Drugs online research references
Biochemistry. 1997 Mar 25;36(12):3700-12.
Kinetic mechanism of human hypoxanthine-guanine phosphoribosyltransferase: rapid phosphoribosyl transfer chemistry.
Xu Y, Eads J, Sacchettini JC, Grubmeyer C.
Department of Biochemistry, Temple University School of Medicine, Philadelphia, Pennsylvania 19140, USA.
Hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) is the locus of Lesch-Nyhan syndrome, the activator of the prodrugs 6-mercaptopurine and allopurinol, and a target for antiparasitic chemotherapy. The three-dimensional structure of the recombinant human enzyme in complex with GMP has recently been solved [Eads, J., Scapin, G., Xu, Y., Grubmeyer, C., & Sacchettini, J. C. (1994) Cell 78, 325-334]. Here, ligand binding, pre-steady state kinetics, isotope trapping, and isotope exchange experiments are presented which detail the sequential kinetic mechanism of the enzyme. In the forward reaction, in which a base (hypoxanthine or guanine) reacts with PRPP to form nucleoside monophosphate and PPi, binding of PRPP precedes that of the base, and in the reverse direction, IMP binds first. Compared to k(cat), phosphoribosyl group transfer is rapid in both the forward (131 vs 6.0 s(-1)) and reverse (9 vs 0.17 s(-1)) directions. In the forward direction, product pyrophosphate dissociates rapidly (> 12 s(-1)) followed by release of IMP (6.0 s(-1)). In the reverse direction, Hx dissociates rapidly (9.5 s(-1)) and PRPP dissociates slowly (0.24 s(-1)). The more rapid rate of utilization of guanine than hypoxanthine in the forward reaction is the result of the faster release of product GMP rather than the result of differences in the rate of the chemical step. The kinetic mechanism, with rapid chemistry and slow product dissociation, accounts for the previously observed ability of the alternative product guanine to stimulate, rather than inhibit, the pyrophosphorolysis of IMP. The overall equilibrium for the hypoxanthine phosphoribosyl transfer reaction lies far toward nucleotide product (Keq approximately 1.6 x 10(5)), at the high end for PRPP-linked nucleotide formation. The three-dimensional structure of the HGPRTase x IMP complex has been solved to 2.4 A resolution and is isomorphous with the GMP complex. The results of the ligand binding and kinetic studies are discussed in light of the structural data.
online pharmacy ref source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9132023&dopt=Abstract
bidmc.harvard.edu
Many of the cytopathic effects of nitric oxide (NO*) are mediated by peroxynitrite (PN), a product of the reaction between NO* and superoxide radical (O2*-). In the present study, we investigated the role of PN, O2*- and hydroxyl radical (OH*) as mediators of epithelial hyperpermeability induced by the NO* donor, S-nitroso-N-acetylpenicillamine (SNAP), and the PN generator, 3-morpholinosydnonimine (SIN-1). Caco-2BBe enterocytic monolayers were grown on permeable supports in bicameral chambers. Epithelial permeability, measured as the apical-to-basolateral flux of fluorescein disulfonic acid, increased after 24 h of incubation with 5.0 mM SNAP or SIN-1. Addition of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, an NO* scavenger, or Tiron, an O2*- scavenger, reduced the increase in permeability induced by both donor compounds. The SNAP-induced increase in permeability was prevented by allopurinol, an inhibitor of xanthine oxidase (a source of endogenous O2*-). Diethyldithiocarbamate, a superoxide dismutase inhibitor, and pyrogallol, an O2* generator, potentiated the increase in permeability induced by SNAP. Addition of the PN scavengers deferoxamine, urate, or glutathione, or the OH* scavenger mannitol, attenuated the increase in permeability induced by both SNAP and SIN-1. Both donor compounds decreased intracellular levels of glutathione and protein-bound sulfhydryl groups, suggesting the generation of a potent oxidant. These results support a role for PN, and possibly OH*, in the pathogenesis of NO* donor-induced intestinal epithelial hyperpermeability.
online pharmacy ref source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9813320&dopt=Abstract
Cancer Chemother Pharmacol. 1986;17(3):274-6.
Hyperuricemia and hypoalbuminemia predispose to cisplatin-induced nephrotoxicity.
Nanji AA, Stewart DJ, Mikhael NZ.
The usefulness of pretreatment biochemical parameters in the prediction of nephrotoxicity associated with cisplatin treatment was studied. Twenty-two patients, who received 29 cycles of cisplatin, were evaluated. Cisplatin was given every 3-4 weeks with saline and mannitol. Azotemia occurred in almost all patients and was transient, peaking 1-2 weeks after therapy. The change in serum creatinine from baseline to peak correlated inversely with pretreatment serum albumin (r = -0.73; P less than 0.01) and with pretreatment uric acid (r = 0.76; P less than 0.01). Ten patients with uric acid levels of less than 6 mg/dl were receiving allopurinol. The competition between organic anions and cisplatin for excretion may, in part, explain the protective effects of hypouricemia. Hypoalbuminemia affects peritubular oncotic pressure and may in turn affect platinum excretion. Hypoalbuminemia also reduces the half-life of cisplatin, exposing the kidney to more of the unbound filterable drug.
online pharmacy ref source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3742714&dopt=Abstract
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