Tight-binding inhibitors-IV. Inhibition of adenosine deaminases by various inhibitors

Three ADA (adenosine deaminase) inhibitors, DHMPR (1,6-dihydro-6-hydroxymethyl purine ribonucleoside); EHNA [erythro-9-(2-hydroxy-3 nonyl)adenine] ; and deoxycoformycin [(R)-3-(2-deoxy-β-d-erythro-pento-furanosyl)-3, 6,7,8-tetrahydroimidazo[4,5-d] [1,3-diazepin-8-ol] or Covidarabin, were compared with regard to their inhibitory behavior with ADAs from human erythrocytes and calf intestine. Marked differences in the times required for establishment of steady state between the enzyme and inhibitors were observed, e.g. DHMPR, virtually instantaneous; EHNA, 2-3 min; and deoxycoformycin, many hr. The parameters of the inhibition of human erythrocytic ADA by deoxycoformycin were as follows: the association rate constant (k₁) = 2.6 × 10⁶ M^-1 sec^-1 ; the dissociation rate constant of the enzyme-inhibitor complex (k₂) = 6.6 × 10^-6 sec^-1; K_i (from k₂ k₁) = 2.5 × 10^-12M and K_i (from I₅₀) = 1.5 × 10^-11 M. The K_i values for EHNA and DHMPR, as determined by classical methods after attainment of steady state, were 1.6 × 10^-9 and 1.3 × 10^-6 M, respectively, for human erythrocytic ADA. The kinetic parameters for EHNA and calf intestinal ADA were as follows: K_i = 6.5 × 10^-9 M (by the method of I₅₀); k₁ = 0.7 × 10⁶ M^-1 sec^-1' and k₂ = 4.6 × 10^-3 sec^-1. On the basis of K_i values, the inhibitors. DHMPR, EHNA and deoxycoformycin (a transition state analog), were classified as readily reversible, semi-tight-binding and tight-binding inhibitors. The difficulties encountered in the kinetic analyses of different types of inhibitors and the methods for dealing with the problems of these inhibitors are discussed.