iNOS can simultaneously generate NO (nitric oxide) and superoxide. These react rapidly to yield peroxynitrite (ONOO), which can isomerize to nitrate. Thus measuring nitrate in a sample can tell you something about ONOO production (and indirectly about NO production).
To expand a bit on the previous answer, nitrite and nitrate are both NO metabolites.
Here is a useful review of some of the chemistry, which can be quite complex:
http://www.sciencedirect.com/science/article/pii/S0005272899000201
Briefly, nitrite (NO2-) is the result of the autooxidation of NO and can be considered an index of normal NO bioavailability. See for example:
http://www.ncbi.nlm.nih.gov/pubmed/11606734
Nitrate (NO3-) on the other hand, is produced from the oxidation of nitrite and, importantly, as noted above from the decay of peroxynitrite as well as the scavenging of NO by hemoglobin. Thus, it can be used as a proxy for inflammation/NO quenching. See below an example of inflammatory bowel disease:
http://www.sciencedirect.com/science/article/pii/S1089860309001517
Be aware that while the assays are generally easy to conduct, careful sample collection and preparation are crucial, to preserve nitrite and minimize contamination. Additionally, there can be dietery effects, so proper controls are a must.
Finally, many people measure NOx, or the total NO2- plus NO3- in their samples. In general, this approach sacrifices some nuance for simplicity. I would recommend measuring both species.
The relationship between iNOS and tissue nitrite is that iNOS is an enzyme that is activated due to the transcription of the gene NFKB. This transcription factor(NFKB) is bound to IKB and once IKB undergo phosphorylation by different immune stimulus such as bacteria LPS or cancerous proteins it releases NFKB. NFKB then trans-locates from the cytoplasm to the nucleus. Trans location causes iNOS activation which in turn catalyzes the production of nitric oxide. increased stimulation then causes peroxynitrites that eventually under isomerism to form nitrite.
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