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 N 2 protects the ribosomes considerably against photoinactivation (75% with MOP and 80% with TMP). In air, the ribosome photoinactivation is mainly due to singlet oxygen ( 1 O 2 ), since the presence of NaN 3 and other 1 O 2 quenchers protects the system and the inactivation is enhanced in D 2 O. Although 1 O 2 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 tRNA phc 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.
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Singh et al. (1978) studied this question.