Photochemical evidence for the presence of histidyl residue in the active site of alkaline mesentericopeptidase.

The photosensitized oxidation of alkaline mesentericopeptidase in the presence of methylene blue results in a first-order rate of inactivation. The loss of enzymatic activity towards casein and N-acetyl-L-tyrosine ethyl ester closely correlates with the destruction of one histidyl residue. A pK value of 6.8 is determined from the sigmoid pH-dependence of the photoinactivation rate. This suggests the involvement of a normal titrating imidazole group in the active site of mesentericopeptidase. The competitive inhibitor Na-benzoyl-L-arginine protects the enzyme from photoinactivation. A conclusion is made that the active site histidyl residue is modified. Circular dichroism spectra show no change in the protein conformation during the photodynamic treatment.