Permeability effects of organomercurials

Diffusion pathways to critical cysteines in the vesicular acetylcholine transporter of Torpedo

Previous work had demonstrated that organomercurial-mediated modification of two cysteine residues in the vesicular acetylcholine transporter (VAChT) from Torpedo californica inhibits binding of vesamicol. The cysteines are protected by acetylcholine and vesamicol (Keller et al. 2000. J. Neurochem. 74:1739-1748). Modified "cysteine 1" is accessible to glutathione from the cytoplasmic surface, whereas modified "cysteine 2" is not. Different organomercurials and aqueous environments were used here to characterize diffusion pathway(s) leading to the cysteines. para-Chloromercuriphenylsulfonate modifies VAChT much more slowly than do more hydrophobic p-chloromercuribenzoate and phenylmercury chloride. Permeabilization of vesicles with cholate detergent increases the rate of modification by p-chloromercuriphenylsulfonate. Permeabilization does not affect the ability of glutathione to reverse modification by p-chloromercuriphenylsulfonate. Higher ionic strength causes about four-fold increase in the rate of modification. The results suggest that hydrophobic and electrostatic barriers inhibit modification of Torpedo VAChT by negatively charged organomercurials and glutathione cannot reach cysteine 2 from either side of the membrane.

Modification of cysteines reveals linkage to acetylcholine and vesamicol binding sites in the vesicular acetylcholine transporter of Torpedo californica

Properties of cysteinyl residues in the vesicular acetylcholine transporter (VAChT) of synaptic vesicles isolated from Torpedo californica were probed. Cysteine-specific reagents of different size and polarity were used and the effects on [3H]vesamicol binding determined. The vesamicol dissociation constant increased 1,000-fold after reaction with p-chloromercuriphenylsulfonate or phenylmercury acetate, but only severalfold after reaction with relatively small methylmercury chloride or methylmethanethiosulfonate (MMTS). Methylmercury chloride, but not MMTS, protected binding from phenylmercury acetate. Thus, two classes of cysteines react to affect vesamicol binding. Class 1 reacts with only organomercurials, and class 2 reacts with both organomercurials and MMTS. Quantitative analysis of the competition between p-chloromercuriphenylsulfonate and VAChT ligands was possible after defining second-order reaction conditions. The results indicate that each cysteinyl class probably contains a single residue. Acetylcholine protects cysteine 1, but apparently does not protect cysteine 2. Vesamicol, which binds to a different site than acetylcholine does, apparently protects both cysteines, suggesting that it induces a conformational change. The relatively large reagent glutathione removes a substituent from cysteine 1, but not cysteine 2, suggesting that cysteine 2 is deeper in the transporter than cysteine 1 is. The complete sequence of T. californica VAChT is given, and possible identities of cysteines 1 and 2 are discussed.

Properties of the inner membrane anion channel in intact mitochondria

The mitochondrial inner membrane possesses an anion channel (IMAC) which mediates the electrophoretic transport of a wide variety of anions and is believed to be an important component of the volume homeostatic mechanism. IMAC is regulated by matrix Mg2+ (IC50 = 38 microM at pH 7.4) and by matrix H+ (pIC50 = 7.7). Moreover, inhibition by Mg2+ is pH-dependent. IMAC is also reversibly inhibited by many cationic amphiphilic drugs, including propranolol, and irreversibly inhibited by N,N'-dicyclohexylcarbodiimide. Mercurials have two effects on its activity: (1) they increase the IC50 values for Mg2+, H+, and propranolol, and (2) they inhibit transport. The most potent inhibitor of IMAC is tributyltin, which blocks anion uniport in liver mitochondria at about 1 nmol/mg. The inhibitory dose is increased by mercurials; however, this effect appears to be unrelated to the other mercurial effects. IMAC also appears to be present in plant mitochondria; however, it is insensitive to inhibition by Mg2+, mercurials, and N,N'-dicyclohexylcarbodiimide. Some inhibitors of the adenine nucleotide translocase also inhibit IMAC, including Cibacron Blue, agaric acid, and palmitoyl CoA; however, atractyloside has no effect.

Evidence for the existence of an inner membrane anion channel in mitochondria

Mitochondria normally exhibit very low electrophoretic permeabilities to physiologically important anions such as chloride, bicarbonate, phosphate, succinate, citrate, etc. Nevertheless, considerable evidence has accumulated which suggests that heart and liver mitochondria contain a specific anion-conducting channel. In this review, a postulated inner membrane anion channel is discussed in the context of other known pathways for anion transport in mitochondria. This anion channel exhibits the following properties. It is anion-selective and inhibited physiologically by protons and magnesium ions. It is inhibited reversibly by quinine and irreversibly by dicyclohexylcarbodiimide. We propose that the inner membrane anion channel is formed by inner membrane proteins and that this pathway is normally latent due to regulation by matrix Mg2+. The physiological role of the anion channel is unknown; however, this pathway is well designed to enable mitochondria to restore their normal volume following pathological swelling. In addition, the inner membrane anion channel provides a potential futile cycle for regulated non-shivering thermogenesis and may be important in controlled energy dissipation.