INHIBITION OF OXYGEN EVOLUTION IN CHLOROPLAST PHOTOSYSTEM-II BY THE PROTEIN-MODIFYING AGENT TETRANITROMETHANE

The protein-modifying agent tetranitromethane (TNM) reacts with tyrosine residues and -SH groups. It was found to inhibit photosynthetic electron transport on the water splitting side of photosystem II (P. V. Sane and U. Johanningmeier, Z. Naturforsch. 35c, 293-297, 1979). In the present work the inhibition by TNM is studied in detail using photosystem II submembrane fractions. It is shown that the action of TNM with membrane-bound proteins could imply the modification of tyrosine residues. At concentrations below 30 mu M and with short incubation periods (<2 min), TNM produces the release of the extrinsic polypeptides involved in the stabilization of the water-splitting complex, this being correlated with inhibition of electron transport at a site prior to H2O2 electron donation even though the inhibition cannot be prevented by the addition of Cl- or Ca2+, which are known cofactors for oxygen evolution. As the incubation period or the concentration of TNM is increased, photosynthetic pigments are bleached, starting with aggregates absorbing at relatively long wavelengths. The inhibition by low concentrations of TNM differs from the effect of most of the previously reported inhibitors acting at the oxygen-evolving complex of photosystem II.