Activities of mitochondrial electron transport chain complex I, complex II, and complex IV
were performed (Mitosciences, USA), and activities were expressed as fold change as described earlier .
NADH ubiquinone oxidoreductase (complex I), succinate-cytochrome c reductase (complex II/III), and cytochrome c oxidase (complex IV
) were measured using the substrates coenzyme Q2, succinate-cytochrome c, and reduced cytochrome c, respectively, by the methods of Birch-Machin and Turnbull  as described before [35,36].
The majority of complex I is found bound with a complex III dimer and complex IV
(CI, [CIII.sub.2], and CIV) termed respirasome or with a complex III dimer alone (CI, [CIII.sub.2]).
Shinde and Pasupathy  reported disturbances in mitochondrial respiration in lymphocytes in patients with PD on the basis of the activity measurements of succinate cytochrome c reductase (complexes II and III), rotenone-sensitive NADH cytochrome c reductase (complexes I and III), citrate synthase, succinate dehydrogenase (complex II), and cytochrome c oxidase (complex IV
In a following step the cytochrome c oxidase was maximally activated by reduction of cytochrome c with TMPD and ascorbic acid and the Na[N.sub.3]-insensitive portion of stimulation was used to indicate complex IV
 found that, in the synaptic mitochondria of hAPP mice, both complex IV
(cytochrome c oxidase) activity and respiratory control ratio decreased, while oxidative stress (measured as 4-hydroxynonenal and hydrogen peroxide levels) increased in comparison with wild-type (WT) mice.
Otherwise, artificial substrates such as ascorbate/TMPD (10 mM and 100 [micro]M, respectively, in the presence of 2 [micro]M antimycin) were used for complex IV
activity, and malonate and KCN were added to inhibit complex IV
and complex II (10 mM and 5 mM, respectively).
These complexes are known as nicotinamide adenine dinucleotide (NADH): ubiquinone oxidoreductase (complex I), succinate dehydrogenase (complex II), ubiquinol-cytochrome c oxidoreductase (complex III), cytochrome c oxidase (complex IV
) and ATP synthase (complex V).
H2S is a readily water-soluble gas, which at higher concentrations becomes toxic by binding to and inhibiting cytochrome C oxidase in complex IV
of the electron transport chain, the last complex in the chain prior to ATP synthesis.
activity was measured at 550 nm in a buffer containing 10 mM potassium phosphate (pH 7.2), 1mg/mL BSA, and the cell/tissue lysate.