Hyperhomocysteinemia is a risk factor for a number of cardiovascular and neurodegenerative processes as well as a complicating factor in normal pregnancy. Toxic effects of homocysteine and the product of its spontaneous oxidation, homocysteic acid, are based on their ability to activate NMDA receptors, increasing intracellular levels of ionized calcium and reactive oxygen species. Even a short-term exposure of cells to homocysteic acid at concentrations characteristic of hyperhomocysteinemia induces their apoptotic transformation. The discovery of NMDA receptors both in neuronal tissue and in several other tissues and organs (including immunocompetent cells) makes them a target for toxic action of homocysteine. The neuropeptide carnosine was found to protect the organism from homocysteine toxicity. Treatment of pregnant rats with carnosine under conditions of alimentary hyperhomocysteinemia increases viability and functional activity of their progeny.

Short-term incubation of lymphocytes with homocysteine or its oxidation product homocysteinic acid increased the formation of reactive oxygen species and cell necrosis (in case of homocysteinic acid). Effective concentration of homocysteine and homocysteinic acid (500 microM) significantly surpassed the level observed during hyperhomocysteinemia. The addition of homocysteinic acid in a nontoxic concentration of 100 microM potentiated the toxic effect of NMDA and led to massive cell death. During hyperhomocysteinemia the amount of these metabolites in the blood was much higher than in the brain. Oxidative stress produced by these substances can result from activation of NMDA glutamate receptors that were recently detected on lymphocytes.

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