Study of isotope effects on protein binding by gas chromatography/mass spectrometry of theophylline-phenobarbitone and 2H, 13C, 15N isotopomers

Deuterium isotope effects on noncovalent interactions between molecules

The topic of deuterium isotope effects is usually concerned with the effects on chemical reactions that are caused by the substitution of deuterium atoms for protium, or hydrogen, atoms in a molecule. These effects include changes in the rate of cleavage of covalent bonds to deuterium, or to an atom located adjacent to deuterium, in a reactant molecule. Deuterium isotope effects on other, noncovalent, interactions between molecules are known to occur, but they are generally considered to be insignificant, especially in biological experiments where deuterium substituted molecules are used as tracers. Noncovalent interactions between molecules include hydrogen bonding, and ionic and van der Waals interactions. This article reviews evidence for deuterium isotope effects on noncovalent interactions, with an emphasis on binding interactions between molecules of biological interest, but also including examples of nonbiological molecules in order to demonstrate the generality of these effects. The reality of this effect relies on the assumption that the only difference between the isotopomers considered is the presence of deuterium or hydrogen; there are no impurities present. The physical basis of the effect may be due to differences in the polarities and/or sizes of deuterated versus nondeuterated isomers, and the extent of a deuterium isotope effect on a noncovalent interaction depends on the site of deuteration within a biomolecule. The presence of this effect requires careful interpretation of results obtained in experiments with deuterium labeled compounds.

Isotopic effects on retention times of caffeine and its metabolites 1,3,7-trimethyluric acid, theophylline, theobromine and paraxanthine

Physicochemical parameters that influence gas chromatographic separation are numerous. Consequently, isotope labelling, because it modifies physicochemical properties, can induce isotopic effects on retention time. Caffeine has been chosen to study this influence because as itself and its metabolites, it allows the preparation of different methylxanthine isotopomers and thus is one of the best models to study isotopic effects induced by stable isotope labelling. Using a caffeine molecule labelled with deuterium at different positions and rat hepatocytes to obtain metabolites, it was possible to study the influence of labelling on retention time [(14% cyanopropylphenyl)methylpolysiloxane] and to point out the role of each labelled site. It appears that isotopic effects induced by the labelling depend not only on the number of labelling atoms but also on whether this labelling is at position 1, 3 or 7 and, consequently, on the role of the labelled site on the function of the molecule.

Pharmacokinetic equivalence of 5(ethyl(2H)5)- and unlabelled phenobarbitone

The present study shows the absence of in vivo pharmacokinetic isotope effect on phenobarbitone (PB) C5-ethyl deuteration (PBd5) following oral administration to man of equimolar PB/PBd5 mixtures (0.40 mmol each). Plasma PB and PBd5 (17 days) and urine PB, PBd5 and parahydroxy-metabolites (PBOH, PBHOd5) levels were determined by GC-MS. Isotope effect research includes comparison of pharmacokinetic parameters, study of time-dependence of isotope ratios (IRs) in plasma and urine (linearity test), comparison of IRs between samples and administered mixtures (Mann Whitney's test) and comparison of PBOH/PBOHd5 ratios before and after urine enzymatic hydrolysis (Student's two tailed t-test). No significant isotope effect was observed on pharmacokinetic parameters, PB hydroxylation or PBOH conjugation (x less than or equal to 5%); which the absence of pentadeuteration-induced alteration in PB's HSA binding parameters (binding mode, Ka, N) corroborates (x less than or equal to 5%). These results establish bioequivalence of PB and PBd5; the latter can be used with benefit in stable-isotope clinical pharmacology (steady state pharmacokinetics, drug interactions...) investigations as well as bioavailability studies of PB preparations.