The binding of dicyclohexylcarbodiimide to uncoupling protein in brown adipose tissue mitochondria

Czech Footprints in the Bioenergetics Research

Life manifests as growth, movement or heat production that occurs thanks to the energy accepted from the outside environment. The basis of energy transduction attracted the Czech researchers since the beginning of the 20th century. It further accelerated after World War II, when the new Institute of Physiology was established in 1954. When it was found that energy is stored in the form of adenosine triphosphate (ATP) that can be used by numerous reactions as energy source and is produced in the process called oxidative phosphorylation localized in mitochondria, the investigation focused on this cellular organelle. Although the Czech scientists had to overcome various obstacles including Communist party leadership, driven by curiosity, boldness, and enthusiasm, they characterized broad spectrum of mitochondrial properties in different tissues in (patho)physiological conditions in collaboration with many world-known laboratories. The current review summarizes the contribution of the Czech scientists to the bioenergetic and mitochondrial research in the global context. Keywords: Mitochondria, Bioenergetics, Chemiosmotic coupling.

Undesirable effects of chemical inhibitors of NAD(P)+ transhydrogenase on mitochondrial respiratory function

NAD(P)⁺ transhydrogenase (NNT) is located in the inner mitochondrial membrane and catalyzes a reversible hydride transfer between NAD(H) and NADP(H) that is coupled to proton translocation between the intermembrane space and mitochondrial matrix. NNT activity has an essential role in maintaining the NADPH supply for antioxidant defense and biosynthetic pathways. In the present report, we evaluated the effects of chemical compounds used as inhibitors of NNT over the last five decades, namely, 4-chloro-7-nitrobenzofurazan (NBD-Cl), N,N'-dicyclohexylcarbodiimide (DCC), palmitoyl-CoA, palmitoyl-l-carnitine, and rhein, on NNT activity and mitochondrial respiratory function. Concentrations of these compounds that partially inhibited the forward and reverse NNT reactions in detergent-solubilized mouse liver mitochondria significantly impaired mitochondrial respiratory function, as estimated by ADP-stimulated and nonphosphorylating respiration. Among the tested compounds, NBD-Cl showed the best relationship between NNT inhibition and low impact on respiratory function. Despite this, NBD-Cl concentrations that partially inhibited NNT activity impaired mitochondrial respiratory function and significantly decreased the viability of cultured Nnt^-/- mouse astrocytes. We conclude that even though the tested compounds indeed presented inhibitory effects on NNT activity, at effective concentrations, they cause important undesirable effects on mitochondrial respiratory function and cell viability.

Evidence for two distinct chloride uniport pathways in brown adipose tissue mitochondria

Chloride permeability of the inner membrane of brown adipose tissue mitochondria was analyzed by monitoring mitochondrial swelling in KCl salts in the presence of K+ ionophores. The results indicate that the high anion conductance observed in these mitochondria is due to the presence of two separate pathways: 1) a Cl-conducting pathway that is inhibited by guanosine 5'-diphosphate (GDP) but neither by N,N'-dicyclohexylcarbodiimide (DCCD) nor by amphiphilic amines and that is found uniquely in brown adipose tissue mitochondria and 2) an inner membrane anion uniport channel that is inhibited both by DCCD and by amphiphilic amines but not by GDP and that is opened either by depletion of matrix Mg2+ or by alkalinization of the matrix.

Control of uncoupling protein in brown-fat mitochondria by purine nucleotides. Chemical modification by diazobenzenesulfonate

The uncoupling protein (UP) of isolated brown adipose tissue mitochondria was studied with respect to the mechanism of control of UP function by purine nucleotides. Passive transport of H+ and Cl- was followed simultaneously in a KCl medium. With both GDP and ATP a higher sensitivity of Cl- transport (apparent Ki = 2.2 microM and 4.7 microM respectively) than of H+ transport (apparent Ki = 7.7 microM and 34 microM respectively) was observed. Chemical modification of isolated mitochondria by diazobenzenesulfonate (DABS) up to 75 mumol/mg protein did not affect the transport, its ionic selectivity and regulation by endogenous free fatty acids. In contrast, the sensitivity to purine nucleotides of both H+ and Cl- translocation was decreased (apparent Ki increased 71 and 47 times respectively). DABS decreased the affinity of [3H]GDP for the specific nucleotide-binding site on mitochondria (Kd increased from 2.7 microM to 13 microM) and depressed, to a smaller extent, the GDP-binding capacity. Correlation between occupancy of the specific nucleotide-binding site by GDP and inhibition of transport yielded a linear relationship for Cl- transport in control mitochondria. For H+ transport in the control, and for both H+ and Cl- transports in DABS-treated mitochondria, a biphasic correlation was obtained. The results show that different structural parts of UP are involved in transport and its control by the regulatory ligands and that, in addition to binding of purine nucleotides to UP, the inhibition of ion transport by purine nucleotides depends on an intrinsic factor modulating the inhibitory effect.

Sulfhydryl groups are involved in H+ translocation via the uncoupling protein of brown adipose tissue mitochondria

Mersalyl inhibits H+ transport via the uncoupling protein (UP) in brown adipose tissue (BAT) mitochondria estimated as swelling in potassium acetate (Ki 67 microM) or as valinomycin-induced H+ extrusion in K2SO4 (Ki 55 microM) and KCl. The swelling in KCl is depressed only slightly. Some other SH-reagents (p-hydroxymercuribenzoate, 5,5'-dithiobis(2-nitrobenzoate) and thiolyte DB), but not hydrophobic reagents (N-ethylmaleimide and eosin-5-maleimide), exhibit analogous inhibition. Thus an essential SH-group localized at the water-accessible cytosolic surface of UP was found to be involved in H+ transport via UP but not in Cl- transport.

Chemical modification of the brown fat mitochondrial uncoupling protein with tetranitromethane

Tetranitromethane reacts with the uncoupling protein of intact brown fat mitochondria. The chloride permeability in the absence of the inhibitory nucleotide GDP is not affected, but the affinity with which GDP binds is decreased, and the coupling between binding of nucleotide and inhibition of chloride permeation is broken.

Specific interaction of fatty acyl-CoA esters with brown adipose tissue mitochondria

The ability of low concentrations of long-chain fatty acyl coenzyme A (CoA) esters to act as inhibitors of purine nucleotide action in hamster brown adipose tissue mitochondria was observed to result from a specific interaction. Palmitoyl-CoA was found to be a competitive inhibitor of nucleotide binding with an apparent Ki of 2.46 +/- 1.09 microM for GDP and 2.98 +/- 0.538 microM for ATP and was able to counteract GDP-inhibited mitochondrial swelling. A minimum acyl-CoA carbon chain length of 12 was necessary for any significant inhibition of GDP binding or induction of swelling to be observed. The effect of palmitoyl-CoA on reversing GDP-inhibited chloride permeability of brown adipose tissue mitochondria was found to be the result of a specific interaction with the brown adipose tissue mitochondrial uncoupling protein. Mitochondria pretreated with N,N'-dicyclohexylcarbodiimide, which binds covalently to the uncoupling protein and partially inhibits brown adipose tissue mitochondrial swelling, underwent a nonspecific increase in swelling in the presence of phenylmercuric acetate. However, with palmitoyl-CoA no further increase in permeability could be mediated. In addition, under certain experimental conditions, palmitoyl-CoA was found to partially inhibit the high halide permeability of brown adipose tissue mitochondria but to a lesser extent than that observed with GDP, suggesting it may be acting as a partial agonist.

Thermodynamic and steady-state-kinetic investigation of the effect of NN'-dicyclohexylcarbodi-imide on H+ translocation by the mitochondrial cytochrome bc1 complex

Steady-state kinetic measurements showed that NN'-dicyclohexylcarbodi-imide decreased the observed H+/2e ratio of H+ transport by mitochondria respiring on succinate, acting mainly at the cytochrome bc1 complex. Thermodynamic assessment of the H+/2e ratio by measuring the force ratio across the bc1 complex showed that the inhibitor did not affect H+ translocation. Possible explanations of this disagreement between methods are examined; we conclude that the inhibitor does not alter the mechanistic stoichiometry of H+ pumping by the bc1 complex.

Evidence for the presence of one carboxyl group in the catalytic center of D-beta-hydroxybutyrate dehydrogenase. Inactivation and binding studies with carbodiimide reagents

D-beta-Hydroxybutyrate dehydrogenase D-3-hydroxybutyrate: NAD+ oxidoreductase, EC 1.1.1.30), a phosphatidylcholine-requiring enzyme, was irreversibly inactivated by a water-soluble carbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDAC) or a hydrophobic carbodiimide, N,N'-dicyclohexylcarbodiimide (DCCD). The inactivation is pseudo-first-order with a kinetic stoichiometry of about 1. Phospholipid-free apoenzyme was more sensitive towards these reagents than reconstituted phospholipid-enzyme or membrane-bound enzyme forms. Reduced coenzyme (NADH) protected the enzyme against the inactivation, while oxidized coenzyme (NAD+) in presence of 2-methylmalonate (a pseudo-substrate) gave a better protection. It was found that the phospholipid-free apoenzyme bound about 1 mol [14C]DCCD. This incorporation was prevented by EDAC, indicating that both reagents react at the same site. [14C]Glycine ethyl ester, a nucleophilic compound which reacts specifically with the carboxylcarbodiimide derivative was incorporated to the enzyme (1 mol [14C]glycine ethyl ester per polypeptide chain), whatever its form, in the presence of DCCD or EDAC. These results indicate the presence of one carboxyl group probably located at or near the coenzyme-binding site and near the interacting domain of the enzyme with phospholipid.

The possible proton translocating activity of the mitochondrial uncoupling protein of brown adipose tissue. Reconstitution studies in liposomes

Loose coupling of thermogenic mitochondria of brown adipose tissue is related to a high proton (or hydroxyl) conductance of the inner membrane and to the presence of a unique 32 kDa uncoupling protein. Reconstitution experiments of the purified protein in liposomes are reported which suggest that this component could form proton channels in the membrane.

Inhibition of electrogenic anion entry into rat liver mitochondria by N,N'-dicyclohexylcarbodiimide

The carboxyl group reagent dicyclohexylcarbodiimide inhibits the electrogenic entry of Cl- and NO3-into rat liver mitochondria at alkaline pH. The inhibition is time dependent and 50% inhibition is obtained by the addition of 3-4 nmol DCCD/mg protein. The blockage of the pH-dependent anion-conducting pore appears to be unrelated to the other known actions of DCCD on rat liver mitochondria but seems similar to its effect on the uncoupling protein of brown adipose tissue.