Purification and partial characterization of bovine heart mitochondrial pyridine dinucleotide transhydrogenase

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.

Phospholipid dependence of the reversible, energy-linked, mitochondrial transhydrogenase in Manduca sexta

Midgut mitochondria from fifth larval instar Manduca sexta exhibit a membrane-associated transhydrogenase that catalyzes hydride ion transfer between NADP(H) and NAD(H). The NADPH-forming transhydrogenations occur as nonenergy- and energy-linked activities. The energy-linked activities couple with electron transport-dependent utilization of NADH/succinate, or with Mg(2+)-dependent ATPase. These energy-linked transhydrogenations have been shown to be physiologically and developmentally significant with respect to insect larval/pupal maturation. In the present study, isolated mitochondrial membranes were lyophilized and subjected to organic solvent or phospholipase treatments. Acetone extraction and addition of Phospholipase A(2) proved to be effective inhibitors of the insect transhydrogenase. Liberation of phospholipids was reflected by measured phosphorous release. Addition of phospholipids to organic solvent- and phospholipase-treated membranes was without effect. Employing a partially lipid-depleted preparation, phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine were reintroduced and transhydrogenase activity assessed. Of the phospholipids tested, only phosphatidylcholine significantly stimulated transhydrogenase activity. The results of this study suggest a phospholipid dependence of the M. sexta mitochondrial transhydrogenase.

Proton-translocating transhydrogenase: an update of unsolved and controversial issues

Proton-translocating transhydrogenases, reducing NADP(+) by NADH through hydride transfer, are membrane proteins utilizing the electrochemical proton gradient for NADPH generation. The enzymes have important physiological roles in the maintenance of e.g. reduced glutathione, relevant for essentially all cell types. Following X-ray crystallography and structural resolution of the soluble substrate-binding domains, mechanistic aspects of the hydride transfer are beginning to be resolved. However, the structure of the intact enzyme is unknown. Key questions regarding the coupling mechanism, i.e., the mechanism of proton translocation, are addressed using the separately expressed substrate-binding domains. Important aspects are therefore which functions and properties of mainly the soluble NADP(H)-binding domain, but also the NAD(H)-binding domain, are relevant for proton translocation, how the soluble domains communicate with the membrane domain, and the mechanism of proton translocation through the membrane domain.

The heterotrimer of the membrane-peripheral components of transhydrogenase and the alternating-site mechanism of proton translocation

Transhydrogenase undergoes conformational changes to couple the redox reaction between NAD(H) and NADP(H) to proton translocation across a membrane. The protein comprises three components: dI, which binds NAD(H); dIII, which binds NADP(H); and dII, which spans the membrane. Experiments using isothermal titration calorimetry, analytical ultracentrifugation, and small angle x-ray scattering show that, as in the crystalline state, a mixture of recombinant dI and dIII from Rhodospirillum rubrum transhydrogenase readily forms a dI(2)dIII(1) heterotrimer in solution, but we could find no evidence for the formation of a dI(2)dIII(2) tetramer using these techniques. The asymmetry of the complex suggests that there is an alternation of conformations at the nucleotide-binding sites during proton translocation by the complete enzyme. The characteristics of nucleotide interaction with the isolated dI and dIII components and with the dI(2)dIII(1) heterotrimer were investigated. (a) The rate of release of NADP(+) from dIII was decreased 5-fold when the component was incorporated into the heterotrimer. (b) The binding affinity of one of the two nucleotide-binding sites for NADH on the dI dimer was decreased about 17-fold in the dI(2)dIII(1) complex; the other binding site was unaffected. These observations lend strong support to the alternating-site mechanism.

The crystal structure of an asymmetric complex of the two nucleotide binding components of proton-translocating transhydrogenase

BACKGROUND

Membrane-bound ion translocators have important functions in biology, but their mechanisms of action are often poorly understood. Transhydrogenase, found in animal mitochondria and bacteria, links the redox reaction between NAD(H) and NADP(H) to proton translocation across a membrane. Linkage is achieved through changes in protein conformation at the nucleotide binding sites. The redox reaction takes place between two protein components located on the membrane surface: dI, which binds NAD(H), and dIII, which binds NADP(H). A third component, dII, provides a proton channel through the membrane. Intact membrane-located transhydrogenase is probably a dimer (two copies each of dI, dII, and dIII).

RESULTS

We have solved the high-resolution crystal structure of a dI:dIII complex of transhydrogenase from Rhodospirillum rubrum-the first from a transhydrogenase of any species. It is a heterotrimer, having two polypeptides of dI and one of dIII. The dI polypeptides fold into a dimer. The loop on dIII, which binds the nicotinamide ring of NADP(H), is inserted into the NAD(H) binding cleft of one of the dI polypeptides. The cleft of the other dI is not occupied by a corresponding dIII component.

CONCLUSIONS

The redox step in the transhydrogenase reaction is readily visualized; the NC4 atoms of the nicotinamide rings of the bound nucleotides are brought together to facilitate direct hydride transfer with A-B stereochemistry. The asymmetry of the dI:dIII complex suggests that in the intact enzyme there is an alternation of conformation at the catalytic sites associated with changes in nucleotide binding during proton translocation.

The unusual transhydrogenase of Entamoeba histolytica

We have expressed and purified a protein fragment from Entamoeba histolytica. It catalyses transhydrogenation between analogues of NAD(H) and NADP(H). The characteristics of this reaction resemble those of the reaction catalysed by a complex of the NAD(H)- and NADP(H)-binding subunits of proton-translocating transhydrogenases from bacteria and mammals. It is concluded that the complete En. histolytica protein, which, along with similar proteins from other protozoan parasites, has an unusual subunit organisation, is also a proton-translocating transhydrogenase. The function of the transhydrogenase, thought to be located in organelles which do not have the enzymes of oxidative phosphorylation, is not clear.

Physiological roles of nicotinamide nucleotide transhydrogenase

From the foregoing considerations, the energy-linked transhydrogenase reaction emerges as a powerful and flexible element in the network of redox and energy interrelationships that integrate mitochondrial and cytosolic metabolism. Its thermodynamic features make it possible for the reaction to respond readily to challenges, either on the side of NADPH utilization or on the side of energy depletion. Yet, the kinetic features are designed to prevent a wasteful input of energy when other sources of reducing equivalents to NADP are available, or to deplete the redox potential of NADPH in other than emergency conditions. By virtue of these characteristics, the energy-linked transhydrogenase can act as an effective buffer system, guarding against an excessive depletion of NADPH, preventing uncontrolled changes in key metabolites associated with NADP-dependent enzymes and calling on the supply of reducing equivalents from NAD-linked substrates only under conditions of high demand for NADPH. At the same time, it can provide an emergency protection against a depletion of energy, especially in situations of anoxia where a supply of reducing equivalents through NADP-linked substrates can be maintained. The flexibility of this design makes it possible that the functions of the energy-linked transhydrogenase vary from one tissue to another and are readily adjustable to different metabolic conditions.

Energy-linked nicotinamide nucleotide transhydrogenase: hydrodynamic properties and active form of purified and membrane-bound mitochondrial transhydrogenase from beef heart

The mitochondrial nicotinamide nucleotide transhydrogenase from beef heart was investigated with respect to minimal assembly of the purified enzyme and of the enzyme in the mitochondrial inner membrane. Studies of the hydrodynamic properties of the purified enzyme in the presence of 0.3% Triton X-100 allowed determination of the Stokes radius, sedimentation constant, partial specific volume, frictional ratio, and molecular weight. Under these conditions transhydrogenase existed as an inactive monomer, suggesting that monomerization may be accompanied by inactivation. Radiation inactivation was used to determine the functional molecular size of purified detergent-dispersed transhydrogenase and transhydrogenase in beef heart submitochondrial particles. Under these conditions the catalytic activity of both the purified and the membrane-bound enzyme was found to be catalyzed by a dimeric form of the enzyme. These results suggest for the first time that the minimal functional assembly of detergent-dispersed as well as membrane-bound transhydrogenase is a dimer, which is not functionally associated with, for example, complex I or ATPase. In addition, the results are consistent with the possibility that the two subunits of transhydrogenase are catalytically active in an alternating fashion according to a previously proposed half-of-the-sites reactivity model.

Expression of the cloned subunits of Escherichia coli transhydrogenase from separate replicons

The pntA and pntB genes of Escherichia coli, encoding the alpha- and beta-subunits of the pyridine nucleotide transhydrogenase, were cloned individually in two different compatible plasmids into Escherichia coli mutants lacking transhydrogenase activity. Energy-linked and non-energy-linked transhydrogenase activities were produced only in cells carrying both plasmids thus showing that the products of both genes are required for the formation of an active enzyme. ATP-energized transhydrogenase activity was not increased in cells containing amplified levels of the transhydrogenase when the cell membrane ATPase was also amplified. It is suggested that the excess transhydrogenase is effectively uncoupled from the ATPase by compartmentalization in the cell.

Affinity chromatography of mitochondrial nicotinamide nucleotide transhydrogenase

The binding of mitochondrial nicotinamide nucleotide transhydrogenase to NAD+ and NADP+ immobilized to agarose through different parts of the nicotinamide nucleotide molecule was investigated. NADP+ bound at the C8 atom in the adenine moiety proved to be the most efficient ligand whereas that bound at the C3 atom of the ribose moiety was relatively inefficient. NAD+ ligands were generally inactive independently of the site of attachment. Previous results suggest, however, that binding to immobilized NAD+ may be influenced by the detergent in which transhydrogenase is dispersed. Binding to neither ligand was affected by the presence of the second substrate.

Purification and properties of reconstitutively active nicotinamide nucleotide transhydrogenase of Escherichia coli

The nicotinamide nucleotide transhydrogenase of Escherichia coli has been purified from cytoplasmic membranes by pre-extraction of the membranes with sodium cholate and Triton X-100, solubilization of the enzyme with sodium deoxycholate in the presence of 1 M potassium chloride, and centrifugation through a 1.1 M sucrose solution. The purified enzyme consists of two subunits, alpha and beta, of apparent Mr 50000 and 47000. During transhydrogenation between NADPH and 3-acetylpyridine adenine dinucleotide by both the purified enzyme reconstituted into liposomes and the membrane-bound enzyme, a pH gradient is established across the membrane as indicated by the quenching of the fluorescence of 9-aminoacridine. Treatment of transhydrogenase with N,N'-dicyclohexylcarbodiimide results in an inhibition of proton pump activity and transhydrogenation, suggesting that proton translocation and catalytic activities are obligatory linked. NADH protected the enzyme against inhibition by N,N'-dicyclohexylcarbodiimide, while NADP, and to a lesser extent NADPH, appeared to increase the rate of inhibition. [14C]Dicyclohexylcarbodiimide preferentially labelled the 50000-Mr subunit of the transhydrogenase enzyme. The presence of an allosteric binding site which reacts with NADH, but not with reduced 3-acetylpyridine adenine dinucleotide, has been demonstrated.

Cloning and expression of the transhydrogenase gene of Escherichia coli

Based on the rationale that Escherichia coli cells harboring plasmids containing the pnt gene would contain elevated levels of enzyme, we have isolated three clones bearing the transhydrogenase gene from the Clarke and Carbon colony bank. The three plasmids were subjected to restriction endonuclease analysis. A 10.4-kilobase restriction fragment which overlapped all three plasmids was cloned into the PstI site of plasmid pUC13. Examination of several deletion derivatives of the resulting plasmid and subsequent treatment with exonuclease BAL 31 revealed that enhanced transhydrogenase expression was localized within a 3.05-kilobase segment. This segment was located at 35.4 min in the E. coli genome. Plasmid pDC21 conferred on its host 70-fold overproduction of transhydrogenase. The protein products of plasmids carrying the pnt gene were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of membranes from cells containing the plasmids. Two polypeptides of molecular weights 50,000 and 47,000 were coded by the 3.05-kilobase fragment of pDC11. Both polypeptides were required for expression of transhydrogenase activity.

Studies of the ferricyanide reductase activities of the mitochondrial reduced nicotinamide adenine dinucleotide-ubiquinone reductase (complex I) utilizing arylazido-beta-alanyl NAD+ and arylazido-beta-alanyl NADP+

The NADH and NADPH ferricyanide reductase activities present in mitochondrial NADH-CoQ reductase preparations have been studied utilizing two photoaffinity pyridine nucleotide analogues: arylazido-beta-alanyl NAD+ (A3'-O-[3-[N-(4-azido-2-nitrophenyl)amino]propionyl]NAD+) and arylazido-beta-alanyl NADP+ (N3'-O-[3-[N-(4-azido-3-nitrophenyl)amino]propionyl]NADP+). For the NADH-K3Fe(CN)6 reductase activity, arylazido-beta-alanyl NAD+ was found to be, in the dark, a competitive inhibitor with respect to both NADH and K3Fe(CN)6 with Ki,app values of 9.7 and 15.5 microM, respectively. In comparison the NADP+ analogue exhibited weak noncompetitive inhibitor activity for this reaction against both substrates. Upon photoirradiation arylazido-beta-alanyl NAD+ inhibited NADH-K3Fe(CN)6 reductase up to 70% in the presence of a 25-fold molar excess of analogue over the enzyme concentration. This photodependent inhibition could be prevented by the presence, during irradiation, of the natural substrate NADH. In contrast complex kinetic results were obtained with studies of the effects of the pyridine nucleotide analogues of NADPH-K3Fe(CN)6 reductase activity in the dark. Photoirradiation of either analogue in the presence of the enzyme complex resulted in an activation of NADPH-dependent activity. The possibility that the NADPH-K3Fe(CN)6 reductase activity of complex I represents a summation of the combined ferricyanide reductase activity of the NADPH-NAD+ transhydrogenase and NADH oxidoreductase is suggested.

Why are two different types of pyridine nucleotide transhydrogenase found in living organisms?

Two types of pyridine nucleotide transhydrogenases have been reported in living organisms. The energy-linked transhydrogenase is found in mitochondria and in certain heterotrophic and photosynthesizing bacteria, while the non-energy-linked transhydrogenase is found in certain heterotrophic bacteria. The presence of a structurally similar non-energy-linked transhydrogenase in Azotobacter vinelandii, Pseudomonas aeruginosa and Pseudomonas fluorescens is readily shown in extracts from these bacteria with Western (protein) blotting. This non-energy-linked enzyme is lacking in Escherichia coli, while the presence of a structurally similar energy-linked enzyme in E. coli and in beef heart mitochondria is indicated with the Western blotting technique. Spinach (Spinacia oleracea) lacks the non-energy-linked transhydrogenase occurring in bacteria. The chloroplast enzyme ferredoxin:NADP+ oxidoreductase, which exhibits non-energy-linked transhydrogenase activity, is immunologically distinct from the bacterial transhydrogenases. In order to provide a rationale for the distribution of the two types of pyridine nucleotide transhydrogenases, the steady-state degrees of reduction of the NADP(H) and NAD(H) pools in A. vinelandii (R'NADP(H) and R'NAD(H)) have been measured for cells metabolizing sucrose at a variable oxygen flux (phi O2). It is found that the degree of reduction of the NADP(H) pool is always higher than that of the NAD(H) pool (R'NADP(H) greater than R'NAD(H)) except when phi O2 goes to zero (R'NADP(H) approximately equal to R'NAD(H)). Comparison of these results with literature values indicates that the inequality R'NADP(H) greater than R'NAD(H) is always found in a membrane-enclosed compartment, irrespective of the type of transhydrogenase present. This allows an understanding of the function of the two types of pyridine nucleotide transhydrogenases in vivo. The physiological role of non-energy-linked transhydrogenase is to catalyze the reaction NADPH + NAD+ leads to NADP+ + NADH, that of energy-linked transhydrogenase to catalyze the reaction NADH + NADP+ leads to NADPH + NAD+. Since at equilibrium R'NADP(H) approximately equal to R'NAD(H) the inequality R'NADP(H) greater than R'NAD(H) under steady-state conditions explains the energy requirement in the latter reaction. The dependence of the non-energy-linked transhydrogenase activity of ferredoxin:NADP+ oxidoreductase on R'NADP(H) is compared with that of A, vinelandii transhydrogenase. The results indicate that this activity is unlikely to be of physiological importance in plant chloroplasts.

The effects of some mitochondrial translocase inhibitors on the activity of rat liver transhydrogenase

Non-energy dependent transhydrogenase activity in submitochondrial particles is stimulated at pH 8 by some inhibitors of mitochondrial carboxylate translocases. On solubilising the enzyme or assaying it at pH 6 these inhibitors have little effect. Km values remain unchanged under all these conditions. Conversely, energy-dependent transhydrogenase activity is inhibited by these substances possibly due to their direct action on proton transport. It is suggested that the observed effects may all be due to changes in membrane conformation induced by the inhibitors.

Modification of bovine heart mitochondrial transhydrogenase with tetranitromethane

Modification of pyridine dinucleotide transhydrogenase with tetranitromethane resulted in inhibition of its activity. Development of a membrane potential in submitochondrial particles during the reduction of 3-acetylpyridine adenine dinucleotide (AcPyAD+) by NADPH decreased to nearly the same extent as the transhydrogenase rate on tetranitromethane treatment of the membrane. Kinetics of the inactivation of homogeneous transhydrogenase and the enzyme reconstituted into phosphatidylcholine liposomes indicate that a single essential residue was modified per active monomer. NADP+, NADPH and NADH gave substantial protection against tetranitromethane inactivation of both the nonenergy-linked and energy-linked transhydrogenase reactions of submitochondrial particles and the NADPH leads to AcPyAD+ reaction of reconstituted enzyme. NAD+ had no effect on inactivation. Tetranitromethane modification of reconstituted transhydrogenase resulted in a decrease in the rate of coupled H+ translocation that was comparable to the decrease in the rate of NADPH leads to AcPyAD+ transhydrogenation. It is concluded that tetranitromethane modification controls the H+ translocation process solely through its effect on catalytic activity, rather than through alteration of a separate H+-binding domain. Nitrotyrosine was not found in tetranitromethane-treated transhydrogenase. Both 5,5'-dithiobis(2-nitrobenzoate)-accessible and buried sulfhydryl groups were modified with tetranitromethane. NADH and NADPH prevented sulfhydryl reactivity toward tetranitromethane. These data indicate that the inhibition seen with tetranitromethane results from the modification of a cysteine residue.

The subunit structure of bovine heart mitochondrial transhydrogenase

Reaction of purified bovine heart transhydrogenase with bifunctional cross-linking reagents dimethyl adipimidate, dimethyl pimelimidate, dimethyl suberimidate, and dithiobis(succinimidyl propionate) results in the appearance of a dimer band on sodium dodecyl sulfate polyacrylamide gels with no higher oligomers formed. Treatment of the enzyme with 6 M urea led to inactivation and prevented cross-linking by dimethyl suberimidate. Transhydrogenase reconstituted into phosphatidylcholine proteoliposomes also yielded a dimer band on cross-linking. These data indicate that soluble and functionally reconstituted transhydrogenase possesses a dimeric structure.

Steady-state kinetics and the inactivation by 2,3-butanedione of the energy-independent transhydrogenase of Escherichia coli cell membranes

Kinetic measurements indicate that the energy-independent transhydrogenation of 3-acetylpyridine-NAD+ by NADPH in membranes of Escherichia coli follows a rapid equilibrium random bireactant mechanism. Each substrate, although reacting preferentially with its own binding site, is able to interact with the binding site of the other substrate to cause inhibition of enzyme activity. 5'-AMP (and ADP) and 2'-AMP interact with the NAD+- and NADP+-binding sites, respectively. Phenylglyoxal and 2,3-butanedione in borate buffer inhibit transhydrogenase activity presumably by reacting with arginyl residues. Protection against inhibition by 2,3-butanedione is afforded by NADP+, NAD+, and high concentrations of NADPH and NADH. Low concentrations of NADPH and NADH increase the rate of inhibition by 2,3-butanedione. Similar effects are observed for the inactivation of the transhydrogenase by tryptic digestion in the presence of these coenzymes. It is concluded that there are at least two conformations of the active site of the transhydrogenase which differ in the extent to which arginyl residues are accessible to exogenous agents such as trypsin and 2,3-butanedione. One conformation is induced by low concentrations of NADH and NADPH. Under these conditions the coenzymes could be reacting at the active site or at an allosteric site. The stimulation of transhydrogenase activity by low concentrations of the NADH is consistent with the latter possibility.