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D. S. Bredt is a neuroscience researcher who, with S. H. Snyder, investigated production of nitric oxide. Their 1990 paper isolated nitric oxide synthase and established its requirement for calmodulin.^[ref]

The researchers followed conversion of arginine into citrulline, associated with formation of nitric oxide. Using affinity chromatography, they purified the activity approximately 6000-fold from rat cerebellum. This tied a chemical transformation to an isolated protein.^[ref]

The example illustrates activity-guided separation: measuring a reaction product lets one track a catalyst through successive fractions. Testing the isolated activity with different added components then investigates regulation. A purification factor describes enrichment of activity relative to total protein; it is not a 6000-fold increase in every biological response.

S. H. Snyder is a researcher in neuroscience and pharmacology whose laboratory investigates neurotransmitters, intracellular messengers and mechanisms of drug action. His research includes nitric oxide, a signaling molecule whose production can be coupled to intracellular calcium ions.^[ref]

With D. S. Bredt, S. H. Snyder isolated nitric oxide synthase from rat cerebellum. The 1990 experiments established a requirement for calmodulin, alongside calcium ions and NADPH, in the measured activity. This connected production of nitric oxide to a molecular calcium-sensing mechanism.^[ref]

The example shows how identifying the enzyme producing a messenger can change the explanation of signaling. Rather than imagining a fixed store simply being emptied, one can ask which inputs activate synthesis and how fast production changes. A messenger's chemical identity and the mechanism controlling its supply are complementary parts of the biological explanation.