The relation between the functions of 9-beta-D-arabinofuranosyladenine as inactivator and substrate of S-adenosylhomocysteine hydrolase

S Helland and PM Ueland

9-beta-D-Arabinofuranosyladenine (ara-A) was converted to adenine in the presence of S-adenosylhomocysteine hydrolase (EC 3.3.1.1.) and to adenine and S-[5'-(9-arabinofuranosyladenyl)]-L-homocysteine (ara-AHcy) when the incubation mixture contained L-homocysteine. The formation of adenine proceeded until 3.3 mol of adenine was formed per mol of enzyme, and at this point the enzyme was totally inactivated. In the presence of homocysteine, the rate of ara-AHcy formation was about half the rate of adenine formation. The association of the conversion of ara- A to adenine with the inactivation process was further demonstrated by the kinetics of these processes and by the observation that in the presence of homocysteine both inactivation of the enzyme and formation of adenine were reduced by 30%, i.e. by a factor corresponding to the synthesis of ara-AHcy. Inactivation of the enzyme was associated with reduction of enzyme bound NAD+. Adenine or the substance liberating adenine was tightly bound to the enzyme, whereas ara-AHcy was dissociable. These data suggest that inactivation, adenine formation and reduction of NAD+ result from an abortive catalytic cycle, whereas enzyme entering a complete catalytic cycle leading to formation of ara- AHcy is not inactivated. The fact that the inactivation and the enzyme catalysis occur simultaneously at about equal rates offers an opportunity to demonstrate the competing relationship between these two processes.

Volume 218, Issue 3, pp. 758-763, 09/01/1981
Copyright © 1981 by American Society for Pharmacology and Experimental Therapeutics