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Related Experiment Videos

Ca2+/calmodulin-regulated nitric oxide synthases.

H H Schmidt1, J S Pollock, M Nakane

  • 1Northwestern University Medical School, Chicago.

Cell Calcium
|June 1, 1992
PubMed
Summary
This summary is machine-generated.

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Nitric oxide synthase (NOS) activity, crucial for NO production, is regulated by intracellular calcium levels and calmodulin. Prolonged calcium increases can reduce NO formation through enzyme phosphorylation.

Area of Science:

  • Biochemistry
  • Molecular Biology
  • Physiology

Background:

  • Nitric oxide synthase (NOS) synthesizes nitric oxide (NO) from L-arginine.
  • Three major NOS isoforms (types I-III) have been identified.
  • NOS activity is implicated in various physiological processes.

Purpose of the Study:

  • To investigate the regulatory mechanisms of NOS isoforms.
  • To understand the role of intracellular calcium and calmodulin in NOS activity.
  • To explore other redox activities of NOS.

Main Methods:

  • Purification of type I (brain) and type III (endothelial) NOS isoforms.
  • Enzyme kinetics assays to measure NOS activity.
  • In vitro phosphorylation studies using Ca2+/calmodulin-dependent protein kinase II.

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Main Results:

  • Type I and III NOS activity are dependent on intracellular calcium concentration ([Ca2+]i) and calmodulin.
  • Both isoforms are inactive at resting [Ca2+]i and fully active at ≥500 nM Ca2+.
  • In vitro phosphorylation by Ca2+/calmodulin protein kinase II reduces the Vmax of NOS, suggesting downregulation.
  • Type I NOS exhibits Ca2+/calmodulin-dependent generation of H2O2 and reduction of cytochrome c/P450.
  • Other redox activities, like NADPH-diaphorase and dihydrobiopterin reduction, appear Ca2+/calmodulin-independent.

Conclusions:

  • Intracellular calcium and calmodulin are key regulators of type I and III NOS activity.
  • Sustained increases in [Ca2+]i can lead to NOS downregulation via phosphorylation.
  • NOS possesses diverse redox capabilities, some calcium-dependent and others independent.