The microsomal electron transport complex, in particular the segment associated with cytochrome P-450 function, is both qualitatively and quantitatively an important contributor to the cellular respiration of tissues such as liver. Although our knowledge is still limited, it is apparent that oxygen plays a pivitol role in dictating the mode of substrate hydroxylation or the generation of hydrogen peroxide. As illustrated in Figure 7, current evidence suggests that hydrogen peroxide is formed by the dismutation of the superoxide anion resulting from the dissociation of oxycytochrome P-450. Of interest are recent studies demonstrating the ability of hydrogen peroxide to initiate a cytochrome P-450 dependent peroxidatic reaction competent for supporting substrate hydroxylation. The fact that the function of cytochrome P-450 is sensitive to changes in oxygen tension establishes its role as an "oxygen sensor" for cellular metabolism.