Reactive Oxygen Species Are Involved in Arsenic Trioxide Inhibition of Pyruvate Dehydrogenase Activity

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Abstract
Arsenite was shown to inhibit pyruvate dehydrogenase (PDH) activity through binding to vicinal dithiols in pure enzyme and tissue extract. However, no data are available on how arsenite inhibits PDH activity in human cells. The IC(50) values for arsenic trioxide (As(2)O(3)) to inhibit the PDH activity in porcine heart pure enzyme preparation and in human leukemia cell line HL60 cells were estimated to be 182 and 2 microM, respectively. Thus, As(2)O(3) inactivation of PDH activity was about 90 times more potent in HL60 cells than in purified enzyme preparation. The IC(50) values for As(2)O(3) and phenylarsine oxide to reduce the vicinal thiol content in HL60 cells were estimated to be 81.7 and 1.9 microM, respectively. Thus, As(2)O(3) is a potent PDH inhibitor but a weak vicinal thiol reacting agent in HL60 cells. Antioxidants but not dithiol compounds suppressed As(2)O(3) inhibition of PDH activity in HL60 cells. Conversely, dithiol compounds but not antioxidants suppressed the inhibition of PDH activity by phenylarsine oxide. As(2)O(3) increased H(2)O(2) level in HL60 cells, but this was not observed for phenylarsine oxide. Mitochondrial respiration inhibitors suppressed the As(2)O(3)-induced H(2)O(2) production and As(2)O(3) inhibition of PDH activity. Moreover, metal chelators ameliorated whereas Fenton metals aggravated As(2)O(3) inhibition of PDH activity. Treatment with H(2)O(2) plus Fenton metals also decreased the PDH activity in HL60 cells. Therefore, it seems that As(2)O(3) elevates H(2)O(2) production in mitochondria and this may produce hydroxyl through the Fenton reaction and result in oxidative damage to the protein of PDH. The present results suggest that arsenite may cause protein oxidation to inactivate an enzyme and this can occur at a much lower concentration than arsenite binding directly to the critical thiols.