Homocysteine impairs the nitric oxide synthase pathway: role of asymmetric dimethylarginine.
BACKGROUND
Hyperhomocysteinemia is a putative risk factor for cardiovascular disease, which also impairs endothelium-dependent vasodilatation. A number of other risk factors for cardiovascular disease may exert their adverse vascular effects in part by elevating plasma levels of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase. Accordingly, we determined if homocysteine could increase ADMA levels.


RESULTS
When endothelial or nonvascular cells were exposed to DL-homocysteine or to its precursor L-methionine, ADMA concentration in the cell culture medium increased in a dose- and time-dependent fashion. This effect was associated with the reduced activity of dimethylarginine dimethylaminohydrolase (DDAH), the enzyme that degrades ADMA. Furthermore, homocysteine-induced accumulation of ADMA was associated with reduced nitric oxide synthesis by endothelial cells and segments of pig aorta. The antioxidant pyrrollidine dithiocarbamate preserved DDAH activity and reduced ADMA accumulation. Moreover, homocysteine dose-dependently reduced the activity of recombinant human DDAH in a cell free system, an effect that was due to a direct interaction between homocysteine and DDAH.


CONCLUSIONS
Homocysteine post-translationally inhibits DDAH enzyme activity, causing ADMA to accumulate and inhibit nitric oxide synthesis. This may explain the known effect of homocysteine to impair endothelium-mediated nitric oxide-dependent vasodilatation.
Stühlinger
M C
MC
Section of Vascular Medicine, Stanford University, Stanford, California, USA.
Tsao
P S
PS
Her
J H
JH
Kimoto
M
M
Balint
R F
RF
Cooke
J P
JP
D000581:Amidohydrolases; D000818:Animals; D001011:Aorta / Q000187:drug effects / Q000378:metabolism; D001120:Arginine / Q000031:analogs & derivatives* / Q000096:biosynthesis / Q000502:physiology*; D001161:Arteriosclerosis / Q000209:etiology; D002417:Cattle; D002460:Cell Line; D046508:Culture Techniques; D004730:Endothelium, Vascular / Q000187:drug effects / Q000378:metabolism*; D006710:Homocysteine / Q000378:metabolism / Q000494:pharmacology*; D006867:Hydrolases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D020138:Hyperhomocysteinemia / Q000150:complications; D007700:Kinetics; D008715:Methionine / Q000494:pharmacology; D009569:Nitric Oxide / Q000378:metabolism; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015398:Signal Transduction
D011993:Recombinant Fusion Proteins; D006710:Homocysteine; D009569:Nitric Oxide; C024917:symmetric dimethylarginine; C018524:N,N-dimethylarginine; D001120:Arginine; D008715:Methionine; D019001:Nitric Oxide Synthase; D006867:Hydrolases; D000581:Amidohydrolases; C059999:dimethylargininase