Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0023418 (leukemia)
 93,477 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The bifunctional enzyme 5-aminoimidazole-4-carboxamide ribotide (AICAR) transformylase-IMP cyclohydrolase has been purified 780-fold to apparent homogeneity from human CCRF-CEM leukemia cells, completed with chromatography on Affi-Gel Blue followed by AICAR-Sepharose 4B. Using a sensitive radioassay, IMP cyclohydrolase has a Ks value for 5-formamidoimidazole-4-carboxamide ribotide (FAICAR) at pH 7.4 of 0.87 +/- 0.11 microM. The following purine nucleotide derivatives were potent competitive inhibitors of IMP cyclohydrolase: 2-mercaptoinosine 5'-monophosphate (Ki = 0.094 +/- 0.024 microM), xanthosine 5'-monophosphate (Ki = 0.12 +/- 0.01 microM), 2-fluoroadenine arabinoside 5'-monophosphate (Ki = 0.16 +/- 0.02 microM), 6-mercaptopurine riboside 5'-monophosphate (Ki = 0.20 +/- 0.02 microM), adenosine N1-oxide 5'-monophosphate (Ki = 0.28 +/- 0.03 microM), and N6-(carboxymethyl)adenosine 5'-monophosphate (Ki = 1.7 +/- 0.42 microM). The pH dependencies of Vmax and Vmax/Ks values for IMP cyclohydrolase are consistent with a single ionizable amino acid residue (pKa = 7.57 +/- 0.09) of the enzyme which must be unprotonated for catalysis to occur and a residue (pKa = 7.57 +/- 0.14) which must be unprotonated for FAICAR to bind. The pKa values of 5.81 +/- 0.03 and 9.41 +/- 0.04 determined for FAICAR indicate that ionization of the substrate does not contribute significantly to the pH effects observed. Chemical modification of IMP cyclohydrolase provides evidence for arginine and cysteine residues at the active site, and roles for these residues in the mechanism of catalysis are proposed.
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PMID:5-Aminoimidazole-4-carboxamide ribotide transformylase-IMP cyclohydrolase from human CCRF-CEM leukemia cells: purification, pH dependence, and inhibitors. 794 35

The catalytic mechanism for the enzyme, IMP cyclohydrolase, may involve a reaction intermediate with negative charge in the 2-position of the purine ring (Szabados, E., Hindmarsh, E., Phillips, L., Duggleby, R.G. & Christopherson, R.I. (1994) Biochemistry 33, 14237-14245). Three analogues of IMP have been synthesised where fluorine, chlorine or bromine has been substituted in the 2-position on the purine ring. These analogues with an electronegative substituent may resemble a reaction intermediate for IMP cyclohydrolase; 2-fluoro IMP is a potent inhibitor of the enzyme with a Ki value of 0.19 microM, while 2-chloro IMP has a Ki of 1.9 microM and 2-bromo IMP is not inhibitory. However, IMP cyclohydrolase is not inhibited in human CCRF-CEM leukaemia cells exposed to 2-fluoro inosine although it is toxic to these cells with an IC50 value of 4.9 microM.
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PMID:Inosine-5'-monophosphate analogues as inhibitors of human IMP cyclohydrolase and cellular growth. 955 23