Nitric Oxide Circulates in Mammalian Plasma Primarily as an S-Nitroso Adduct of Serum Albumin

• Published in: Proceedings of the National Academy of Sciences - PNAS 1992-08, Vol.89 (16), p.7674-7677
• Main Authors: Stamler, Jonathan S., Jaraki, Omar, Osborne, John, Simon, Daniel I., Keaney, John, Vita, Joseph, Singel, David, Valeri, C. Robert, Loscalzo, Joseph
• Format: Article
• Language: English
• Subjects: Adducts; albumin; Albumins; Analytical, structural and metabolic biochemistry; Animals; Arginine - analogs & derivatives; Arginine - pharmacology; Biochemistry; Biological and medical sciences; Blood; Blood plasma; Blood pressure; Blood Pressure - drug effects; Chemiluminescence; Fundamental and applied biological sciences. Psychology; Intermediary metabolites. Miscellaneous; levels; Luminescent Measurements; Mammals; man; Medical research; nitric oxide; Nitric Oxide - blood; Nitroso Compounds - blood; omega-N-Methylarginine; Other biological molecules; Oxides; Photolysis; plasma; Protein Binding; Proteins; Rabbits; Serum Albumin - metabolism; Serum albumins; Thiols
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.89.16.7674

We have recently shown that nitric oxide or authentic endothelium-derived relaxing factor generated in a biologic system reacts in the presence of specific protein thiols to form S-nitrosoprotein derivatives that have endothelium-derived relaxing factor-like properties. The single free cysteine of serum albumin, Cys-34, is particularly reactive toward nitrogen oxides (most likely nitrosonium ion) under physiologic conditions, primarily because of its anomalously low pK; given its abundance in plasma, where it accounts for ≈0.5 mM thiol, we hypothesized that this plasma protein serves as a reservoir for nitric oxide produced by the endothelial cell. To test this hypothesis, we developed a methodology, which involves UV photolytic cleavage of the S-NO bond before reaction with ozone for chemiluminescence detection, with which to measure free nitric oxide, S-nitrosothiols, and S-nitrosoproteins in biologic systems. We found that human plasma contains ≈7 μM S-nitrosothiols, of which 96% are S-nitrosoproteins, 82% of which is accounted for by S-nitroso-serum albumin. By contrast, plasma levels of free nitric oxide are only in the 3-nM range. In rabbits, plasma S-nitrosothiols are present at ≈1 μM; 60 min after administration of NG-monomethyl-L-arginine at 50 mg/ml, a selective and potent inhibitor of nitric oxide synthetases, S-nitrosothiols decreased by ≈40% (>95% of which were accounted for by S-nitrosoproteins, and ≈80% of which was S-nitroso-serum albumin); this decrease was accompanied by a concomitant increase in mean arterial blood pressure of 22%. These data suggest that naturally produced nitric oxide circulates in plasma primarily complexed in S-nitrosothiol species, principal among which is S-nitroso-serum albumin. This abundant, relatively long-lived adduct likely serves as a reservoir with which plasma levels of highly reactive, short-lived free nitric oxide can be regulated for the maintenance of vascular tone.