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.[ref]
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
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.[ref]
Isolation of nitric oxide synthetase, a calmodulin-requiring enzyme by D. S. Bredt and S. H. Snyder. Abstract: affinity purification and requirement for calmodulin
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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