On the interpretation of equilibrium binding studies

Understanding cooperativity in human p450 mediated drug-drug interactions

Multiple drugs can interact, often leading to adverse side effects. One well documented site for these interactions includes the group of cytochrome P450 monoxygenases. Several human hepatic systems are known to bind more than a single substrate molecule which can give rise to the terms "homotropic and heterotopic cooperativity" to define the resultant thermodynamic and kinetic properties observed in drug metabolism investigations. We provide a means for understanding and quantitating these drug-drug interactions by documenting the functional properties of the various states of the enzyme and show that, even in the absence of true binding cooperativity, significant non-Michaelis metabolic profiles are possible.

Binding of nucleotides to guanylate kinase, p21(ras), and nucleoside-diphosphate kinase studied by nano-electrospray mass spectrometry

The binding of nucleotides to three different nucleotide-binding proteins and to a control protein was studied by means of nano-electrospray mass spectrometry applied to aqueous nondenaturing solutions. The method leads to unambiguous identification of enzyme complexes with substrates and products but does not allow the determination of dissociation constants or even stoichiometries relevant to the binding in solution. For guanylate kinase (EC 2.7.4. 8), the transfer of HPO(3) between nucleotides was observed whenever a ternary complex with adenylate or guanylate nucleotides was formed. Guanosine 5'-tetraphosphate was generated after prolonged incubation with GDP or GTP. Mg(2+) binding was considerably enhanced in functional high affinity complexes, such as observed between guanylate kinase and its bisubstrate inhibitor P(1)-(5'-guanosyl)-P(5)-(5'-adenosyl) pentaphosphate or with the tight nucleotide-binding protein p21(ras) and GDP. Nucleoside-diphosphate kinase (EC 2.7.4.6) itself was phosphorylated in accordance to its known ping-pong mechanism. All nucleotide-binding proteins were shown to bind sulfate (SO(4)(2-)) with presumably high affinity and slow exchange rate. The binding of phosphate (PO(4)(3-)) could be inferred indirectly from competition with SO(4)(2-).

Recovery of beta-adrenoceptors and cyclic AMP response after long term treatment of intact heart cells with beta-blockers

We have studied the recovery of receptor binding and of isoprenaline-stimulated cyclic AMP responses after chronic (2-5 days) exposure of tissue-cultured living rat heart cells to several beta-adrenoceptor antagonists. Most experiments were performed with [3H]- (+/-)-carazolol and [3H]-(+/-)-CGP 12177, as prototypes of high affinity lipophilic and hydrophilic ligands respectively. Chronic antagonist treatment did not alter the total number of receptors nor did it cause intracellular accumulation of the ligands. At the end of the treatment, radiolabelled antagonists were displaced either by 'infinite' dilution of the incubation medium or by competitive displacement with the non-labelled ligand (-)-timolol. In dilution assays dissociation of carazolol from specific sites was biphasic with t 1/2 values of 41 +/- 14 and 219 +/- 15 min. Dissociation of CGP 12177 was monophasic with t 1/2 of 102 +/- 2 min. Timolol enhanced the dissociation rates of both radioligands and suppressed the slow phase of carazolol dissociation. Isoprenaline-stimulated cyclic AMP formation did not recover in parallel with the release of the two antagonists from receptor binding sites. To reach about 80% of control values for receptor availability or cyclic AMP response required 3 h and 24 h washout periods, respectively, after carazolol (0.2 nM) treatment, or 1.5 and 12 h washout periods after CGP 12177 (4 nM) treatment. Such a 'decoupling' effect was not observed during recovery from chronic exposure to the antagonists, timolol and propranolol. We conclude that some antagonists cause a novel form of desensitization that is not linked to their partial agonistic potency. Moreover, carazolol-type drugs seem to induce an additional isomeric form of the beta-receptor that is not recognized by other antagonists. These observations could explain the well known discrepancy between long duration of action and rapid removal from the circulation of several antagonists in current therapeutic use.