Inhibition of select mitochondrial enzymes in PC12 cells exposed to S-(1,1,2,2-tetrafluoroethyl)-L-cysteine

Many halogenated foreign compounds are detoxified by conversion to the corr esponding cysteine S-conjugate, which is N-acetylated and excreted. However , several halogenated cysteine S-conjugates [e.g. S-(1,1,2,2-tetrafluoroeth y)-L-cysteine (TFEC)] are converted to mitochondrial toxicants by cysteine S-conjugate P-lyases. In the present work, we showed that TFEC appreciably inactivated highly purified a-ketoglutarate dehydrogenase complex (KGDHC) i n the presence of a cysteine S-conjugate beta-lyase. Incubation of PC12 cel ls (which contain endogenous cysteine S-conjugate beta-lyase activity) with TFEC led to a concentration- and time-dependent loss of endogenous KGDHC a ctivity. A 24-hr exposure to 1 mM TFEC decreased KGDHC activity in the cell s by 90%. Although treatment with TFEC did not inhibit intrinsic pyruvate d ehydrogenase complex (PDHC) activity, it inhibited dichloroacetate/Mg2+-med iated activation/dephosphorylation of PDHC in the PC12 cells by 90%. To det ermine the selectivity of enzymes targeted by TFEC, several cytosolic and m itochondrial enzymes involved in energy metabolism [malate dehydrogenase, g lyceraldehyde 3-phosphate dehydrogenase, glutamate dehydrogenase, lactate d ehydrogenase, cytosolic and mitochondrial aspartate aminotransferases (AspA T)] were also assayed in the PC12 cells exposed to 1 mM TFEC fur 24 hr. Of these enzymes, only mitochondrial AspAT, a key enzyme of the malate-asparta te shuttle, was inhibited. The present results demonstrate a selective vuln erability of mitochondrial enzymes to toxic cysteine S-conjugates. The data indicate that TFEC may be a useful cellular/mitochondrial toxicant for elu cidating the consequences of the diminished mitochondrial function that acc ompanies numerous neurodegenerative diseases. (C) 1999 Elsevier Science Inc .