On the mechanism of citrate and isocitrate protective action on rat liver mitochondria

Enzymatic reduction of 5,5'-dithiobis-(2-nitrobenzoic acid) by lysate of rat liver mitochondria

The oxidation of mitochondrial sulphydryl groups is known to increase the permeability of mitochondrial membranes and to be a key event in oxidative stress. Resistance to this damage is thought to involve thioredoxin reductase. In this study, the reduction of 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) by a lysate of rat liver mitochondria was used to assay the mitochondrial disulphide reducing capacity. NADPH-dependent reduction of DTNB was used to distinguish enzymatic reduction from the non-enzymatic reduction. Enzymatic reduction by the mitochondrial lysate was suppressed by DTNB at concentrations exceeding 0.25 mM and by pH above 7.0. It was strongly inhibited by Zn2+ and Mn2+ (IC50 about 2.5 and 20 microM, respectively) and was more weakly inhibited by Mg2+ and Ca2+ (IC50 about 1.8 and 2.1 mM, respectively). As a consequence of inhibition by divalent cations, the reaction was stimulated by both physiological (ATP, ADP, pyrophosphate and citrate) and non-physiological (EDTA and EGTA) chelators. Reduction of insulin disulphides by the mitochondrial lysate was dependent on the presence of a divalent cation chelator during the isolation of mitochondria and during the enzyme assay. Our results suggest that stimulation of mitochondrial disulphide reducing activity by lowered pH, as well as by increased levels of ATP, ADP and citrate, has the potential to contribute to the maintenance of mitochondrial sulphydryl groups under oxidative conditions.

A complex effect of arsenite on the formation of alpha-ketoglutarate in rat liver mitochondria

This investigation presents disturbances of the mitochondrial metabolism by arsenite, a hydrophilic dithiol reagent known as an inhibitor of mitochondrial alpha-keto acid dehydrogenases. Arsenite at concentrations of 0.1-1.0 mM was shown to induce a considerable oxidation of intramitochondrial NADPH, NADH, and glutathione without decreasing the mitochondrial membrane potential. The oxidation of NAD(P)H required the presence of phosphate and was sensitive to ruthenium red, but occurred without the addition of calcium salts. Mitochondrial reactions producing alpha-ketoglutarate from glutamate and isocitrate were modulated by arsenite through various mechanisms: (i) both glutamate transaminations, with oxaloacetate and with pyruvate, were inhibited by accumulating alpha-ketoglutarate; however, at low concentrations of alpha-ketoglutarate the aspartate aminotransferase reaction was stimulated due to the increase of NAD+ content; (ii) the oxidation of isocitrate was stimulated at its low concentration only, due to the oxidation of NADPH and NADH; this oxidation was prevented by concentrations of citrate or isocitrate greater than 1 mM; (iii) the conversion of isocitrate to citrate was suppressed, presumably as a result of the decrease of Mg2+ concentration in mitochondria. Thus the depletion of mitochondrial vicinal thiol groups in hydrophilic domains disturbs the mitochondrial metabolism not only by the inhibition of alpha-keto acid dehydrogenases but also by the oxidation of NAD(P)H and, possibly, by the change in the ion concentrations.

The participation of NADP, the transmembrane potential and the energy-linked NAD(P) transhydrogenase in the process of Ca2+ efflux from rat liver mitochondria

The pyridine nucleotide specificity, the participation of delta psi, and the energy-linked transhydrogenase in the process of Ca2+ efflux stimulated by the oxidized state of NAD(P) were examined in rat liver mitochondria energized by ascorbate + TMPD. The following observations were made: The Ca2+ efflux rate is independent of the redox state of mitochondrial NAD, but is at a minimum when mitochondrial NADP is in the reduced state and accelerated several-fold when it is in the oxidized state. When the redox state of NADP is shifted to a more oxidized state, the steady-state level of Ca2+ in the medium increased and delta psi decreased in proportion to the mitochondrial NADP+ level. The activity of the energy-linked NAD(P) transhydrogenase seems to be a key element in determining the redox state of NADP and thus of Ca2+ retention and efflux from mitochondria.

Ca2+ transport in isolated mouse liver mitochondria; role of reductive carboxylation and citrate?

The uptake of Ca2+ in isolated mouse liver mitochondria respiring on succinate in the presence of rotenone and added Pi, was inhibited by dibucaine, fluorocitrate, p-hydroxymercuribenzoate (PMB), malonate, palmitoyl-CoA, succinyl-CoA and trifluoroperazine. The release of accumulated Ca2+ was stimulated by arsenite, malonate, PMB, palmitoyl-CoA and succinyl-CoA, whereas the release was inhibited by dibucaine, fluorocitrate, trifluoroperazine, and by oligomycin, especially in the presence of ADP. The pyridine nucleotides were oxidized in mitochondria incubated with PMB. The observations suggest a possible contributory role of reductive carboxylation for the uptake of Ca2+, and a possible role of citrate for the retention of Ca2+ in isolated mouse liver mitochondria.

Citrate effects on the Ca2+-loading capacity of isolated rat liver mitochondria: interaction of citrate and ATP

The maximal amounts of Ca2+ being accumulated (delta Ca2+max) and H+ emitted (delta H+max) by Ca2+-loading mitochondria, with succinate (+rotenone) as respiratory substrate, were evaluated. delta Ca2+max was increased by providing either citrate or ATP to a Pi- and Mg2+-free medium. With citrate, delta H+max was only scarcely increased, so that the effect of the proton-carrying anion resulted essentially from an increase in the Ca2+/H+ ratio, i.e., from preservation of membrane potential. With ATP (+/- oligomycin), the Ca2+/H+ ratio was unaltered; i.e., the increase of delta Ca2+max was paralleled by a related increase in delta H+max. Mitochondria appeared to retain Ca at higher delta pH, i.e., at lower membrane potential, in the presence of ATP. With citrate and ATP together, both the Ca2+/H+ ratio and delta H+max were largely increased, and the product of these two terms, delta Ca2+max, was considerably enlarged. The effect of either citrate or ATP was markedly reinforced in the presence of the other anion. In addition to increasing the Ca2+/H+ ratio, citrate contributed to increasing delta H+max in the presence of ATP, i.e., apparently sensitized mitochondria to the action of ATP. A citrate-induced depression of Ca2+ cycling across the inner membrane, even though pronounced, did not account for the sensitization. Supraadditive effects of citrate and ATP persisted in the presence of MgCl2 and Pi, under conditions of massive Ca2+ loading, and may contribute to the high capacity of mitochondria, in situ, to retain calcium.

Alloxan effects on mitochondria: study of oxygen consumption, fluxes of Mg2+, Ca2+, K+ and adenine nucleotides, membrane potential and volume change in vitro

Isolated mouse liver mitochondria incubated with alloxan showed stimulated resting (state 4) respiration with succinate, and inhibited resting respiration with pyridine-linked substrates, whereas active (state 3) respiration was decreased with both kinds of substrates. The effects were dependent on the concentration of alloxan, on the energy state, and on transport of inorganic phosphate and uptake of Ca2+. Using succinate as substrate, the effects of alloxan on endogenous Mg2+, K+ and adenine nucleotides, uptake of K+, accumulated Ca2+, membrane potential and volume were studied in liver mitochondria, and in addition efflux of endogenous K+ and accumulated Ca2+ were investigated in mouse islet mitochondria. High concentrations of alloxan (greater than or equal to 3 mmol/l) induced efflux of endogenous Mg2+, K+ and adenine nucleotides, efflux of accumulated Ca2+, inhibition of uptake of K+, loss of membrane potential, and swelling. Low concentrations of alloxan (less than 3 mmol/l) had similar effects only in the presence of added Ca2+ and inorganic phosphate. The influence of potentially protective agents was studied mainly with regard to alloxan induced swelling. Complete or partial protection was offered by antimycin A, malonate, La3+, Ni2+, ruthenium red, mersalyl and N-ethylmaleimide, suggesting requirement for energized transport of Ca2+ and uptake of inorganic phosphate. The start of the respiratory changes, decrease of membrane potential and loss of Mg2+ preceded the release of accumulated Ca2+, which occurred in parallel with efflux of K+ and swelling. The loss of Ca2+ in association with swelling agrees with data previously obtained using qualitative and quantitative electron microscopy and X-ray microanalysis of islet beta cells from alloxan-treated mice.(ABSTRACT TRUNCATED AT 250 WORDS)