Homocysteine and Asymmetric Dimethylarginine (ADMA) in Neurological Diseases

Abstract

Homocysteine (Hcy) is formed from methionine (Met) and is distributed in two metabolic pathways: in the process of remethylation to Met and in the process of transsulfuration to cysteine. Hyperhomocysteinemia (HHcy) is a risk factor for cardiovascular and neurological diseases such as: Alzheimer’s and Parkinson’s diseases, multiple sclerosis, and stroke. Increased Hcy level may lead to endothelial dysfunction due to impaired bioavailability of endothelium-derived nitric oxide (NO). The molecular mechanism decreasing the levels of NO in HHcy conditions is incompletely understood, but it seems that asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, may be a key factor. ADMA is formed from L-arginine by enzymes in the family of protein N-methyltransferases (PRMT) and may undergo hydrolysis to L-citrulline and dimethylamine with the participation of dimethylaminohydrolase (DDAH). In pathological conditions such as neurodegenerative diseases, Hcy may lead to increased ADMA concentrations by inhibiting the activity of DDAH. Several drugs, such L-dopa, antiepileptic drugs, and lipid-lowering drugs, may interfere with the metabolic pathways of thiols, leading to an alteration of plasma Hcy and ADMA levels. It seems that administration of L-arginine, in conjunction with B vitamins, to patients with HHcy may be a new method in the treatment of neurodegenerative diseases.