Metabolism of diadenosine tetraphosphate (Ap4A) and related nucleotides in plants; review with historical and general perspective.

This review presents our knowledge of potential biochemical conversions of minor mononucleotides, such as adenosine-5'-tetraphosphate (p4A) and adenosine-5'-pentaphosphate (p5A), and dinucleotides, such as diadenosine-5',5"'-P1,P3-triphosphate (Ap3A) and diadenosine-5',5"'-P1,P4-tetraphosphate (Ap4A), in plants. Although the occurrence of p4A, Ap3A and/or Ap4A has been demonstrated in various bacteria, fungi and animals, identification of these compounds in plants has not been reported as yet. However, the ubiquity of both the compounds and enzymes that can synthesize them (certain ligases and transferases), the demonstration that certain plant ligases can synthesize pnAs and ApnNs in vitro, and the existence in plants of specific and nonspecific degradative enzymes strongly suggest that these various pnNs and NpnN's do indeed occur and play a biological role in plant cells. In fact, some of the plant enzymes involved in the synthesis and degradation of these minor mono- and dinucleotides have been studied even more thoroughly than their counterparts from other organisms.