title: Sensitivity of glycogen phosphorylase isoforms to indole site inhibitors is markedly dependent on the activation state of the enzyme.
abstract: OBJECTIVE
  Inhibition of hepatic glycogen phosphorylase is a potential treatment for glycaemic control in type 2 diabetes. Selective inhibition of the liver phosphorylase isoform could minimize adverse effects in other tissues. We investigated the potential selectivity of two indole site phosphorylase inhibitors, GPi688 and GPi819.
  
  
  METHODS
  The activity of glycogen phosphorylase was modulated using the allosteric effectors glucose or caffeine to promote the less active T state, and AMP to promote the more active R state. In vitro potency of indole site inhibitors against liver and muscle glycogen phosphorylase a was examined at different effector concentrations using purified recombinant enzymes. The potency of GPi819 was compared with its in vivo efficacy at raising glycogen concentrations in liver and muscle of Zucker (fa/fa) rats.
  
  
  RESULTS
  In vitro potency of indole site inhibitors depended upon the activity state of phosphorylase a. Both inhibitors showed selectivity for liver phosphorylase a when the isoform specific activities were equal. After 5 days dosing of GPi819 (37.5 micromol kg(-1)), where free compound levels in plasma and tissue were at steady state, glycogen elevation was 1.5-fold greater in soleus muscle than in liver (P < 0.05).
  
  
  CONCLUSIONS
  The in vivo selectivity of GPi819 did not match that seen in vitro when the specific activities of phosphorylase a isoforms are equal. This suggests T state promoters may be important physiological regulators in skeletal muscle. The greater efficacy of indole site inhibitors in skeletal muscle has implications for the overall safety profile of such drugs.
authors.affiliation: Cardiovascular and Gastrointestinal Discovery Department, AstraZeneca, Macclesfield, Cheshire, UK. sue.freeman@astrazeneca.com
authors.forename: S
authors.initials: S
authors.lastname: Freeman
authors.forename: J B
authors.initials: JB
authors.lastname: Bartlett
authors.forename: G
authors.initials: G
authors.lastname: Convey
authors.forename: I
authors.initials: I
authors.lastname: Hardern
authors.forename: J L
authors.initials: JL
authors.lastname: Teague
authors.forename: S J G
authors.initials: SJ
authors.lastname: Loxham
authors.forename: J M
authors.initials: JM
authors.lastname: Allen
authors.forename: S M
authors.initials: SM
authors.lastname: Poucher
authors.forename: A D
authors.initials: AD
authors.lastname: Charles
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D024981:Glycogen Phosphorylase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D007211:Indoles / Q000494:pharmacology*; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000201:enzymology; D051381:Rats; D011924:Rats, Zucker; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism
chemical_list: D004791:Enzyme Inhibitors; D007211:Indoles; D007527:Isoenzymes; D011994:Recombinant Proteins; D002110:Caffeine; D024981:Glycogen Phosphorylase