Note that thiol-DNIC is formed as an intermediate in both synthesis (V1) as well as decomposition (V2) of RS-NO. Both processes are irreversible under normal physiological conditions due to formation of N2O or disulfides, respectively (see Chapter 2). The equilibrium V3—V4 between DNIC and its constituents is considered as a reversible reaction.

The balance between free thiols and free NO molecules is an important factor in this chemical system. The coupling between the NO and thiol pool is largely determined by the presence of free iron, and involves the formation of intermediate DNIC. As a result, the reaction system reaches a steady-state equilibrium. However, two irreversible processes destroy the stationary state of the reaction system after all. Such irreversible steps include formation of disulfide and dinitrogen monoxide (N2O), respectively. It means that finally the system ensures reduction of NO to N2O under consumption of thiols, i.e. the thiols function as a "fuel" of the system. Evidently, oxygen addition to the system could influence it by decreasing the amount of thiols due to their oxidation.

Numerical simulation [19] of the coupled reaction equations described by Scheme 11 showed that reaction conditions can be chosen such that the DNIC concentration starts to oscillate with time. Such oscillations are well-known from other chemical systems and are caused by the nonlinear concentration dependence of higher order reaction rates. Fig. 19 shows the result of such a simulation.

Oscillatory time dependence of the DNIC concentration was experimentally observed with real-time EPR spectroscopy of 50 ^l aliquots of reaction volume (Fig. 19). These experiments involved the anaerobic decomposition of Cys-NO by ferrous iron in HEPES-buffered solutions at room temperature. We expect that similar oscillatory reaction kinetics will be observed in vitro for RS-NO synthesis in anaerobic mixtures of ferrous iron, free NO radicals and other thiols. The onset, frequency and amplitude of such oscillations depends sensitively on the choice of initial concentrations. It would be very interesting to investigate whether such conditions are within the physiological range as found in cells and tissues.

Was this article helpful?

0 0

Post a comment