title: Increased activity, amount, and altered kinetic properties of IMP dehydrogenase from mycophenolic acid-resistant neuroblastoma cells.
abstract: Mouse wild-type neuroblastoma cells (NB cells) were stepwise selected for 10,000-fold increased resistance to mycophenolic acid (NB-Myco cells), an inhibitor of IMP dehydrogenase (IMP:NAD+ oxidoreductase, EC 1.1.1.205). IMP dehydrogenase activity was increased 25-fold, from 3.1 to 75 nmol/min.mg of protein; and a 56.7-kDa peptide was increased in abundance 200-500-fold in NB-Myco as compared to NB cells. Purification and sequence analysis confirmed that the abundant protein was IMP dehydrogenase. The stepwise selection, increased activity and protein abundance, and unstable phenotype are indirect evidence for a process of gene amplification. Kinetic findings consistent with an Ordered Bi Bi mechanism were indicative of IMP dehydrogenase having undergone mutation. The Michaelis constants were unchanged for IMP (14 and 13 microM) and increased 4-fold for NAD from 25 to 94 microM for NB and NB-Myco cells, respectively. The Ki for mycophenolic acid was increased 2400-fold from 1.4 nM to 3.4 microM for the enzyme from NB versus NB-Myco cells, and the Ki for XMP was increased 4-fold from 78 to 336 microM. Mycophenolic acid exhibited uncompetitive inhibition with IMP, consistent with the formation of a dead end E-XMP-inhibitor complex. The cellular GTP concentration was increased 2-fold in resistant cells and, upon removal of mycophenolic acid, further increased to 4.5-fold that of NB cells.
authors.affiliation: Department of Medical Biochemistry, Faculty of Medicine, University of Calgary, Alberta, Canada.
authors.forename: S D
authors.initials: SD
authors.lastname: Hodges
authors.forename: E
authors.initials: E
authors.lastname: Fung
authors.forename: D J
authors.initials: DJ
authors.lastname: McKay
authors.forename: B S
authors.initials: BS
authors.lastname: Renaux
authors.forename: F F
authors.initials: FF
authors.lastname: Snyder
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D004351:Drug Resistance; D006801:Humans; D007168:IMP Dehydrogenase / Q000302:isolation & purification / Q000378:metabolism*; D007658:Ketone Oxidoreductases / Q000378:metabolism*; D007700:Kinetics; D051379:Mice; D008969:Molecular Sequence Data; D009173:Mycophenolic Acid / Q000494:pharmacology*; D009447:Neuroblastoma; D010446:Peptide Fragments / Q000302:isolation & purification; D012689:Sequence Homology, Nucleic Acid; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000201:enzymology*
chemical_list: D010446:Peptide Fragments; D007168:IMP Dehydrogenase; D007658:Ketone Oxidoreductases; D009173:Mycophenolic Acid