The relation of the pH and concentration-dependent dissociation of porcine heart mitochondrial malate dehydrogenase

Amide hydrogen exchange shows that malate dehydrogenase is a folded monomer at pH 5

Although there is general agreement that native mitochondrial malate dehydrogenase (MDH) exists as a dimer at pH 7, its aggregation state at pH 5 is less certain. The present amide hydrogen exchange study was performed to determine whether MDH remains a dimer at pH 5. To detect pH-induced changes in solvent accessibility, MDH was exposed to D(2)O at pH 5 or 7, then fragmented with pepsin into peptides that were analyzed by mass spectrometry. Even after adjustments for the effect of pH on the intrinsic rate of hydrogen exchange, large increases in deuterium levels were found at pH 5 only in peptic fragments derived from the subunit binding surface of MDH. In parallel experiments, elevated deuterium levels were also found in the same regions of MDH monomer trapped inside a mutant form of the chaperonin GROEL: This selective increase in hydrogen exchange rates, which was attributed to increased solvent accessibility of these regions, provides new evidence that MDH is a monomer at pH 5.

Dissociation of mitochondrial malate dehydrogenase into active soluble subunits

Gel exclusion chromatographic studies demonstrate that cytosolic and mitochondrial malate dehydrogenases (cMDH and mMDH) dissociate into subunits in the presence of 0.1% of the non-ionic detergent Triton X-100 (TX-100). The presence of cofactor and catalytically competent cofactor-substrate pairs does not protect mMDH against this dissociation. In contrast, cMDH dimers resist dissociation in the presence of either addition. Since steady state kinetic studies indicate both enzymes are fully active in the presence of 0.1% TX-100, we conclude that quaternary structure is not a kinetically important feature of mMDH structure and cooperativity does not account for mMDH kinetic anomalies. In contrast, cooperativity is a reasonable explanation for cMDH kinetic properties even in the presence of 0.1% TX-100, since this enzyme's subunits associate in the presence of active site ligands. The existence of fully active mMDH subunits raises the possibility that this species rather than the dimer may be a constituent of proposed multi-enzyme complexes of the mitochondrion. Preliminary chromatographic experiments involving gently disrupted mitochondria have found MDH activity in differently sized complexes, all with molecular weights larger than the mMDH dimer but smaller than complexes anticipated for multi-enzyme complexes involving other enzymes and the mMDH dimer.

Aggregation states of mitochondrial malate dehydrogenase

The oligomeric state of fluorescein-labeled mitochondrial malate dehydrogenase (L-malate NAD+ oxidoreductase; mMDH; EC 1.1.1.37), as a function of protein concentration, has been examined using steady-state and dynamic polarization methodologies. A "global" rotational relaxation time of 103 +/- 7 ns was found for micromolar concentrations of mMDH-fluorescein, which is consistent with the reported size and shape of mMDH. Dilution of the mMDH-fluorescein conjugates, prepared using a phosphate buffer protocol, to nanomolar concentrations had no significant effect on the rotational relaxation time of the adduct, indicating that the dimer-monomer dissociation constant for mMDH is below 10(-9) M. In contrast to reports in the literature suggesting a pH-dependent dissociation of mMDH, the oligomeric state of this mMDH-fluorescein preparation remained unchanged between pH 5.0 and 8.0. Application of hydrostatic pressure up to 2.5 kilobars was ineffective in dissociating the mMDH dimer. However, the mMDH dimer was completely dissociated in 1.5 M guanidinium hydrochloride. Dilution of a mMDH-fluorescein conjugate, prepared using a Tris buffer protocol, did show dissociation, which can be attributed to aggregates present in these preparations. These results are considered in light of the disparities in the literature concerning the properties of the mMDH dimer-monomer equilibrium.

Protein ligand interactions:isoquinoline alkaloids as inhibitors for lactate and malate dehydrogenase

Kinetic analysis has shown that isoquinoline, papaverine and berberine act as reversible competitive inhibitors to muscle lactate dehydrogenase and mitochondrial malate dehydrogenase with respect to the coenzyme NADH. The inhibitor constants Ki vary from 7.5 microM and 12.6 microM berberine interaction with malate dehydrogenase and lactate dehydrogenase respectively to 91.4 microM and 196.4 microM with papaverine action on these two enzymes. Isoquinoline was a poor inhibitor with Ki values of 200 microM (MDH) to 425 microM (LDH). No inhibition was observed for both enzymes in terms of their respective second substrate (oxaloacetic acid - malate dehydrogenase; pyruvate - lactate dehydrogenase). A fluorimetric analysis of the binding of the three alkaloids show that the dissociation constants (Kd) for malate dehydrogenase are 2.8 microM (berberine), 46 microM (papaverine) and 86 microM (isoquinoline); the corresponding values for lactate dehydrogenase are 3.1 microM, 52 microM and 114 microM. In all cases the number of binding sites averaged at 2 (MDH) and 4 (LDH). The binding of the alkaloids takes place at sites close to the coenzyme binding site. No conformational non equivalence of subunits is evident.

Factors affecting malate dehydrogenase activity in freezing-thawing processes

Malate dehydrogenases from several sources show different behaviour when frozen-thawed in 100 mM sodium phosphate buffer, pH 7.4, containing chaotropic ions. The effects produced by the addition of various metabolites, protein concentration and buffer medium used on the loss of activity induced by the freezing-thawing process are reported. The major part of the loss of activity is caused by the formation of "wrong" aggregates of high mol. wt.

Purification and properties of malate dehydrogenase from the thermoacidophilic archaebacterium Thermoplasma acidophilum

Malate dehydrogenase from the thermoacidophilic archaebacterium Thermoplasma acidophilum is purified 50-fold to electrophoretic homogeneity. The purified enzyme crystallizes readily. Native malate dehydrogenase shows a relative molecular mass of 144 000. It is a tetramer of identical subunits with a relative molecular mass of 36 600. Malate dehydrogenase from Thermoplasma uses both NADH and NADPH as coenzyme to reduce oxaloacetate. The enzyme shows A-side (pro-R) stereospecificity for both coenzymes. The pH optimum for the reduction of oxaloacetate in the presence of NADH is found to be at pH 8.1. At pH 7.4 the Km value for oxaloacetate is found to be 5.6 microM while for NADH a value of 11.7 microM is found. The homogeneous enzyme shows a turnover number of kcat = 108 s-1.

Reassociation of dimeric cytoplasmic malate dehydrogenase is determined by slow and very slow folding reactions

Malate dehydrogenase occurs in virtually all eucaryotic cells in mitochondrial and cytoplasmic forms, both of which are composed of two identical subunits. The reactivation of the mitochondrial isoenzyme has been the subject of previous studies [Jaenicke, R., Rudolph, R., & Heider, I. (1979) Biochemistry 18, 1217-1223]. In the present study, the reconstitution of cytoplasmic malate dehydrogenase from porcine heart after denaturation by guanidine hydrochloride has been determined. The enzyme is denatured by greater than 1.2 M guanidine hydrochloride; upon reconstitution, approximately 60% of the initial native enzyme can be recovered. The kinetics of reconstitution after maximum unfolding by 6 M guanidine hydrochloride were analyzed by fluorescence, far-ultraviolet circular dichroism, chemical cross-linking with glutaraldehyde, and activity measurements. After fast folding into structured intermediates (less than 1 min), formation of native enzyme is governed by two parallel slow and very slow first-order folding reactions (k1 = 1.3 X 10(-3) S-1 and k2 = 7 X 10(-5) S-1 at 20 degrees C). The rate constant of the association step following the slow folding reaction (determined by k1) must be greater than 10(6) M-1 S-1. The energy of activation of the slow folding step is of the order of 9 +/- 1 kcal/mol; the apparent rate constant of the parallel very slow folding reaction is virtually temperature independent. The intermediates of reassociation must be enzymatically inactive, since reactivation strictly parallels the formation of native dimers. Upon acid dissociation (pH 2.3), approximately 35% of the native helicity is preserved, as determined by circular dichroism.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of mitochondrial malate dehydrogenase: kinetic modulation independent of subunit interaction

Porcine heart mitochondrial malate dehydrogenase (EC 1.1.1.37), a dimeric enzyme of Mr = 70,000, is both allosterically activated and inhibited by citrate. Using an affinity elution procedure based upon citrate binding to malate dehydrogenase, the isolation of pure heterodimer (a dimeric species with one active subunit and one iodoacetamide-inactivated subunit) has been achieved. Investigations utilizing this heterodimer in conjunction with resin-bound monomers of malate dehydrogenase have allowed the formulation of a definite conclusion concerning the role of subunit interactions in catalysis and regulation of this enzyme. The citrate kinetic effects, oxaloacetate inhibition, malate activation, and the effects of 2-thenoyl-trifluoroacetone (TTFA) are shown to be independent of interaction between catalytically active subunits. Previous kinetic data thought to support a reciprocating catalytic mechanism for this enzyme may be reinterpreted upon closer analysis in relation to an allosteric, conformationally specific binding model for malate dehydrogenase.

Dissociation of mitochondrial malate dehydrogenase

The kinetics of the dissociation reaction under acidic conditions of the dimeric pig and chicken mitochondrial malate dehydrogenases (EC 1.1.1.37) have been studied. The dissociation of the pig enzyme is completely reversible. The pK for dissociation determined by light-scattering measurements agrees within experimental error with the pK value of 5.25 measured for a tyrosine-carboxylate pair. The rate constants for the dissociation reaction and for the protonation process of this tyrosine are in close agreement. Thus, the tyrosine-carboxylate pair can be used as indicator of the dissociation reaction. The dissociation of the chicken enzyme proceeds around pH 4.5 at a much lower rate. A true equilibrium between dimer and monomers is not found, since the monomer gradually unfolds at this pH. The monomers of both enzymes, pig and chicken mitochondrial malate dehydrogenase, show the same stability towards acid. The difference in stability of the dimeric forms, therefore, must be due to an altered subunit contact area.

Synthesis and properties of 5-membered heterocyclic disulfides: application for enzyme modification

Synthesis of the disulfides 2,2'-dithiodithiazole, 2,2'-dithiobis(4-methylthiazole), and 5,5'-dithiobis(3-methylisothiazole), which are thiol-specific reagents, is described and the uv-absorption spectra of the disulfides and corresponding thiols and/or thiones are presented. The utility of these and several other reagents for the modification of thiol groups in enzymes has been demonstrated with lactate dehydrogenase and malate dehydrogenase and compared with the frequently applied modification reagents 5,5'-dithiobis(2-nitrobenzoic acid) and 2,2'-dithiodipyridine. The 5-membered heterocyclic disulfides permit the spectrophotometric determination of all sulfhydryl groups of malate dehydrogenase and lactate dehydrogenase without requiring their prior denaturation. Application of these reagents makes possible the determination of SH groups in the pH range less than or equal to 7.

Importance of tyrosine for structure and function of mitochondrial malate dehydrogenases

Mitochondrial malate dehydrogenase from pig and chicken both contain one tyrosine/subunit with highly red-shifted spectrum, most probably involved in a hydrogen bond with a carboxylate group. The spectral changes of this tyrosine can be used as an indicator for alkaline denaturation, acid transition and coenzyme binding. Acid transition is coupled with breaking of this bond by protonation as monitored by loss of absorbance at 290 nm. Activity is lost and fluorescence intensity is increased at slightly higher pH, thus indicating increased mobility of the indicator and most probably of the whole protein prior to protonation of the indicator-tyrosine.

Stability of dehydrogenases. III. Malate dehydrogenases

Cytoplasmic and mitochondrial malate dehydrogenases from pig and chicken were studied by chemical modification of amino groups, hybridization of immobilization. Determination of thermal stability was used to characterize the different species. Modification of amino groups was found to decrease thermal stability especially when neutralization of the positive charges occurred. Decreased thermal stability correlated with decreased reassociation of immobilized monomers and modification in the monomeric state completely inhibited reassociation. Thus some lysines seem to be implicated within the subunit contacts. Active monomers of the mitochondrial forms as demonstrated earlier (Jürgensen, S.R., Wood, D.C., Mahler, J.C. and Harrison, J.H. (1981) J. Biol Chem. 256, 2383-2388) were found to display unaltered kinetic properties. From hybridizations the mechanism of thermal denaturation of malate dehydrogenases was concluded to contain a rate-limiting cooperative transaction of both monomers within the dimer, as was found earlier for tetrameric lactate dehydrogenase (Müller, J. (1981) Biochim. Biophys. Acta 669, 210-215 and Müller, J. and Klein, C. (1981) Biochim. Biophys. Acta 671, 38-41).

The effect of ionic environment on pig heart mitochondrial malate dehydrogenase

1. The effect of ionic environment on pig heart mitochondrial malate dehydrogenase was investigated by means of two low resolution conformational probes, thermal stability and proteolytic liability. 2. High ionic strength and high enzyme concentrations stabilize the enzyme towards thermal inactivation, probably by maintaining the enzyme in the dimeric form. 3. Proteolysis of malate dehydrogenase under conditions where the enzyme is thermally stable indicates a secondary ionic effect on structure. 4. Attempts to demonstrate dissociation of native malate dehydrogenase dimer into its constituent subunits using gel filtration on Sephacryl S-200 proved inconclusive. At low ionic strength malate dehydrogenase does not exhibit normal gel filtration behaviour.

Hydrophobic interaction between the monomer of mitochondrial malate dehydrogenase and phospholipid membranes

Porcine mitochondrial malate dehydrogenase (EC 1.1.1.37) dissociates into subunits on dilution. The enzyme monomer caused large increases in the surface pressure of monolayers of 1:1 phosphatidylserine/phosphatidylcholine at air/water and oil/water interfaces. The monomer increased the permeability of phospholipid vesicles to 22Na+. Both effects were significantly greater than the corresponding effects of ribonuclease A, cytochrome c and the dimeric form of malate dehydrogenase. Changes in the circular-dichroism spectra of the enzyme indicated that conformational changes may be associated with dimer formation or when monomer interacts with lysophosphatidyl-choline. Similar interactions to those described may occur in situ when mitochondrial malate dehydrogenase is transported to the mitochondrial matrix from its site of synthesis on cytosolic ribosomes.