Trypsin was irradiated as an envelope surrounding colloidal silica which was suspended in aqueous borate buffer. The results obtained from studies of the residual enzymatic activity, and of constituent amino acids destroyed, were compared with those from the enzyme irradiated as a dilute solution in the same buffer. The enzymatic activity to a series of esters and amides was protected at least fifty-fold by the complexing with silica. The relative reactivities, to the synthetic substrates examined, were different for the suspended trypsin from those found with the enzyme freely dissolved. Changes in the three-dimensional structure of the protease on formation of the enveloping monolayer about the colloidal silica, compared with that pertaining in solutions of the enzyme, are the probable causes of the different responses to γ-irradiation observed. Aromatic amino-acid residues were those first affected on irradiation of both the free and complexed trypsin.

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