Extraction and kinetic characteristics of chicken liver mitochondrial malate dehydrogenase

Acute sources of mitochondrial NAD+ during respiratory chain dysfunction

It is a textbook definition that in the absence of oxygen or inhibition of the mitochondrial respiratory chain by pharmacologic or genetic means, hyper-reduction of the matrix pyridine nucleotide pool ensues due to impairment of complex I oxidizing NADH, leading to reductive stress. However, even under these conditions, the ketoglutarate dehydrogenase complex (KGDHC) is known to provide succinyl-CoA to succinyl-CoA ligase, thus supporting mitochondrial substrate-level phosphorylation (mSLP). Mindful that KGDHC is dependent on provision of NAD⁺, hereby sources of acute NADH oxidation are reviewed, namely i) mitochondrial diaphorases, ii) reversal of mitochondrial malate dehydrogenase, iii) reversal of the mitochondrial isocitrate dehydrogenase as it occurs under acidic conditions, iv) residual complex I activity and v) reverse operation of the malate-aspartate shuttle. The concept of NAD⁺ import through the inner mitochondrial membrane as well as artificial means of manipulating matrix NAD⁺/NADH are also discussed. Understanding the above mechanisms providing NAD⁺ to KGDHC thus supporting mSLP may assist in dampening mitochondrial dysfunction underlying neurological disorders encompassing impairment of the electron transport chain.

Determination of the catalytic mechanism for mitochondrial malate dehydrogenase

The kinetics of malate dehydrogenase (MDH) catalyzed oxidation/reduction of L-malate/oxaloacetate is pH-dependent due to the proton generated/taken up during the reaction. Previous kinetic studies on the mitochondrial MDH did not yield a consensus kinetic model that explains both substrate and pH dependency of the initial velocity. In this study, we propose, to our knowledge, a new kinetic mechanism to explain kinetic data acquired over a range of pH and substrate concentrations. Progress curves in the forward and reverse reaction directions were obtained under a variety of reactant concentrations to identify associated kinetic parameters. Experiments were conducted at physiologically relevant ionic strength of 0.17 M, pH ranging between 6.5 and 9.0, and at 25 °C. The developed model was built on the prior observation of proton uptake upon binding of NADH to MDH, and that the MDH-catalyzed oxidation of NADH may follow an ordered bi-bi mechanism with NADH/NAD binding to the enzyme first, followed by the binding of oxaloacetate/L-malate. This basic mechanism was expanded to account for additional ionic states to explain the pH dependency of the kinetic behavior, resulting in what we believe to be the first kinetic model explaining both substrate and pH dependency of the reaction velocity.

Hormonal effects and the control of gluconeogenesis from sorbitol, xylitol and glycerol in perfused chicken liver

Addition of sorbitol or xylitol to perfused chicken liver caused a biphasic increase in the rate of glucose production. The second increase correlated with a decrease in the lactate to pyruvate ratio. Increased glucose production in response to the addition of glycerol was not biphasic. Aminooxyacetate inhibited both the inherent second increase in glucose production and stimulatory effects of alanine and pyruvate. The stimulatory effects of norepinephrine and glucagon on gluconeogenesis from sorbitol decreased in the presence of methylene blue. Only the stimulatory effect of norepinephrine was inhibited by aminooxyacetate.

NAD-linked L(+)-lactate dehydrogenase from the strict aerobe alcaligenes eutrophus. 2. Kinetic properties and inhibition by oxaloacetate

The L(+)-lactate dehydrogenase (EC 1.1.1.27) of Alcaligenes eutrophus catalyzes the NADH-dependent reduction of pyruvate and a few other 2-oxoacids. The Km values for NADH, NAD, pyruvate and L(+)-lactate are 0.075 mM, 0.130 mM, 0.820 mM and 7.10 mM, respectively. The reaction follows a rapid equilibrium ordered bi-bi mechanism and involves the formation of a dead-end EBQ complex. The competitive inhibition of pyruvate reduction caused by NAD (with respect to NADH) is regarded to be of physiological importance. The enzyme is strongly inhibited by oxaloacetate, oxalate and to a less extent by oxamate. Oxaloacetate was found to be the most powerful inhibitor of the enzyme and exerts an almost complete inhibition of the reduction of pyruvate and some 2-oxoacids at concentrations of 1 microM and less. At 0.1 microM oxaloacetate the inhibition of pyruvate reduction is about 90%. The kinetics of pyruvate reduction in the presence of oxaloacetate is characterized by a burst phase followed by a decreased steady-state velocity. During the burst phase, which lasts from several seconds to some minutes, the enzyme undergoes transition to a less active enzyme form. The inhibition studies revealed the lactate dehydrogenase to be a hysteretic enzyme, due to its slow response to the ligand. The characteristics of the transient were examined. The inhibition of lactate dehydrogenase from A. eutrophus by oxaloacetate is considered to be of great physiological importance, allowing its function only at a low oxaloacetate concentration and consequently at high NADH/NAD ratios.

Kinetic mechanism of the molecular forms of chicken liver mitochondrial malate dehydrogenase

1. The reaction kinetic mechanism (pH 7.4) of the molecular forms of chicken liver m-MDH is of the order bi-bi ternary complex type with the existence of the E-oxaloacetate, E-L-malate, E-NAD+-oxaloacetate, E-NADH-L-malate, E-NAD+-NADH, E-NAD+-NAD+, E-NADH-NAD+ and E-NAD-NADH abortive complexes. 2. The saturating concentration values of the substrates are notably modified, in certain cases, in the presence of the reaction products.

Intramitochondrial location of the molecular forms of chicken liver mitochondrial malate dehydrogenase

1. The two molecular forms of mitochondrial malate dehydrogenase are partly bound to the mitochondrial membranes. 2. The A form is located on the outer surface of the inner mitochondrial membrane and also in the intermembrane space. 3. The B form of the enzyme appears in the matrix and bound in part, probably, to the inner surface of the inner mitochondrial membrane. 4. Glutamate dehydrogenase, glutamate oxaloacetate transaminase, fumarase and lactate dehydrogenase are bound, to a greater or lesser extent, to the mitochondrial membranes, the fumarase having the highest degree of binding.