ABSTRACT
The intracellular level of dinucleoside polyphosphates is determined by both their rates of synthesis and degradation. This chapter deals exclusively with specific and nonspecific enzymes which, depending on the organism from which they are derived, cleave NpnN's either hydrolytically or phosphorolytically. In 1963, Warner and Finamore reported the occurrence of diguanosine tetraphosphate in extracts of Artemia cysts. The first studies on substrate specificity were carried out using a variety of compounds with unmodified phosphate residues. During the course of a systematic screening of the effect of nucleoside 5’-phosphates on the initial velocity of the reaction, it was observed that GTP was a particularly strong inhibitor of the reaction catalyzed by the Np4 N’ hydrolase from Artemia cysts. Later experiments were carried out on the distribution of dinucleoside triphosphatase activity in the cytosol and particulate fractions from rat liver. Using standard techniques, rat liver was homogenized and fractionated into nuclear, mitochondrial, mitochondrial/microsomal, microsomal and cytosolic fractions.
