Nitric oxide circulates in mammalian plasma primarily as an S-nitroso adduct of serum albumin.
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 approximately 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 approximately 7 microM 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 approximately 1 microM; 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 approximately 40% (greater than 95% of which were accounted for by S-nitrosoproteins, and approximately 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.
Stamler
J S
JS
Department of Medicine, Harvard University, Cambridge, MA 02138.
Jaraki
O
O
Osborne
J
J
Simon
D I
DI
Keaney
J
J
Vita
J
J
Singel
D
D
Valeri
C R
CR
Loscalzo
J
J
D000818:Animals; D001120:Arginine / Q000031:analogs & derivatives / Q000494:pharmacology; D001794:Blood Pressure / Q000187:drug effects; D008163:Luminescent Measurements; D009569:Nitric Oxide / Q000097:blood*; D009603:Nitroso Compounds / Q000097:blood*; D010782:Photolysis; D011485:Protein Binding; D011817:Rabbits; D012709:Serum Albumin / Q000378:metabolism*; D019323:omega-N-Methylarginine
D009603:Nitroso Compounds; D012709:Serum Albumin; D019323:omega-N-Methylarginine; D009569:Nitric Oxide; D001120:Arginine