THE RELATION BETWEEN THE FUNCTIONS OF 9-BETA-D-ARABINOFURANOSYLADENINE AS INACTIVATOR AND SUBSTRATE OF S-ADENOSYLHOMOCYSTEINE HYDROLASE

Abstract

9-.beta.-D-Arabinofuranosyladenine (ara-A) was converted to adenine in the presence of S-adenosylhomocysteine hydrolase (EC 3.3.1.1.) [isolated from mouse liver] 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 mole of enzyme; and at this point, the enzyme was totally inactivated. In the presence of homocysteine, the rate of ara-AHcy formation was .apprx. 1/2 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; ara-AHcy was dissociable. Inactivation, adenine formation and reducation of NAD+ may result from an abortive catalytic cycle; 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 2 processes.