THE EARLY ROLE OF NITRIC-OXIDE IN EVOLUTION

Nitric oxide (NO), which today serves many different purposes in regulating complex cellular functions, must have played a crucial role in the early stages of the evolution of life. The formation of NO may have been a critical defence mechanism for primitive microrganisms at a time when life faced the problem of rising atmospheric levels of ozone (O-3) formed upon photolysis of oxygen (O-2), which occurred shortly after the development of respiration in cyanobacteria. The production of NO by organisms would have allowed neutralization of toxic O-3 by chemical reaction outside the cell, thus acting as a protective mechanism against oxidative destruction, allowing evolutionary advantage. Later, NO production might have allowed the control of reactive O-2 species within cells before the development of specific electron-accepting enzymes. The pathway of NO formation was then consequently developed further to serve other useful functions. Although mammalian cells produce NO from L-arginine, the origin of this ability might have arisen from the essential process of either nitrification or denitrification in prokaryotic cells.