= Nitric oxide synthase
A <nitric oxide synthase> is an <enzyme> that produces <nitric oxide> from <arginine>, with <citrulline> as the accompanying amino-acid product. The reaction couples oxidation chemistry to reducing equivalents supplied by <NADPH>. The resulting <nitric oxide> can act as a short-range biological signal, so controlling its synthesis is different from releasing a pre-existing stored transmitter.<Isolation of nitric oxide synthetase, a calmodulin-requiring enzyme by D. S. Bredt and S. H. Snyder>[Abstract: enzymatic formation from arginine, citrulline production and NADPH requirement]{ref}
The purification of a brain enzyme provided an important mechanistic experiment. In 1990, the authors enriched activity from rat cerebellum with an affinity column and showed that the purified activity required <calmodulin> under their calcium-dependent assay conditions. This connected a protein preparation with a regulated chemical conversion, rather than identifying the signal solely from a tissue's relaxation response.<Isolation of nitric oxide synthetase, a calmodulin-requiring enzyme by D. S. Bredt and S. H. Snyder>[Abstract: affinity purification and requirement for calmodulin]{ref}
An illuminating assay comparison holds enzyme amount fixed and changes a required reaction component. If adding <calmodulin> restores activity to a suitable preparation, a weak initial signal need not mean that the enzyme protein was absent. Protein abundance, catalytic competence and the availability of substrates or <enzyme cofactors> are separate quantities.
Different nitric-oxide-synthase forms and cellular contexts have different regulatory behavior. The calcium dependence established for the purified brain preparation should therefore not be turned into a claim that every nitric-oxide-synthase response in every tissue follows the same calcium-control rule.
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