Protein binding of homocysteine and other thiols in HeLa cell cultures after addition of homocysteine and copper ions.

Homocysteine can be viewed as the first risk factor for atherosclerosis, believed to exert its effects through a mechanism involving oxidative damage. Oxygen radicals are known to interact with a variety of macromolecules leading to lipid peroxidation, DNA strand breakage and a variety of changes in proteins, including thiol oxidation. The present study deals with the protein-binding of homocysteine, cysteine and glutathione in a human cell line culture exposed to homocysteine and copper ions in order to elucidate the possible role of homocysteine in cell injury and atherogenesis. It is shown that homocysteine has the highest tendency of the thiols investigated to create disulfide bonds with proteins. The interaction with the protein cysteine thiol groups, which are involved in the function of many enzymes, structural proteins and receptors might disturb many metabolic functions in the cell. This finding might therefore be one reason for the cell-damaging effects of homocysteine. Addition of reduced homocysteine to cell cultures decreased the intra- and extracellular proportions of protein-bound thiols except that of intracellular glutathione. In agreement with the pro-oxidative effects of copper ions, the findings in the present study showed an increase of the protein-bound fractions of all thiols after the addition of copper ions. Another finding is that increased (in the presence of 100 mumol/l of copper ions) or decreased (in the presence of 100-2000 mumol/l of homocysteine) proportions of protein-bound fractions of the thiols in these short-term experiments did not seriously affect the cells since the cell growth was unchanged.