Evidence for the extracellular reduction of ferricyanide by rat liver. A trans-plasma membrane redox system

1. Reduction of ferricyanide by the isolated perfused rat liver and by isolated rat hepatocytes was studied. 2. Ferricyanide was reduced to ferrocyanide by the perfused liver at a linear rate of 0.22mumol/min per g of liver. Ferricyanide was not taken up by the liver and the perfusate concentration of ferricyanide+ferrocyanide remained constant throughout the perfusion. Perfusate samples from livers perfused without ferricyanide did not reduce ferricyanide. 3. Isolated hepatocytes reduced ferricyanide in a biphasic manner. The initial rate of 2.3mumol/min per g of cells proceeded for approx. 3min and derived from low-affinity sites (apparent K(m)>1.3mm). The secondary rate of 0.29mumol/min per g of cells was maintained for the remainder of the incubation and derived from higher affinity sites (apparent K(m)0.13mm). Disruption of the cells resulted in an increase in the low-affinity rate and a decrease in the high-affinity rate. 4. Ferrocyanide was oxidized by isolated hepatocytes but not by perfused liver. The apparent K(m) for ferrocyanide oxidation by hepatocytes was 1.3mm. 5. Oxidized cytochrome c was reduced by isolated hepatocytes in the presence of 1mm-KCN but at a rate less than that of the reduction of ferricyanide. 6. Properties of the ferricyanide-reducing activities of intact hepatocytes and the perfused liver were examined. The low-affinity rate, present only in cell and broken cell preparations, was inhibited by 1mum-rotenone and 0.5mm-ferrocyanide, and stimulated by 0.1mm-KCN. The mitochondrial substrate, succinate, also stimulated this rate. The perfused liver showed only a high-affinity activity for ferricyanide reduction. This activity was also present in liver cells and was unaffected by rotenone, antimycin A, KCN, NaN(3), or p-hydroxymercuribenzoate but was inhibited by 2.6mm-CaCl(2), 2-heptyl-4-hydroxyquinoline-N-oxide and ferrocyanide. Overall, these results are consistent with the occurrence of a trans-plasma membrane redox system of liver that reduces extracellular ferricyanide to ferrocyanide. The reduction process shows properties which are similar to that of the NADH:ferricyanide oxidoreductase found in isolated liver plasma membranes but different from that of mitochondria.