Irreversible inhibition of S-adenosylhomocysteine hydrolase by nucleoside analogs

A large number of nucleoside analogs have been found to inactivate S-adenosylhomocysteine (AdoHcy) hydrolase in a time-dependent irreversible manner. There are two classes of these irreversible inhibitors: (A) analogs that inactivate the enzyme in a pseudofirst-order process and are devoid of any side chain at the 5′-OH group; (B) analogs that inactivate the enzyme in a time-dependent but curvilinear process, and generally have a side chain at the 5′ position. Among the more potent irreversible inhibitors are 2-chloroadenosine, 9-β-d-arabinofuranosyladenine (Ara-A), and (±)aristeromycin. Release of adenine base from adenosine or Ara-A in the presence of AdoHcy hydrolase was observed, thus supporting the proposed catalytic mechanism of AdoHcy hydrolase, that entails the transient formation of 3′-ketoadenosine during enzymatic catalysis of either the formation or hydrolysis of AdoHcy. Both Ara-A and adenosine may exert their irreversible inactivation by a suicide mechanism, but nucleosides such as 5′-iodo-5′-deoxyadenosine and 3′-deoxyadenosine are probably strictly irreversible inhibitors per se in view of the catalytic mechanism proposed for AdoHcy hydrolase. Labeling of AdoHcy hydrolase, perhaps covalent in nature, by radioactive Ara-A and adenosine was demonstrated by gel electrophoresis.