In order to evaluate the contribution of the single tryptophyl residues to the fluorescence emission of elastase, the latter was analyzed as a function of pH, as well as in the presence of some perturbants and quenchers. Moreover, specific tryptophyl residues were converted to formyl-kynurenine by photosensitized oxidation. Our results suggest that the two external tryptophans 26 and 164 account for at least 50% of the total emission; out of the two partially buried tryptophans, the residue 83

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... s practically non-fluorescent, whereas the residue 12 contributes about one fourth of the elastase fluorescence. The residual fluorescence (about 25% of the total emission) is to be ascribed to the three deeply buried tryptophans 39, 132 and 232.