The electron transport mechanism in the bovine sub-mitochondrial particles (SMP) complex I assay (as in Jewess et al [49]). In a buffer enriched with excess NADH, Coenzyme Q1 and DCIP, SMP containing bovine complex I enzyme oxidize NADH to NAD+ and reduce Coenzyme Q1 to Coenzyme Q1H2, resulting in a decline of NADH absorbance at 340nm. Coenzyme Q1H2 reduces DCIP, resulting in a decrease in absorbance at 620nm. The decrease in absorbance at 340nm, and also at 620nm continues until all exogenously added NADH is oxidized. Antimycin A (an inhibitor of complex III) prevents Coenzyme Q1H2 from passing reducing components to complex III, and KCN (an inhibitor of complex IV) prevents cytochrome oxidase activity of complex IV.