Superoxide can reduce cytochrome c in the intermembrane space (Wegerich et al. 2013, and inferred from other mammals in Butler et al. 1975, Koppenol et al. 1976, Butler et al. 1982). Superoxide has been shown in rat and mouse mitochondria to be released into the intermembrane space by the complex III of the respiratory chain (Han et al. 2001, Muller et al. 2004).