The protonated form of 1-N6-etheno-[erythro-9-(2-hydroxy-3-nonyl)] adenine is identified at the active site of adenosine deaminase.

A novel fluorescent competitive inhibitor of adenosine deaminase (EC 3.5.4.4) (ADA), 1-N6-etheno-[erythro-9-(2-hydroxy-3-nonyl)] adenine (epsilon-EHNA), is protonated at the active site of the enzyme. In epsilon-EHNA [K1 = (4.06 +/- 1.00) 10(-6) M] part of the competitive inhibition of EHNA is combined with spectroscopic properties of etheno-adenines. Computer subtraction of the fluorescence excitation spectrum of ADA from that of its equimolar complex with epsilon-EHNA yielded the corrected excitation spectrum of epsilon-EHNA at the active site of the enzyme. This spectrum mimics that of epsilon-EHNA at pH 5.5 in buffer solution and is suggested to indicate a shift in protonation equilibrium at the active site.