Bovine pancreatic ribonuclease A was inactivated by photodynamic treatment at pH 8 with eosin Y, FMN and methylene blue as sensitizers. Enzyme inactivated with methylene blue showed partial aggregation. Tyrosyl fluorescence of the enzyme decreased rapidly during photoinactivation with eosin Y and methylene blue; this resulted from a change in conformation of the enzyme rather than from a destruction of tyrosyl residues. Photodynamic treatment with FMN, but not with eosin Y or with methylene blue, produced a substantial fraction of active enzyme with unusual sensitivity to 5 M urea. Treatment with all three dyes as photosensitizers gave active forms of the enzyme which were more sensitive to heat than native enzyme. These results suggest that ribonuclease, like lysozyme, loses catalytic activity on photodynamic treatment at least in part as a result of conformational changes.

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