Spectrofluorimetric analysis of the interaction of amyloid peptides with neuronal nitric oxide synthase: implications in Alzheimer's disease.

Background: The deposition of aggregated β-amyloid peptide senile plaques and the accumulation of arginine within the astrocytes in the brain of an Alzheimer's patient are classic observations in the neuropathology of the disease. It would be logical, in the aetiology and pathogenesis, to investigate arginine-metabolising enzymes and their intimate association with amyloid peptides.

Methods: Neuronal nitric oxide synthase (nNOS) was isolated, purified and shown, through fluorescence quenching spectroscopy and fluorescence resonance energy transfer (FRET), to interact with structural fragments of Aβ(1-40) and be catalytic towards amyloid fibril formation.

Results: Only one binding site on the enzyme was available for binding. Two amyloid peptide fragments of Aβ(1-40) (Aβ(17-28) and Aβ(25-35)) had Stern-Volmer values (K(SV)) of 0.111μM(-1) and 0.135μM(-1) indicating tight binding affinity to nNOS and easier accessibility to fluor molecules during binding. The polarity of this active site precludes binding of the predominantly hydrophobic amyloid peptide fragments contained within Aβ(17-28) and within two glycine zipper motifs [G-X-X-X-G-X-X-X-G] [Aβ(29-37)] and bind to the enzyme at a site remote to the active region.

Conclusions: The interaction and binding of Aβ(17-28) and Aβ(25-35) to nNOS causes the movement of two critical tryptophan residues of 0.77nm and 0.57nm respectively towards the surface of the enzyme.

General Significance: The binding of Aβ-peptide fragments with nNOS has been studied by spectrofluorimetry. The information and data presented should contribute towards understanding the mechanism for deposition of aggregated Aβ-peptides and fibrillogenesis in senile plaques in an AD brain.