Enzymology of Nitric Oxide Synthesis

In general, the NOS isoforms can be grouped into two classes as determined by their expression in cells. Certain NOS isoforms, termed constitutive, appear to be expressed at a fairly constant level in their host cells. However, these isoforms are inactive in their native state and require that the Ca -binding protein calmodulin associate with them in order to generate NO (Schmidt et al., 1991 Busse and Mulsch, 1990 Bredt and Snyder, 1994). Thus NO production by constitutive NOS isoforms is often linked to Ca +-mediated signal transduction cascades that involve soluble guanylate cyclase, which is a hemeprotein that is activated by NO (Arnold et al., 1977). 

Other NOS isoforms, termed inducible, are typically not present in cells until the cell is exposed to immunostimulants of bacterial or host origin (Nathan, 

Expression of these NOS isoforms is controlled primarily at the level of transcription and translation (Xie et al., 1992 Lorsbach et al., 1993 Deng et al., 

Calmodulin has been discovered to associate with inducible macrophages 

NOS isoforms have now been purified from several sources, some of their properties are listed in Table 1. 

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