INVOLVEMENT OF ESSENTIAL HISTIDINE RESIDUE(S) IN THE ACTIVITY OF EHRLICH CELL PLASMA-MEMBRANE NADH-FERRICYANIDE OXIDOREDUCTASE

The existence of histidine residue(s) implicated in the active site of NADH-ferricyanide oxidoreductase in plasma membrane vesicles isolated from Ehrlich ascites tumour cells is investigated. The shape of the p H-dependence curve of the enzyme activity suggests that one or more hi stidine residues are located at (or near) the active site of the enzym e. This hypothesis is supported by the following experimental data: th e loss of activity after treatment with diethyl pyrocarbonate (DEPC) o r photooxidation by using Rose bengal, and the strong inhibition cause d by Zn2+ ions at micromolar concentrations. The combined arguments su pport the statement that histidine plays an essential role in the cata lytic activity of NADH-ferricyanide oxidoreductase from Ehrlich ascite s tumour cells.