EN
This review is a summary of our knowledge on uncommon mononucleotide, the adenosine 5-tetraphosphate (p4A). It covers the very first communications on p4A and other nucleoside 5-tetraphosphates, methods that allow to identify p4A and estimate it quantitatively. It also presents the occurrence of p4A in biological material, enzymatic reactions which can lead to the accumulation of p4A in the cells, catabolic reactions, the properties of three specific p4A hydrolases from rabbit, yellow lupin and humans, and of the yeast exopolyphosphatase for which p4Ns are very good substrates. Finally, inhibition exerted by p4A on some enzymatic reactions and the effects the compound exerts on blood vessels and nervous system are discussed.