Mitochondrial Physiology Network 10.9: 49-50 (2005) - download pdf

A toxic halogenated cysteine S-conjugate has deleterious effects on mitochondrial physiology.

Boris F Krasnikov^2,3, AJL Cooper^1,2,3

¹Departments of Biochemistry and ²Neurology & Neuroscience, Weil Medical College of Cornell University, 1300 York Avenue, New York, NY 10021, USA; ³Burke Medical Research Institute, 785 Mamaroneck Avenue, White Plains, NY 10605, USA. – bkrasnik@burke.org

Many toxicants impair mitochondrial physiology leading to a decrease in energy production. Such a malfunction could lead to progressive organ failure. Humans are continuously exposed to reactive electrophiles derived from the environment, medications and food, and as natural metabolites within the body. Many are detoxified through acetylation of the corresponding cysteine S-conjugate to the mercapturate, which is excreted. However, if the cysteine S-conjugate contains a good leaving group at the sulfur, a β-elimination reaction may compete with the acetylation reaction.  Enzymes that catalyze this elimination reaction are known as cysteine S-conjugate β-lyases. These enzymes convert susceptible cysteine S-conjugates to pyruvate, ammonium and a sulfur-containing fragment (RSH). If the eliminated fragment is not particularly reactive, the parent cysteine S-conjugate is not generally toxic. On the other hand, if RSH is reactive, the parent cysteine S-conjugate is potentially toxic. Halogenated alkenes, such as tetrafluoroethylene, are examples of environmental and workplace contaminants that are toxified (bioactivated) at least in part by the action of cysteine S-conjugate β-lyase(s) on the corresponding cysteine S-conjugate. The cysteine S-conjugate derived from tetrafluoroethylene ((1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC)) is converted by cysteine S-conjugate β-lyases to an RSH fragment that is a thioacylating agent, particularly of protein lysine residues. TFEC (and other halogenated cysteine S-conjugates) are mitochondrial toxicants, presumably as a result of the presence of cysteine S-conjugate β-lyase activity in these organelles [1, 2]. A major cysteine S-conjugate β-lyase in mitochondria is mitochondrial aspartate aminotransferase (mitAspAT) [3]. In the presence of TFEC the enzyme is inactivated on average after several thousand turnover events [3]. Exposure of rat kidney in vivo and PC12 cells in culture to TFEC results in selective inhibition of mitochondrial enzymes of energy metabolism, including mitAspAT, aconitase, and the E2 and E3 components of the α-ketoglutarate dehydrogenase complex (KGDHC) [reviewed in 4].

In the present work, isolated rat liver mitochondria energized with succinate were incubated in the presence or absence of TFEC. Four physiologically important mitochondrial parameters (O₂ uptake, Ca²⁺ flux, mitochondrial membrane potential (Y), and swelling) were simultaneously measured using a uniquely designed multiparameter chamber. A concentration- and time-dependent disruption of these parameters by TFEC was observed: (1) inhibition of mitochondrial respiration, (2) Ca²⁺ release from mitochondrial matrix, and (3) dissipation of Δψ_m. Both the lag-period and the degree of maximal swelling of mitochondria induced by TFEC were decreased with increasing concentration and time of pre-incubation. KGDHC and mitAspAT were both inhibited by ~20 % in liver mitochondria exposed to TFEC. No change was found in the activities of glutamate- or malate dehydrogenases.

Possible exposure to endogenous and exogenous electrophiles (that can be bioactivated via the cysteine S-conjugate β-lyase pathway) may lead to selective loss of enzymes involved in energy metabolism. This loss may contribute to (or exacerbate) the mitochondrial dysfunction associated with aging and many neurodegenerative diseases.

1.  Lash LH, Elfarra AA, Anders MW (1986) Renal cysteine conjugate -lyase: Bioactivation of nephrotoxic cysteine S-conjugates in mitochondrial outer membrane. J. Biol. Chem. 261: 5930-5935.

2.  Stevens JL, Ayoubi N, Robbins JD (1988) The role of mitochondrial matrix enzymes in the metabolism and toxicity of cysteine conjugates. J. Biol. Chem. 263: 3395-3401.

3.  Cooper AJL, Bruschi SA, Iriarte A, Martinez-Carrion M (2002) Rat liver mitochondrial aspartate aminotransferase catalyses cysteine S-conjugate β-lyase reactions. Biochem. J. 368: 253-261.

4.  Cooper AJL, Bruschi SA, Anders MW (2002) Toxic, halogenated cysteine S-conjugate and targeting of mitochondrial enzymes of energy metabolism. Biochem. Pharmacol. 64: 553-564.