Jean-Pierre Calmon

Transplasma-membrane ferricyanide reduction in sycamore cells. Characterization of the system and inhibition by some phenyl biscarbamates

Abstract Evidence is presented for the presence of a transplasma-membrane redox system in sycamore cells, which were found able to reduce external ferricyanide. This reduction induced a simultaneous acidification of the external medium, with a H+/e− stoichiometry of 0.925. Ferricyanide reduction was accompanied by a slight reduction of potassium uptake (15% inhibition). The transmembrane electron transport was inhibited by oligomycin and quinacrine; the effect of the latter inhibitor was only temporary, owing to its progressive absorption by the cells. At 100 μM, several derivatives of the herbicide phenmedipham were able to inhibit the growth of sycamore cells. These compounds inhibited the external acidification induced by fusicoccin, without interfering with the integrity of the plasmalemma. They had no effect on the phosphohydrolase activity of a microsomal fraction enriched in plasmalemma ATPase. They were not uncouplers of oxidative phosphorylation, and appeared as poor inhibitors of mitochondrial electron transfer (I50 > 100 μM). In intact cell suspension, they inhibited both the reduction of ferricyanide and the accompanying acidification of the external medium (I50 = 32 μM). Moreover, they could induce a reversal of the functioning of the redox system, which consequently induced ferrocyanide oxidation.

A nonlinear hammett relationship as evidence for a change-over in mechanism in the alkaline hydrolysis of methyl carbanilates

Abstract A nonlinear Hammett relationship could be used as evidence for a change-over in mechanism in the alkaline hydrolysis of methyl carbanilates. The electron-withdrawing substituted compounds hydrolyse via an A–E pathway (ϱ 1.06) whereas the hydrolysis of the electron-donating substituted compounds involves an E-A scheme.

Uncoupling activity of some N-phenylcarbamates

Abstract In the N-phenylcarbamate family several compounds inhibit cell division and some others prevent the light-driven electron transfer in thylakoids. In this family (general formula R1-NH-COO-R2 where R1 is a phenyl), 18 compounds were investigated for their possible uncoupling activity. Potent uncouplers, able to uncouple fully the oxidative phosphorylation or the photophosphorylation between 1 and 10 μM, were obtained when R2 was 4-NO2C6H4, CH2CHCl2 or CH2CF3 and R1 was a 3,4-Cl2C6H3. The -NH-group of the carbamate function is probably involved in the proton transfer through the two types of biological membranes studied here (mitochondria and thylakoids).

Effects of N,N′-bis-(4-trifluoromethylphenyl)-urea on isolated plant mitochondria and thylakoid membranes

Abstract The N,N′-bis-(4-trifluoromethylphenyl)-urea is demonstrated to be a powerful uncoupler of ATP formation in plant mitochondria and thylakoid membranes. In mitochondria, the full uncoupling effect was obtained at a concentration of 3 μM uncoupling agent in the medium, which corresponded to an inner mitochondrial concentration of 7.2 nmol uncoupling agent mg−1 protein. The full uncoupling concentration was displaced from 3 to 0.6 μM when bovine serum albumin was omitted from the reaction medium. Up to 10 μM, the studied compound acted as a strictly selective uncoupler. At concentrations higher than 10 μM, a selective inhibition of electron transfer occurred at the level of the external NADH dehydrogenase of the inner mitochondrial membrane; the I50 value was 25 μM. In thylakoid membranes, the full uncoupling effect was obtained for a concentration of 0.6 μM uncoupling agent in the medium, which corresponded to a concentration of 25 nmol uncoupling agent mg−1 chlorophyll. The addition of bovine serum albumin (0.1%) to the reaction medium reversed this effect. For concentrations greater than 2 μM, inhibition of PSII electron transfer was observed. Full inhibition was obtained at 15 μM uncoupling agent and corresponded to 650 nmol uncoupling agent mg−1 chlorophyll. In marked contrast with classical uncouplers such as CCCP (m-chlorophenylhydrazone) or substituted phenols, the N,N′-bis-(4-trifluoromethylphenyl)-urea cannot be classified among the protonophoric class of uncouplers, due to its inability to exchange protons at pHs between 2 and 9. Its uncoupling action must therefore be attributed either to conformational changes of a transmembrane protein present both in mitochondria and in thylakoid membranes, or to a modification of the arrangement of the phospholipid membrane bilayers.

Kinetics of the alkaline hydrolysis of several alkyl phenylcarbazates

The kinetics in 25%(v/v) dioxane–water of the alkaline hydrolysis, followed by UV spectrometry, of several alkyl phenylcarbazates (Ar–NH–NHCOOR, with Ar = phenyl, 4-chlorophenyl, 4-fluorophenyl, 3,4-dichlorophenyl and R = methyl, ethyl, propyl, chloroethyl, trichloroethyl) are discussed. The pKa of trichloroethyl phenylcarbazate is 14.12 at 25 °C. The pH profiles, the activation entropy of –40 cal mol–1 K–1, the kinetic solvent isotope effect, kOH–/kOD–, of 3.45, the general-base catalysis and the effect of the leaving group and of the substituent on the aromatic ring are in agreement with the involvement of a BAC2 reaction scheme.

Kinetics and mechanism of hydantoic ring opening. The alkaline hydrolysis of 3-arylimidazolidine-2,4-diones to 5-arylhydantoic acids

The kinetics of hydrolysis of 3-arylimidazolidine-2,4-diones to 5-arylhydantoic acids were studied in aqueous solution over the pH range 8–14 at 25°. In strongly alkaline media (pH > 11.5), an ionisation equilibrium gives rise to an unreactive anion. The pKa values determined both spectrophotometrically and kinetically for the unsubstituted derivative are in good agreement. In mildly alkaline media (pH < 11.5), the observed rates have a first-order dependence on hydroxide ion concentration and no general catalysis is detected. The value of the Hammett parameter (ρ 0.8), the low deuterium isotope solvent effect (kH2O/kD2Oca. 0.7), and the markedly negative values of the entropies of activation (–30 cal mol–1 K–1 < ΔS‡ < –20 cal mol–1 K–1) are rationalized in terms of a rate-determining hydroxide ion attack which is in agreement with the pKa value of the leaving group of 3-arylhydantoins.

Kinetics and mechanism of hydrolysis of 4-phenylallophanates

The kinetics of hydrolysis of aryl and alkyl 4-phenylallophanates to phenylurea and phenol or alchol are studied. Acid–base catalysis, deuterium solvent isotope effects, and entropy of activation provide good evidence for a changeover in mechanism from E1cB for aryl allophanates to BAc2 for alkyl esters. The parameters of the Hammett and Yukawa–Tsuno relationships for aryl esters and the non-linear Bronsted correlation with the pKa values of the leaving groups are in good agreement with the proposed mechanisms.