The adenine analog erythro-9-(2-hydroxy-3-nonyl)adenine, EHNA, a tight reversible inhibitor (K(I) = 1.6 x 10-9 M) of adenosine deaminase (EC 18.104.22.168) (ADase), was modified into the fluorescent etheno derivative ε-EHNA. The latter is a competitive inhibitor of adenosine deaminase [K(I) = (2.80 ± 0.01)10-6 M], having the fluorescent properties of ε-adenines. Affinity to the active site, monitored by both steady-state and dynamic fluorescence polarization, was confirmed by competition experiments with 2'-deoxycoformycin, the substrate adenosine and EHNA. The ε-adenine moiety of ε-EHNA librates at the shallow active site of ADase. The low absorptivity of ε-EHNA required the measurement of fluorescence excitation spectra. Computer subtraction of fluorescence excitation spectrum of ADase from that of its equimolar complex with ε-EHNA revealed the corrected excitation spectrum of ε-EHNA at the active site of the enzyme. This spectrum mimics that of ε-EHNA at pH 5.5 in buffer solution, implying its protonation at the active site of the enzyme. These results are in agreement with the presence of acidic amino acids that are essential to the catalytic mechanism.
- Adenosine deaminase
- Etheno analogues
- Fluorescent competitive inhibitor
- pH sensitive inhibitor of ADase
- Protonation equilibrium
ASJC Scopus subject areas
- Physical and Theoretical Chemistry