Diguanosinetetraphosphatase from Rat Liver: Activity on Diadenosine Tetraphosphate and Inhibition by Adenosine Tetraphosphate

Open Access
Publisher Website
Google Scholar
Abstract
The hydrolysis of diadenosine tetraphosphate, a compound previously described by others to occur in liver at concentrations of around 0.1 μM, is carried out by a specific enzyme. This enzyme has been partially purified from rat liver extracts, and the following properties have been found. The Km, value for diadenosine tetraphosphate is 2 μM; the products of hydrolysis are ATP and AMP; the Km, value for diguanosine tetraphosphate is 2 μM; none of the following substances were substrates of the enzyme: diadenosine triphosphate, diguanosine di and triphosphates, adenosine tetraphosphate, ATP, ADP, NAD+, NADP+ and bis-p-nitrophenylphosphate. Cyclic AMP was not an inhibitor of the reaction. The enzyme requires MG2+ ions, is maximally active at a pH value of approximately 8, and has a molecular weight of 22000 as estimated by filtration on Sephadex G-100. The activation energy of the reaction was of 10250 cal × mol−1 (42886 J × mol−1). Particularly striking is the inhibition by adenosine tetraphosphate (Ki= 48 nM) and guanosine tetraphosphate Ki=14 nM). Other nucleotides tested were also competitive inhibitors with K values in the 10–100 μM range.