Thiols enhance NO formation from nitrate photolysis

Area of Science:

• Biochemistry
• Photochemistry
• Analytical Chemistry

Background:

• Nitrate is typically viewed as a stable end-product of nitric oxide (NO) metabolism.
• While NO generation from nitrate photolysis in water is known, its behavior in biological settings like plasma is unclear.
• Thiols, abundant in biological systems, were hypothesized to influence NO production from nitrate photolysis.

Purpose of the Study:

• To investigate the photochemical reactions of nitrate in human plasma.
• To determine the role of thiols in mediating nitric oxide (NO) and NO-adduct formation from nitrate photolysis.
• To assess the implications for analytical methods relying on nitrate photolysis.

Main Methods:

• Human plasma samples were exposed to UV light.
• Nitric oxide (NO) formation was measured using gas phase chemiluminescence.
• Nitrogen oxides (NOx) and thiol levels were manipulated (removal, reconstitution, blockade) to assess their impact.
• S-nitrosothiol concentrations were measured before and after UV exposure.

Main Results:

• UV irradiation of plasma generated significant amounts of NO, dependent on plasma N-oxides.
• Nitrate, not nitrite, addition restored NO generation in depleted plasma.
• Thiols (dithiothreitol, L-cysteine) increased NO formation, while thiol blockade (NEM, HgCl2) reduced it.
• UV exposure dramatically increased S-nitrosothiol concentrations, indicating their formation from nitrate.

Conclusions:

• Thiols significantly facilitate the photolytic conversion of nitrate to NO and S-nitrosothiols in plasma.
• Photolysis-based analytical methods may overestimate S-nitrosothiols due to this thiol-mediated reaction.
• The in vivo relevance of this novel nitrate photochemistry pathway requires further investigation.