Inhibition by Zn(2+) of A-form monoamine oxidase in monkey brain mitochondria

Inhibition of purified bovine liver glutathione reductase with some metal ions

Glutathione reductase (GR; E.C. 1.6.4.2) is a flavoprotein that catalyzes the NADPH-dependent reduction of oxidized glutathione (GSSG). In this study we tested the effects of Al3+, Ba2+, Ca2+, Li+, Mn2+, Mo6+, Cd2+, Ni2+, and Zn2+ on purified bovine liver GR. In a range of 10 microM-10 mM concentrations, Al3+, Ba2+, Li+, Mn2+, and Mo6+, and Ca2+ at 5 microM-1.25 mM, had no effect on bovine liver GR. Cadmium (Cd2+), nickel (Ni2+), and zinc (Zn2+) showed inhibitory effects on this enzyme. The obtained IC50 values of Cd2+, Ni2+, and Zn2+ were 0.08, 0.8, and 1 mM, respectively. Cd2+ inhibition was non-competitive with respect to both GSSG (Ki(GSSG) 0.221 +/- 0.02 mM) and NADPH (Ki(NADPH) 0.113 +/- 0.008 mM). Ni2+ inhibition was non-competitive with respect to GSSG (Ki(GSSG) 0.313 +/- 0.01 mM) and uncompetitive with respect to NADPH (Ki(NADPH) 0.932 +/- 0.03 mM). The effect of Zn2+ on GR activity was consistent with a non-competitive inhibition pattern when the varied substrates were GSSG (Ki(GSSG) 0.320 +/- 0.018 mM) and NADPH (Ki(NADPH) 0.761 +/- 0.04 mM), respectively.

Inhibition of pea ferredoxin-NADP(H) reductase by Zn-ferrocyanide

Ferredoxin-NADP(H) reductases (FNRs) represent a prototype of enzymes involved in numerous metabolic pathways. We found that pea FNR ferricyanide diaphorase activity was inhibited by Zn2+ (Ki 1.57 microM). Dichlorophenolindophenol diaphorase activity was also inhibited by Zn2+ (Ki 1.80 microM), but the addition of ferrocyanide was required, indicating that the inhibitor is an arrangement of both ions. Escherichia coli FNR was also inhibited by Zn-ferrocyanide, suggesting that inhibition is a consequence of common structural features of these flavoenzymes. The inhibitor behaves in a noncompetitive manner for NADPH and for artificial electron acceptors. Analysis of the oxidation state of the flavin during catalysis in the presence of the inhibitor suggests that the electron-transfer process between NADPH and the flavin is not significantly altered, and that the transfer between the flavin and the second substrate is mainly affected. Zn-ferrocyanide interacts with the reductase, probably increasing the accessibility of the prosthetic group to the solvent. Ferredoxin reduction was also inhibited by Zn-ferrocyanide in a noncompetitive manner, but the observed Ki was about nine times higher than those for the diaphorase reactions. The electron transfer to Anabaena flavodoxin was not affected by Zn-ferrocyanide. Binding of the apoflavodoxin to the reductase was sufficient to overcome the inhibition by Zn-ferrocyanide, suggesting that the interaction of FNRs with their proteinaceous electron partners may induce a conformational change in the reductase that alters or completely prevents the inhibitory effect.