Proton relaxation study of the hog kidney diamine oxidase active center.

Proton relaxation studies of the interactions with hog kidney diamine oxidase of water, substrate-analogue inhibitors, and product analogues indicate that the active site Cu(II) is not located near the oxidizing site of the enzyme, rather near the nonoxidized end of the binding substrate. The studies with histamine derivatives provide evidence for a concentration-dependent occupation of two sites. The site which is populated at high concentrations provides proximity of the imadazole ring nitrogen N1 to the Cu(II). Water binds at the Cu(II) of the native enzyme. However, this water is probably not involved in the hydrolysis of the enzyme-substrate imine bond to eliminate the first reaction product. O2 does not compete with H2O for a site on the Cu(II) ion. In the case of one of the probes, namely the ammonia (product) analogue dimethylamine, the validity of the protein relaxation results was verified by also observing the nitrogen (15N) relaxation rates of ammonia itself. The conclusion that the ammonium ions is not directly bonded to the active site Cu(II) is reached from both the proton and nitrogen relaxation experiments.