Succinate Oxidation In Mitochondria Research Articles

Nucleotide stimulation of the uncoupler-initiated inhibition of mitochondrial succinate oxidation

Tomato fruit mitochondria depended upon adenine nucleotide to relieve the inhibition of succinate oxidation initiated by uncouplers of oxidative phosphorylation. Both ADP and ATP were effective. Magnesium and other divalent cations enhanced this effectiveness. Oligomycin, sufficient to repress state 3 respiration in coupled mitochondria, slightly diminished but did not eliminate the effect of ADP. Relief by ADP depended on orthophosphate, negating the possibility that ATP, synthesized by adenylate kinase, was the basis of relief. The stimulation by the nucleotides thus did not appear to require the energy of ATP or to function through a reversible reaction involving ATP and ADP plus Pi. Stimulation may, therefore, result from a binding by nucleotide which effects a favorable conformation for respiration and/or an unfavorable condition for binding by an inhibitor. Pyruvate prevented the inhibition, indicating that oxaloacetate was the immediate cause of the inhibition.

The inhibitory effects in vitro of phenothiazines and other drugs on lipid-peroxidation systems in rat liver microsomes, and their relationship to the liver necrosis produced by carbon tetrachloride.

1. The effects of several phenothiazine derivatives on lipid-peroxidation systems in rat liver microsomes were studied and the results are considered in relation to the hepatotoxic action of carbon tetrachloride. 2. The lipid-peroxidation system coupled to NADPH(2) oxidation and stimulated by an ADP-Fe(2+) mixture is strongly inhibited in vitro by promethazine (50% inhibition at 29mum). Chlorpromazine and Stelazine also inhibit the peroxidation system but are less effective than promethazine. 3. The effects of promethazine on three other systems involving oxygen uptake (sulphite oxidation, orcinol oxidation and mitochondrial succinate oxidation) were also studied. Promethazine does not inhibit these systems to the same extent as it does the NADPH(2)-ADP-Fe(2+) lipid-peroxidation system. 4. Promethazine also produces an inhibition of the NADPH(2)-ADP-Fe(2+) system in liver microsomes after administration in vivo. It is concluded that the inhibition involves the interaction of the drug (or a metabolite of it) with the microsomal electron-transport chain. 5. Several other compounds known to protect the rat against liver necrosis after the administration of carbon tetrachloride were tested for inhibitory action on the NADPH(2)-ADP-Fe(2+) system. No clear correlation was observed between effectiveness in vivo as a protective agent and inhibitory effects on the NADPH(2)-ADP-Fe(2+) system in vitro. 6. Promethazine was found to inhibit the stimulation of lipid peroxidation produced in rat liver microsomes by low concentrations of carbon tetrachloride. This effect occurs at a concentration similar to that observed in vivo after administration of a normal clinical dose.

Multiple Molecular Forms of Bovine Heart Cytochrome c: V. A COMPARATIVE STUDY OF THEIR PHYSICOCHEMICAL PROPERTIES AND THEIR REACTIONS IN BIOLOGICAL SYSTEMS

Abstract The differences in the physicochemical and electron transfer properties of the multiple forms of bovine heart cytochrome c, Cy I to Cy IV, provide a means of correlating substitutions of aspartyl or glutamyl residues for asparaginyl or glutaminyl residues in the polypeptide chain with the conformation of this heme protein, as well as a new aspect in the study of the relation between its structure and function. The differences in light absorption spectra (at 25° and -190°) and optical rotatory dispersion, as well as in circular dichroic absorption spectra, of the multiple forms indicate that they differ in conformation, and that Cy I represents the native form. This, coupled with the fact that the oxidation-reduction potentials of the various forms are different, indicates that the environment of the heme group, i.e. the hemeprotein interaction, has been affected in Cy II and Cy III. Evidence is presented in support of a difference in conformation in the ferrous as well as in the ferric form, although the difference is clearer in the latter form. The different forms are slowly oxidized by molecular oxygen (278 µm) at neutral pH; the first order rate constant increases in the order Cy I l Cy II l Cy III, and numerically by about 3 times (from 3.0 x 10-5 to 8.3 x 10-5 sec-1). The different forms are reduced by ascorbate at pH 7.0 in a second order reaction. The rate constants decrease in the order Cy I g Cy II g Cy III and numerically by about 3 times (from 7.9 to 2.8 m-1 sec-1). The different forms are all able to restore succinate oxidation in mitochondria depleted of endogeneous cytochrome c. However, they are not isokinetic. Athough the V'max values are essentially the same, the K'm values for cytochrome c increase in the order Cy I l Cy II l Cy III and numerically by about 5.5 times (from 2.9 to 15.9 µm). The results put into perspective earlier studies on various chemically and enzymically modified forms of cytochrome c, and support the view that certain asparagine and glutamine residues are important to achieve the conformation and the biological activity characteristic of native mammalian cytochrome c.

Inhibition of mitochondrial succinate oxidation by alkyl hydroxy naphthoquinones

Summary 1. Effective inhibition by 3-alkyl-1,4-naphthoquinones of succinate oxidation in rat-liver mitochondria requires the presence of a 2-hydroxyl group and a saturated 2′–3′ bond in the 3-alkyl side chain. 2. Basic pH renders the inhibitors less effective and their action may also be reversed by the addition of serum albumin. 3. Inorganic phosphate is required for maximum inhibition in intact mitochondria, appearing to facilitate the penetration of the inhibitor to the site of action. 4. Inhibition induced by naphthoquinones may be released to differing extents by various uncoupling agents.

Energetic aspects of the mitochondrial oxidation of succinate.

Energetic aspects of the mitochondrial oxidation of succinate.

Mitochondrial respiratory control and phosphorylative activities in a magnesium-free medium

“Maximum” P/O vaues have been obtained in a magnesium-free medium for the mitochondrial oxidation of succinate, β-hydroxybutyrate, glutamate and α-ketoglutarate. Lack of magnesium in the medium employed causes a loss of mitochondrial integrity and respiratory control. These phenomena are influenced by ATP, pH, inorganic phosphate and osmotic pressure. Favourable conditions for avoiding a rapid loss are: high concentration of ATP. a pH of about 8.0, low concentration of inorganic phosphate and isotonicity. In the medium used, isolated liver mitochondria from guinea pig are more stable than those from rat. It is assumed that magnesium is specifically involved in the respiratory control mechanism.