Inhibition of chemical oxidation and reduction of cytochromes ƒ and b-559 by carbonylcyanide p-trifluoromethoxy phenylhydrazone

The presence of low (1–4 μM) concentrations of carbonylcyanide p-trifluoromethoxyphenylhydrazone during actinic illumination of chloroplasts generally inhibits the rate of subsequent dark chemical oxidation-reduction reactions of cytochrome ƒ and b-559. Ferricyanide oxidation and ascorbate reduction of cytochromes ƒ and b-559 are inhibited, as is hydroquinone reduction of cytochrome b-559. Inhibition by carbonylcyanide p-trifluoromethoxyphenylhydrazone of hydroquinone reduction of cytochrome ƒ, the most rapid of these chemical oxidation-reduction reactions, cannot be detected. The rate of the chemical redox reactions of the cytochromes in the presence of carbonylcyanide p-trifluoromethoxyphenylhydrazone are all markedly dependent upon the concentration of oxidant or reductant except the hydroquinone reduction of cytochrome b-559 photooxidized in the presence of carbonylcyanide p-trifluoromethoxyphenylhydrazone.

The data is interpreted in terms of an effect of carbonylcyanide p-trifluoromethoxyphenylhydrazone on thylakoid membrane structure which generally inhibits accessibility to the hydrophobic interior of the membrane, possibly through an increase in membrane microviscosity. The question of whether such an effect on membrane structure could be involved in uncoupling or inhibition effects of the carbonylcyanidephenylhydrazone compounds is discussed, as is the special effect of these compounds on the cytochrome b-559 photoreactions at room temperature.