Theory of staircase cyclic voltammetry of two electrode reactions coupled by a chemical reaction

Two reversible electrode reactions that are connected by either reversible or totally irreversible chemical reactions are theoretically analysed by staircase cyclic voltammetry. The dependence of peak potentials on the thermodynamic and kinetic parameters is calculated. If the mechanism is permanently in equilibrium, the stability constant of the reversible chemical reaction can be determined. Furthermore, the critical kinetic parameter is determined and its application to the measurement of the forward rate constant of the chemical reaction is demonstrated. Also, the influence of the kinetics of electrode reactions is discussed. Keywords: ECE mechanism; Cyclic voltammetry; Theory

Spectral and Electrochemical Studies of Phenazopyridine

The spectral and voltammetric behavior of phenazopyridine (1) in aqueous buffers of varied pH is presented. Spectrophotometry, cyclic voltammetry, differential pulse polarography and coulometry were utilized to study its proton and electron transfer characteristics. Relevant thermodynamic and electrochemical data such as pKa, charge transfer co-efficient (αna), forward rate constant (k0f,h),etc., have been evaluated. An excellent electroanalytical method of assaying of 1 has been developed in differential pulse polarography at pH = 8. Molecular modeling on various acid-base conjugates of 1 and their several conformers has been carried out to arrive at the thermodynamic and conformational issues to correlate the spectral and electrochemical observations.

Palytoxin-induced Effects on Partial Reactions of the Na,K-ATPase

The interaction of palytoxin with the Na,K-ATPase was studied by the electrochromic styryl dye RH421, which monitors the amount of ions in the membrane domain of the pump. The toxin affected the pump function in the state P-E2, independently of the type of phosphorylation (ATP or inorganic phosphate). The palytoxin-induced modification of the protein consisted of two steps: toxin binding and a subsequent conformational change into a transmembrane ion channel. At 20°C, the rate-limiting reaction had a forward rate constant of 105 M−1s−1 and a backward rate constant of about 10−3 s−1. In the palytoxin-modified state, the binding affinity for Na+ and H+ was increased and reached values between those obtained in the E1 and P-E2 conformation under physiological conditions. Even under saturating palytoxin concentrations, the ATPase activity was not completely inhibited. In the Na/K mode, ∼50% of the enzyme remained active in the average, and in the Na-only mode 25%. The experimental findings indicate that an additional exit from the inhibited state exists. An obvious reaction pathway is a slow dephosphorylation of the palytoxin-inhibited state with a time constant of ∼100 s. Analysis of the effect of blockers of the extracellular and cytoplasmic access channels, TPA+ and Br2-Titu3+, respectively, showed that both access channels are part of the ion pathway in the palytoxin-modified protein. All experiments can be explained by an extension of the Post-Albers cycle, in which three additional states were added that branch off in the P-E2 state and lead to states in which the open-channel conformation is introduced and returns into the pump cycle in the occluded E2 state. The previously suggested molecular model for the channel state of the Na,K-ATPase as a conformation in which both gates between binding sites and aqueous phases are simultaneously in their open state is supported by this study.

Myocardial creatine kinase kinetics and isoform expression in hearts with severe LV hypertrophy

Left ventricular (LV) hypertrophy (LVH) results in a fetal shift in myocardial creatine kinase (CK) expression. Because CK plays an important role in intracellular energy production, transport, and utilization, this study was performed to characterize changes in CK expression and CK flux in severe pressure-overload LVH. Ascending aortic banding in 8-wk-old dogs resulted in LVH with a 92% increase in relative LV mass. In LVH hearts, CK-M isoform mRNA was decreased by 40% ( P = 0.05) and protein was decreased by 50% ( P < 0.01), whereas mitochondrial CK protein was decreased by 22% ( P < 0.05). CK-B isoform mRNA was undetectable in normal hearts but was prominently expressed in LVH ( P< 0.01); CK-B protein was increased by more than 10-fold in LVH ( P < 0.01). Despite these changes, total CK activity was normal in LVH. Myocardial CK flux was examined using31P magnetic resonance spectroscopy magnetization transfer. The CK forward rate constant was similar in normal and LVH hearts at baseline and did not change in either group during dobutamine treatment. In hearts with LVH, the CK forward flux rate was reduced by ∼60% ( P < 0.05) and decreased further during dobutamine. Thus, although pressure-overload LVH caused alterations of expression of both CK mRNA and protein levels, LV performance and oxygen consumption in response to dobutamine were normal. However, myocardial free ADP was increased in LVH hearts. This finding suggests that the CK alterations result in a need for higher ADP levels to maintain ATP synthesis in the hypertrophied heart.

Volume-Sensitive K+/Cl− Cotransport in Rabbit Erythrocytes

The kinetics of activation and inactivation of K+/Cl− cotransport (KCC) have been measured in rabbit red blood cells for the purpose of determining the individual rate constants for the rate-limiting activation and inactivation events. Four different interventions (cell swelling, N-ethylmaleimide [NEM], low intracellular pH, and low intracellular Mg2+) all activate KCC with a single exponential time course; the kinetics are consistent with the idea that there is a single rate-limiting event in the activation of transport by all four interventions. In contrast to LK sheep red cells, the KCC flux in Mg2+-depleted rabbit red cells is not affected by cell volume. KCC activation kinetics were examined in cells pretreated with NEM at 0°C, washed, and then incubated at higher temperatures. The forward rate constant for activation has a very high temperature dependence (Ea ∼ 32 kCal/mol), but is not affected measurably by cell volume. Inactivation kinetics were examined by swelling cells at 37°C to activate KCC, and then resuspending at various osmolalities and temperatures to inactivate most of the transporters. The rate of transport inactivation increases steeply as cell volume decreases, even in a range of volumes where nearly all the transporters are inactive in the steady state. This finding indicates that the rate-limiting inactivation event is strongly affected by cell volume over the entire range of cell volumes studied, including normal cell volume. The rate-limiting inactivation event may be mediated by a protein kinase that is inhibited, either directly or indirectly, by cell swelling, low Mg2+, acid pH, and NEM.

31P-saturation-transfer nuclear-magnetic-resonance measurements of phosphocreatine turnover in guinea-pig brain slices

The technique of 31P saturation-transfer n.m.r. was used to determine the forward and the reverse rate constants of creatine phosphotransferase in superfused guinea-pig cerebral tissues in vitro. The calculated forward rate constant of 0.22 +/- 0.03s-1 compared well with a previously reported value for rat brain in vivo [Shoubridge, Briggs & Radda (1982) FEBS Lett. 140, 288-292]. The reverse rate constant was found to be 0.55 +/- 0.10s-1. 3. By using concentrations of ATP and phosphocreatine estimated previously for this superfused preparation [Cox, Morris, Feeney & Bachelard (1983) Biochem. J. 212, 365-370], forward and reverse flux rates were calculated to be 0.68 and 0.72 mumol X s-1 X g-1 respectively. The concordance of forward and reverse fluxes contrasts with the situation observed in vitro in other tissues, and suggests that the creatine phosphotransferase reaction is at equilibrium under the conditions used here. 4. Lowering the concentration of glucose in the superfusing medium from 10mM to 0.5mM had no significant effect on phosphocreatine concentration or on the forward (ATP-generating) flux through creatine phosphotransferase. The results indicate that a normal phosphocreatine content in the presence of lowered glucose availability is reflected by an unchanged turnover rate.

Ligand substitution kinetics of iron(III) in aprotic solvents

The complexation reactions of NCS- and Br- with iron(III) have been studied in N,N-dimethyl-formamide, and the temperature activation parameters determined. The observed forward rate constants are 114±5 and 69±31, mol-1 s-1, the activation enthalpies are 48+±3 and 74±4 kJ mol-1, and the activation entropies are -45±8 and 40±10 J K-1 mol-1, respectively, at 298 K and 0.5 M ionic strength. The complexation reactions of NCS- with iron(III) in dimethyl sulfoxide have been studied at ionic strengths of 0.1, 0.3 and 0.5 M, and over a range of temperatures. The observed forward rate constant is 55±31. Mol-1 s-1, the activation enthalpy is 42±3 kJ mol-1, and the activation entropy is -70±30 J K-1 mol-1 at 298 K and 0.5 M ionic strength. It is proposed that the substitution reactions of iron(111) in the aprotic solvents N,N-dimethyl- formamide and dimethyl sulfoxide proceed through an associative interchange (Ia) mechanism.