Autoxidation versus covalent binding of quinones as the mechanism of toxicity of dopamine, 6-hydroxydopamine, and related compounds toward C1300 neuroblastoma cells in vitro
Graham, D.G.; Tiffany, S.M.; Bell, W.R.; Gutknecht, W.F.
Molecular Pharmacology 14(4): 644-653
1978
ISSN/ISBN: 0026-895X PMID: 567274 Document Number: 132784
The mechanism of cytotoxicity of 6-hydroxydopamine, 2,4,5-trihydroxyphenylalanine, dopa, dopamine, norepinephrine and epinephrine was explored by determining whether cytotoxicity was a reflection of the potential for autoxidation of each polyphenol or of the sulfhydryl reactivity of its quinone products. The cytotoxicity of the polyphenols, as measured by inhibition of [3H]thymidine incorporation into DNA by mouse C1300 neuroblastoma cells in tissue culture, correlated with the rate of autoxidation, as measured spectrophotometrically or by O2 electrode studies. Polarographic determinations of the oxidation potentials of the polyphenols were also predictive of cytotoxicity; the most cytotoxic compounds had the most negative half-wave potentials and therefore were the most readily oxidized. The sulfhydryl reactivity of the quinone oxidation products of the polyphenols, as measured by inhibition of purified calf thymus DNA polymerase .alpha., exhibited an inverse relationship to the cytotoxicity of the polyphenols; the most toxic compounds, 6-hydroxydopamine and 2,4,5-trihydroxyphenylalanine, were oxidized to the least reactive quinone products. An alternative mechanism of toxicity was observed with N-acetyldopamine, which was oxidized to 4-(2-N-acetylaminoethyl)-1,2-benzoquinone, a potent sulfhydryl reagent. N-Acetyldopamine was more toxic than predicted by its half-wave potential or its rate of autoxidation. While norepinephrine completely neutralized 6-hydroxydopamine and 2,4,5-trihydroxyphenylalanine as cytotoxic agents, the toxicity of N-acetyldopamine was minimally affected. 6-hydroxydopamine and 2,4,5-trihydroxyphenylalanine kill cells through the production of hydroxydopamine and 2,4,5-trihydroxyphenylalanine kill cells through the production of H2O2, .**GRAPHIC**. and OH.cntdot., while for dopamine and dopa the reaction of quinone oxidation products with nucleophiles probably also contributes to their cytotoxicity.