The opposite effect of bivalent cations on cytochrome b5 reduction by NADH:cytochrome b5 reductase and NADPH:cytochrome c reductase.

The effects of bivalent cations on cytochrome b5 reduction by NADH:cytochrome b5 reductase and NADPH:cytochrome c reductase were studied with the proteinase-solubilized enzymes. Cytochrome b5 reduction by NADH:cytochrome b5 reductase was strongly inhibited by CaCl2 or MgCl2. When 1.2 microM-cytochrome b5 was used, the concentrations of CaCl2 and MgCl2 required for 50% inhibition (I50) were 8 and 18 mM respectively. The inhibition was competitive with respect to cytochrome b5. The extent of inhibition by CaCl2 or MgCl2 was much higher than that by KCl or other alkali halides. In contrast, cytochrome b5 reduction by NADPH:cytochrome c reductase was extremely activated by CaCl2 or MgCl2. In the presence of 5 mM-CaCl2, the activity was 24-fold higher than control when 4.4 microM-cytochrome b5 was used. The magnitude of activation by CaCl2 was 2-3-fold higher than that by MgCl2. The activation by these salts was much higher than that by KCl, indicating that bivalent cations play an important role in this activation. The mechanisms of inhibition and activation by bivalent cations of cytochrome b5 reduction by these two microsomal reductases are discussed.