Mechanism of Enzyme Inactivation by Ultraviolet Light and the Photochemistry of Amino Acids (At 2537 Å)*

Abstract— >The inactivation of the enzymes chymotrypsin, lysozyme, ribonuclease, and trypsin by ultraviolet light can be accounted for quantitatively by summing the products of (1) the probability that light is absorbed by a given amino acid residue, the molecular extinc tion coefficient, and (2) the probability that absorbed light induces a chemical change in the residue, the quantum yield for the residue. The principal residues involved are cystyl and tryptophanyl. Peptide bond rupture is not important. Energy transfer among chromophores within molecules of enzymes need not be invoked in order to account for photochemical inactivation. Quantum yields for the destruction of a number of amino acids at 2537 A have been measured.