SINGLET OXYGEN: A MAJOR REACTIVE SPECIES IN THE FUROCOUMARIN PHOTOSENSITIZED INACTIVATION OF E. COLI RIBOSOMES

— The skin photosensitizing furocoumarins, 8-methoxypsoralen (MOP) and 4,5′,8-trimethylpsoralen (TMP), inactivate E. coli ribosomes in vitro, on UV irradiation at 313 nm. Purging the solutions with N2 protects the ribosomes considerably against photoinactivation (75% with MOP and 80% with TMP). In air, the ribosome photoinactivation is mainly due to singlet oxygen (1O2), since the presence of NaN3 and other 1O2 quenchers protects the system and the inactivation is enhanced in D2O. Although 1O2 dominates as the inactivating species, the possibility of additional (∼15%) minor mechanisms involving free radicals exists. However, O-2 does not appear to be the damaging species, since superoxide dismutase does not provide any protection. Photosensitization of the partially purified enzyme, phe-tRNA-synthetase with MOP or TMP shows inactivation and protection curves similar to those seen with the ribosomes. On the other hand, unfrac-tionated tRNAphc is not photosensitized under similar conditions, although it shows self-photosensitization. It is likely that in the furocoumarin-sensitized ribosomes, the primary events of photoinactivation are associated with the proteins.