Evidence for cooperative effects in the exchange reaction catalysed by the oxoglutarate translocator of rat-heart mitochondria

Biophysical studies on ATP synthase

The isolation of ATP synthase (F0F1) (82) and F0 (83) 34 years ago finally revealed that F0F1 is a motor composed of F0 (ion-motor, abc subunits) and F1 (ATP-motor, alpha 3 beta 3 gamma delta epsilon subunits) (Fig. 1). The single molecule videotape (4, 5, 65, 66) revealed that gamma epsilon axis of F1 rotates counterclockwise, proceeds by each 2 pi/3 step, and is driven by torque of 42 pN.nm (12) with nearly 100% efficiency (5) (Fig. 4). The motor is composed of a rotor (gamma epsilon-F0-c) and a stator (alpha 3 beta 3 delta-F0-ab), and the rotor is connected to a shaft (gamma epsilon). Since F0F1 is driven by delta microH+ (9, 10, 84), biophysical studies on stable TF0F1 (1, 7) are essential to elucidate the mechanism. These include nanomechanics (4, 5) (Fig. 4), crystallography (2, 3) (Figs. 2 and 3), NMR (51, 52), ESR (56), synchrotron analysis (3, 28), and electrophysiology (10, 25). The KmATP value of rotation is 0.8 microM, with the Vmax of 3.9 rps (5). This corresponds to the bi-site catalysis in proton transport by F0F1 (10, 70, 84). X-ray crystallography of MF1 (2) and the alpha 3 beta 3 oligomer of TF1 (3) (Fig. 2) together with mutation analyses revealed the role of residues in the rotation. The idea of elastic energy store is proposed in alpha 3 beta 3 gamma during the stepping time (up to a few sec) after the ATP binding. Biological studies have partially clarified the genetic and kinetic regulation of the rotation in MF1. Both theories (6, 7, 62, 64, 85) and the biological significance (17) of the intramolecular rotation of F0F1 await further studies, especially those of F0 and minor subunits.

Effect of aspartate and glutamate on the oxoglutarate carrier investigated in rat heart mitochondria and inverted submitochondrial vesicles

Interaction of glutamate and aspartate with the oxoglutarate carrier was investigated in rat heart mitochondria or inverted submitochondrial particles. With mitochondria, glutamate and aspartate had no effect on the initial rate of oxoglutarate or malate uptake. With inverted submitochondrial vesicles, binding experiments indicated that aspartate bound to the oxoglutarate carrier on its matricial face and increased the affinity of the substrate binding site for malate but did not change the affinity for oxoglutarate. Glutamate had no effect on both substrate bindings. The dissociation constants of the binary substrate-carrier complexes on the matricial side were determined (1.28 +/- 0.15 mM for oxoglutarate and 2.22 +/- 0.26 mM for malate). These values, compared with those obtained previously on the cytosolic side of intact mitochondria, confirmed the asymmetry of the carrier in the native membrane (higher affinities on the cytosolic face). It is concluded that (1) aspartate and glutamate are not cytosolic effectors of the oxoglutarate carrier, (2) matricial aspartate is a positive effector of the binding of malate on the matricial side of the oxoglutarate carrier, and (3) such a characteristic may play a role in the regulation of the oxoglutarate carrier. Thus, it may be emphasized that (1) this observation is the first clear evidence of a well-defined 'sophisticated regulation' (allosteric) of a mitochondrial metabolite carrier, and (2) this regulation of the oxoglutarate carrier may have important consequences on the efficiency of reducing equivalent import in the matrix space by the malate-aspartate shuttle.

Reaction mechanism of the reconstituted oxoglutarate carrier from bovine heart mitochondria

The transport mechanism of the reconstituted oxoglutarate carrier, purified from bovine heart mitochondria, was studied kinetically. A complete set of half-saturation constants (Km) was established for the two different substrates oxoglutarate and malate on both the external and the internal sides of the membrane. The internal affinities for oxoglutarate (Km 0.17 mM) and malate (Km 0.7 mM) were higher than the corresponding external affinities (Km 0.3 mM and 1.4 mM, respectively). The exclusive presence of a single transport affinity for each substrate on one side of the membrane indicated a unidirectional insertion of the oxoglutarate carrier into the liposomal membrane. The Km values and also the maximum exchange rates (8-11 mumol.min-1.mg protein-1) for oxoglutarate and malate were independent of the nature of the counter substrate on the other side of the membrane. Under these defined conditions we analyzed the antiport mechanism in two-reactant initial velocity studies varying both the internal and external substrate concentrations. From the kinetic patterns obtained, a sequential type of mechanism became evident, implying that one internal and one external substrate molecule form a ternary complex with the carrier before transport occurs. A quantitative analysis of substrate interaction with the unloaded or single-substrate-occupied carrier revealed that rapid-equilibrium random conditions were fulfilled, characterized by a fast and independent binding of internal and external substrate. This kinetic mechanism agrees with previous results obtained in intact mitochondria. Considering also the data available for other mitochondrial carriers, a common kinetic mechanism (sequential type) for this carrier family is suggested.

Spontaneous modification of the oxoglutarate translocator in vivo

In studying the oxoglutarate translocator of rat-heart mitochondria over many years, we have observed an unexpected decrease in its efficiency. It has been divided by 2.48 +/- 0.07, (S.E.M.) for the exchange of external oxoglutarate for internal malate at 2 degrees C when the internal-malate concentration is 4 mM and is accompanied by an increase in its concentration (multiplied by 1.61 +/- 0.02, S.E.M.). The affinity of the external sites of the translocator for the external oxoglutarate is unchanged as well as the binding and kinetic cooperativities of the external oxoglutarate. This shows that the external side of the translocator has not been modified and suggests that its central part has not been modified either. The apparent Michaelis constant of the internal malate is increased (multiplied by 1.74 +/- 0.23, S.E.M.) suggesting that the translocator has been modified on its matricial side. Some control experiments show that a change in the diet of the rats, despite its effect on the fatty-acid content of the mitoplasts, is probably not responsible for the observed modification. As it is nevertheless very likely that changes of the oxoglutarate translocator have occurred in vivo, it is proposed that the observed modification has a genetic origin. The existence of two antagonist changes which are not directly related suggests that one of them is a response of the organism against the other; thus the oxoglutarate translocator may play a regulatory rôle in certain physiological conditions.

Conformational changes and possible structure of the oxoglutarate translocator of rat-heart mitochondria revealed by the kinetic study of malate and oxoglutarate uptake

Initial rates of the exchanges [14C]malateout:malatein, ([14C]oxoglutarate + malate)out:malatein and ([14C]malate + oxoglutarate)out:malatein catalysed by the oxoglutarate carrier of rat-heart mitochondria have been studied under conditions where internal and external substrates may be varied. It is shown that contrary to external oxoglutarate which induces a conformational change of the translocator subunit to which it binds, external malate does not induce conformational changes during its binding and is a Michaelian substrate. The study of the effect of external malate on the rate of oxoglutarate uptake shows that external malate and external oxoglutarate are competitive. External oxoglutarate affects the catalytic rate constant of malate uptake in a modulated way. After substrate binding, the exchange reaction between an external dicarboxylate and an internal dicarboxylate is accompanied by conformational changes. The particular form of the rate equation strongly suggests that during a first step the external substrate bound to an external binding subunit at the external surface of the membrane, and the internal substrate bound to an internal binding subunit at the internal surface of the membrane, are transferred to a catalytic subunit (channel?) deeper in the membrane. Two models, one with a single channel, and the other with several associated channels, are proposed. It is demonstrated that a binding subunit which has transferred its substrate to a catalytic subunit is left in a conformation which does not depend on the substrate that has 'passed through it'. It is also demonstrated that all the catalytic subunits are identical. These theoretical deductions allow a simple description of the complicated effect that external oxoglutarate has on the rate of malate uptake. The fact that all the external binding subunits are equivalent regarding external malate binding and that all the catalytic subunits are identical support the view that the mitochondrial preparation contains a single species of oxoglutarate translocator and not an isozymic mixture.

The alpha-adrenergic-mediated activation of Ca2+ influx into cardiac mitochondria. A possible mechanism for the regulation of intramitochondrial free CA2+

Mitochondria isolated from rat hearts perfused with adrenaline, and from hearts excised from adrenaline-treated rats, showed an enhanced rate of respiration-dependent Ca2+ uptake. Adrenaline pretreatment did not change the activity of the Na+/Ca2+-antiporter of isolated heart mitochondria. Simultaneous measurements of the membrane potential revealed that perfusion with adrenaline has no significant effect on this parameter during Ca2+ accumulation. The activation of Ca2+ uptake was induced also by the alpha-adrenergic agonist, methoxamine, but not by the beta-adrenergic agonist, isoprenaline. Methoxamine pretreatment also increased the sensitivity of alpha-oxoglutarate dehydrogenase in intact mitochondria to 10 nM--300 nM extramitochondrial Ca2+ during steady-state Ca2+ recycling across the inner membrane. Possible implications of these data for the adrenergic regulation of oxidative metabolism are discussed.

Kinetic mechanism of the exchanges catalysed by the adenine-nucleotide carrier

Initial rates of the exchange ADPin/ADPout catalysed by the adenine-nucleotide carrier of rat-heart mitochondria have been studied under conditions where internal and external ADP may be varied. The initial rate was measured within 1 s by the carboxyatractyloside-stop method, using a rapid-mixing technique. The double-reciprocal plots v0(-1) versus [ADP]out-1 at different internal-ADP concentrations and v0(-1) versus [ADP]in-1 at different external-ADP concentrations exhibit straight-line relationships having a common point of intersection on the axis of ordinates. These results demonstrate the essential role of a ternary complex and thus exclude the ping-pong mechanism generally accepted. The kinetic equation implies a strong positive cooperativity in the binding of the two substrates. Two models are proposed: (a) the ternary complex performs the exchange and the transport of the substrates in a single step; (b) the carrier is mobile and transports the substrates one by one, the formation of a ternary complex being needed to release the first product.

Kinetic and binding properties of the oxoglutarate translocator of rat-heart mitochondria

The kinetic study of the oxoglutarateout/malatein exchange through the inner mitochondrial membrane of rat-heart mitochondria has been compelted and extended to higher external-oxoglutarate and to lower internal-malate concentrations. It has been found that the external oxoglutarate inhibits the exchange at high concentration. This excess-substrate inhibition is preceded by four jumps. The kinetic-saturation curve by the internal malate presents an apparent positive cooperativity that may be interpreted in different ways. The independence of the effects of the two substrates on the initial rate has been observed again and supports the conclusions reached in previous work. A method for the determination of oxoglutarate binding to the external face of the inner membrane is described. The binding curve shows four intermediary plateau regions that reflect significant apparent K-effects, alternatively negative and positive. For external-oxoglutarate concentrations below the region of excess-substrate inhibition, the binding-saturation curve and the kinetic-saturation curve are similar, demonstrating that K-effects are predominant. A particularly wide intermediary plateau that seems to correspond to half saturation of the active sites is common to both saturation curves. A clear lack of proportionality between the two curves at low oxoglutarate concentrations seems to indicate that more than one catalytic-rate constant is implied in the exchange kinetics. Two models of the oxoglutarate carrier are presented. Both lead to a minimum degree of 10:10 for the equation of the binding of oxoglutarate to the catalytic sites. In the first model this corresponds to ten subunits associated into a single oligomer while in the second model this results from a mixture of monomeric, dimeric, trimeric and tetrameric associations.