Rat liver nucleotide pyrophosphatase/phosphodiesterase is an efficient adenylyl transferase

Rat liver nucleotide pyrophosphatase/phosphodiesterase I (NPP/PDE) catalyse d efficiently the transfer of adenylate from ATP to alcohols (methanol, eth anol, propanol, ethylene glycol, glycerol, 2,2-dichloroethanol and glycerol 2-phosphate), which acted as adenylate accepters competing with water with different efficiencies. NPP/PDE kinetics in alcohol/water mixtures were ac counted for by rate equations for competitive substrates, modified to inclu de alcohol negative co-operativity and, depending on the nature of the alco hol, enzyme denaturation by high alcohol concentrations or activation by lo w alcohol concentrations. The correlation of alcohol efficiencies with alco hol acidities, the comparison of rat liver with snake venom NPP/PDE, and th e different effects of ionic additives on the efficiencies of glycerol 2-ph osphate and glycerol provided evidence for interaction of the alcohols with a base catalyst, a non-polar and a cationic subsite in the active centre o f rat liver NPP/PDE. The enzyme thus appears to be well suited to act as tr ansferase, and we propose that NPP/PDE could be an adenylylating agent in t he membrane.