Application of carbon fiber composite minielectrodes for measurement of kinetic constants of nitric oxide decay in solution

Carbon fiber microelectrodes and carbon fiber composite minielectrodes (CFM/CFCM) have been generally used for measurements of nitric oxide (NO) concentration in chemical and biological systems. The response time of a CFM/CFCM is usually from milliseconds to seconds depending on the electrode size, the thickness of coating layers on the electrode, and NO diffusion coefficients of the coating layers. As a result, the time course of recoded current changes (I–t curves) by the CFM/CFCM may be different from the actual time course of NO concentration changes (c–t curves) if the half-life of NO decay is close to or shorter than the response time of the electrode used. This adds complexity to the process for determining rate constants of NO decay kinetics from the recorded current curves (I–t curves). By computer simulations based on a mathematical model, an approximation method was developed for determining rate constants of NO decay from the recorded current curves. This method was first tested and valuated using a commercial CFCM in several simple reaction systems with known rate constants. The response time of the CFCM was measured as 4.7 ± 0.7 s (n = 5). The determined rate constants of NO volatilization and NO autoxidation in our measurement system at 37 °C are (1.9 ± 0.1) × 10^?3 s^?1 (n = 4) and (2.0 ± 0.3) × 10³ M^?1 s^?1 (n = 7), which are close to the reported rate constants. The method was then applied to determine the rate of NO decay in blood samples from control and smoking exposed mice. It was observed that the NO decay rate in the smoking group is >20% higher than that in control group, and the increased NO decay rate in the smoking group was reversed by 10 μM diphenyleneiodonium chloride (DPI), an inhibitor of flavin enzymes such as leukocyte NADPH oxidase.