of isotope were found (0.12% in IMP and 0.13% in ITP).

Inosine di- and triphosphate synthesis in erythrocytes and cell extracts

The ability to synthesize inosinetriphosphate was demonstrated in blood cells as well as in a variety of tissue extracts in spite of the presence of ITP pyrophohydrolase. At the expense of having sub-optimal conditions, an assay system was selected that completely repressed the hydrolyzing enzyme, thus permitting the accumulation of ITP. In an attempt to define the biosynthetic pathway of ITP, and since guanylate kinase has been implicated in the formation of ITP, the rate of synthesis of ITP and GTP in cell extracts was compared. The comparison of the specific activities of the [14C]-labeled hypoxanthine and guanine moieties of the inosine and guanosine phosphates formed during incubation with [8-14C]-inosine and [8-14C]-guanosine respectively, revealed striking differences in the relative rates of isotope incorporation. Tentative mechanisms are proposed to explain these differences. The data obtained thus far does not discard the possibility that ITP may be formed by stepwise phosphorylation and (or) by direct pyrophosphorylation of IMP.

Influence of 2,3-diphosphoglycerate on the buffering properties of human blood: role of the red cell membrane

The effect of the concentration of red cell 2,3-diphosphoglycerate (2,3-DPG, 0.5-21 mumoles/g cells) on the buffering properties and on the slope of the relation between the extracellular and intracellular pH (deltapHi/deltapHe) of human blood was studied. The results were evaluated in connection with previous findings concerning the effect of 2,3-DPG on the Donnan ratio rH+ = H+e/H+i. deltapHi/deltapHe decreases with rising red cell 2,3-DPG content as well as with rising extracellular pH. deltapHi/deltapHe and rH+ can be related to each other by the empirical equation deltapHi/deltapHe = 1 + log rH+ = 1 + pHi - pHe. The validity of this equation appears to be restricted to conditions where the Donnan ratio rH+ is altered between 0.3 and 1 either by changes of the red cell concentration of buffering anions such as 2,3-DPG or by changes of the extracellular pH. As determined in suspensions of red cells with intact membranes, the 2,3-DPG-and pH-induced changes of deltapHi/deltapHe lead to proportional changes in the buffering power of the non-bicarbonate buffers of erythrocytes. Due to this effect the buffering power of suspensions of cells containing 5 times the normal concentration of the buffer 2,3-DPG is lower than that of cells with normal 2,3-DPG content (at extracellular pH values above 7). These findings demonstrate that the action of intracellular non-bicarbonate buffers in blood is effectively modulated by the physico-chemical properties of the red cell membrane.

Individual variation in inosine triphosphate accumulation in human erythrocytes

Erythrocytes from 5% of a normal population accumulated relatively high amounts of radioactive inosine triphosphate (i.e., greater than 70 nmoles/10(10) cells in 2 hr) when they were incubated with [14C]hypoxanthine. The incidence of this characteristic in a mentally retarded population was 16%. Inosine triphosphate was synthesized from [14C]hypoxanthine, but not from [14C]adenine or [14C]guanine. The metabolism of [14C]adenine and [14C]guanine was the same in erythrocytes that accumulated "normal" and "high" amounts of inosine triphosphate. Inosine triphosphate did not accumulate in leukocytes.

ITP pyrophosphohydrolase and IDP phosphohydrolase in rat tissue

The existence of a nucleoside triphosphate pyrophosphohydrolase specific for ITP has been demonstrated in the cytosol fraction of a variety of rat tissues. The enzyme, stable to moderate heat treatment, was present in erythrocytes as well as brain, heart, kidney, liver, lung, muscle, ovaries, spleen, testes and thymus. The specific activity of the enzyme ranges from 26 to 150 mumoles/min/g protein. In addition, evidence is given for a heat labile nucleoside diphosphate (IDP) phosphohydrolase present in most rat tissues, and particularly high in the adrenal (137 mumoles/min/g protein). An "ITP-IMP cycle" is proposed as a rgulating mechanism for intracellular levels of ATP.

Adenosine as a source for hypoxanthine nucleotides synthesis in human erythrocytes. The effect of dipyridamole

Incubation of human erythrocytes in the medium containing adenosine, pyruvate and phosphate causes the synthesis of considerable amounts of IMP and ITP. The synthesis of ITP is linear during the 4-hour incubation period. Dipyridamole lowered the synthesis of ITP to a certain extent and greatly increased the synthesis of IMP.