Over the past 25 years, the role of nitric oxide (NO) in biology has evolvedfrom being recognized as an environmental pollutant to an endogenously produced substance involved incell communication and signal transduction. NO is produced by a family of enzymes called nitric oxidesynthases (NOSs), which can be stimulated by a variety of factors that mediate responses to various stimuli.NO can initiate its biological effects through activation of the heterodimeric enzyme, soluble guanylylcyclase (sGC), or through several other chemical reactions. Activation of sGC results in the productionof 3',5'-cyclic guanosine monophosphate (cGMP), an intracellular second messenger signaling molecule,which can subsequently mediate such diverse physiological events such as vasodilatation and immunomodulation.Chemically reactive NO can affect physiological changes through modifications to cellular proteins, oneof which is tyrosine nitration. The demonstration that NO is involved in so many biological pathwaysindicates the importance of this endogenously produced substance, and suggests that there is much moreto be discovered about its role in biology in years to come.

Keywords: NO, cyclic GMP, Nitrotyrosine