Interaction of 9-β-d-Arabinofuranosyladenine, 9-β-d-Arabinofuranosyladenine 5′-Monophosphate, and 9-β-d-Arabinofuranosyladenine 5′-Triphosphate with S-Adenosylhomocysteinase¹

9-β-d-Arabinofuranosyladenine (ara-A), 9-β-d-arabinofuranosyladenine 5′-monophosphate, and 9-β-d-arabinofuranosyladenine 5′-triphosphate competitively inhibit both the synthesis and hydrolysis of S-adenosylhomocysteine catalyzed by S-adenosylhomocysteinase [S-adenosylhomocysteine hydrolase (EC 3.3.1.1.)] from mouse liver, and the inhibitor constants were 5.0 × 10^-6, 1.1 × 10^-4, and 1.0 × 10^-3 m, respectively. A time-dependent inactivation of the enzyme was observed when the enzyme was preincubation with ara-A, 9-β-d-arabinofuranosyladenine 5′-monophosphate, or 9-β-d-arabinofuranosyladenine 5′-triphosphate. ara-A was the most potent inactivator. The inactivation with ara-A was less pronounced in the presence of adenosine, S-adenosylhomocysteine, adenine, adenosine 5′-monophosphate, or adenosine 5′-diphosphate, showed first-order kinetics, saturability, and irreversibility. The rate of inactivation was half-maximal at 5 × 10^-6 m ara-A, and the rate constant of inactivation was 0.43 min^-1 at saturating concentrations of ara-A. ara-A was tightly but not covalently bound to the enzyme. ara-A bound to the enzyme was not available for deamination to 9-β-d-arabinofuranosylhypoxanthine catalyzed by the enzyme adenosine deaminase.