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

The protein-modifying agent tetranitromethane (TNM) reacts with tyrosi ne residues and -SH groups. It was found to inhibit photosynthetic ele ctron transport on the water splitting side of photosystem II (P. V. S ane and U. Johanningmeier, Z. Naturforsch. 35c, 293-297, 1979). In the present work the inhibition by TNM is studied in detail using photosy stem II submembrane fractions. It is shown that the action of TNM with membrane-bound proteins could imply the modification of tyrosine resi dues. At concentrations below 30 mu M and with short incubation period s (<2 min), TNM produces the release of the extrinsic polypeptides inv olved in the stabilization of the water-splitting complex, this being correlated with inhibition of electron transport at a site prior to H2 O2 electron donation even though the inhibition cannot be prevented by the addition of Cl- or Ca2+, which are known cofactors for oxygen evo lution. As the incubation period or the concentration of TNM is increa sed, photosynthetic pigments are bleached, starting with aggregates ab sorbing at relatively long wavelengths. The inhibition by low concentr ations of TNM differs from the effect of most of the previously report ed inhibitors acting at the oxygen-evolving complex of photosystem II.