Biphasic reduction, cytochrome

It has been believed that P-450 reduction by NADPH cytochrome P-450 reductase is a biphasic process, but it was recently shown [7] that some P-450 cytochromes are reduced with single-exponential kinetics and that the presence of substrate is not an obligatory condition for the reduction of all P-450 forms. Thus, the kinetics of reduction of various ferric P-450 cytochromes possibly depends on many factors such as substrate, rate-limiting step, etc. 

Because electrode measurements of O2 uptake can detect intra- and extracellular oxidase activity, this assay can be used to measure the respiratory burst elicited by soluble and particulate stimuli. What is somewhat surprising is that, during stimulation of neutrophils with agonists such as fMet-Leu-Phe, the activated O2 uptake profile is biphasic (Fig. 5.11c). A rapid burst of O2 uptake (which coincides with measurements of cytochrome c reduction) is followed by a more sustained activity of lower magnitude. 

The kinetics of reduction by ejq and COa " radicals of cytochrome from Desulfovibrio vulgaris have been reported. The four haem groups in the molecule are kinetically equivalent towards attack by e q (k-2x 10 M s ), but COa" reduces only two of the haem groups rapidly (A =2.1 x 10 M s ). These biphasic kinetics are analogous to those observed for reduction by SOa (A = 6.8 and 2.1 x 10 s" ), and the data are consistent with lack of electron... 

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