Reassociation and reactivation of lactic dehydrogenase from the unfolded subunits

Enabling Efficient Design of Biological Formulations Through Advanced Characterization

The present review summarizes the use of differential scanning calorimetry (DSC) and scattering techniques in the context of protein formulation design and characterization. The scattering techniques include wide angle X-ray diffractometry (XRD), small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS). While DSC is valuable for understanding thermal behavior of the excipients, XRD provides critical information about physical state of solutes during freezing, annealing and in the final lyophile. However, as these techniques lack the sensitivity to detect biomolecule-related transitions, complementary characterization techniques such as small-angle scattering can provide valuable insights.

A perspective on mechanisms of protein tetramer formation

Homotetrameric proteins can assemble by several different pathways, but have only been observed to use one, in which two monomers associate to form a homodimer, and then two homodimers associate to form a homotetramer. To determine why this pathway should be so uniformly dominant, we have modeled the kinetics of tetramerization for the possible pathways as a function of the rate constants for each step. We have found that competition with the other pathways, in which homotetramers can be formed either by the association of two different types of homodimers or by the successive addition of monomers to homodimers and homotrimers, can cause substantial amounts of protein to be trapped as intermediates of the assembly pathway. We propose that this could lead to undesirable consequences for an organism, and that selective pressure may have caused homotetrameric proteins to evolve to assemble by a single pathway.

Continuous chromatographic protein refolding

Column-based protein refolding requires a continuous processing capability if reasonable quantities of protein are to be produced. A popular column-based method, size-exclusion chromatography (SEC) refolding, employs size-exclusion matrices to separate unfolded protein from denaturant, thus refolding the protein. In this work, we conduct a comparison of SEC refolding with refolding by batch dilution, using lysozyme as a model protein. Lysozyme refolding yield was found to be extremely sensitive to the chemical composition of the refolding buffer and particularly the concentration of dithiothreitol (DTT) introduced from the denatured protein mixture. SEC refolding was not adversely affected by DTT carry-over as small contaminants in the denatured solution are separated from protein during the refolding operation. We also find that, contrary to previous reports, size-exclusion refolding on batch columns leads to refolding yields slightly better than batch dilution refolding yields at low protein concentrations but this advantage disappears at higher protein concentrations. As batch-mode chromatography would be the limiting step in a column based refolding downstream process, the batch column refolding method was translated to a continuously operating chromatography system (preparative continuous annular chromatography, P-CAC). It was shown that the P-CAC elution profile is similar to that of a stationary column, making scale-up and translation to P-CAC relatively simple. Moreover, it was shown that high refolding yields (72%) at high protein concentration (>1 mg ml(-1)) could be obtained.

Effect of rate of chemical or thermal renaturation on refolding and aggregation of a simple lattice protein

We used dynamic Monte Carlo simulation to investigate how changing the rate of chemical or thermal renaturation affects the folding and aggregation behavior of a system of simple, two-dimensional lattice protein molecules. Four renaturation methods were simulated: infinitely slow cooling; slow but finite cooling; quenching; and pulse renaturation. The infinitely slow cooling method, which is equivalent to dialysis or diafiltration, provides refolding yields that are relatively high and aggregates that are relatively small (mostly dimers or trimers). The slow but finite cooling method, which is equivalent to multiple-step dilution, provides refolding yields that are almost as high as those observed in the infinitely slow cooling case, but in a relatively short period of time. Quenching, which is equivalent to one-step dilution or quick quenching, is extremely slow and has low re- folding yields. A maximum appears in the refolding yield as a function of denaturant concentration in the simulation but disappears after a very long duration. Finally, the pulse renaturation method provides refolding yields that are substantially higher than those observed in the other three methods, even at high packing fractions. As in the early stages of quenching, there is a maximum in the refolding yield as a function of denaturant concentration when relatively large numbers of denatured chains are added to the refolding solution at each step.

Effect of glycine on pH changes and protein stability during freeze-thawing in phosphate buffer systems

Previous studies have established that the selective precipitation of a less soluble buffer component during freezing can induce a significant pH shift in the freeze concentrate. During freezing of sodium phosphate solutions, crystallization of the disodium salt can produce a pH decrease as great as 3 pH units which can dramatically affect protein stability. The objective of our study was to determine how the presence of glycine (0-500 mM), a commonly used bulking agent in pharmaceutical protein formulations, affects the pH changes normally observed during freezing in sodium phosphate buffer solutions and to determine whether these pH changes contribute to instability of model proteins in glycine/phosphate formulations. During freezing in sodium phosphate buffers, the presence of glycine significantly influenced the pH. Glycine at the lower concentrations (< or = 50 mM) suppressed the pH decrease normally observed during freezing in 10 and 100 mM sodium phosphate buffer, possibly by reducing the nucleation rate of salt and thereby decreasing the extent of buffer salt crystallization. The presence of glycine at higher concentration (> 100 mM) in the sodium phosphate buffer resulted in a more complete crystallization of the disodium salt as indicated by the frozen pH values closer to the equilibrium value (pH 3.6). Although high concentrations of glycine can facilitate more buffer salt crystallization and these pH shifts may prove to be potentially damaging to the protein, glycine, in its amorphous state, can also act to stabilize a protein via the preferential exclusion mechanism.

Effect of denaturant and protein concentrations upon protein refolding and aggregation: a simple lattice model

We present a study of the competition between protein refolding and aggregation for simple lattice model proteins. The effect of solvent conditions (i.e., the denaturant concentration and the protein concentration) on the folding and aggregation behavior of a system of simple, two-dimensional lattice protein molecules has been investigated via (dynamic Monte Carlo simulations. The population profiles and aggregation propensities of the nine most populated intermediate configurations exhibit a complex dependence on the solution conditions that can be understood by considering the competition between intra- and interchain interactions. Some of these configurations are not even seen in isolated chain simulations; they are observed to be highly aggregation prone and are stabilized primarily by the aggregation reaction in multiple-chain systems. Aggregation arises from the association of partially folded intermediates rather than from the association of denatured random-coil states. The aggregation reaction dominates over the folding reaction at high protein concentration and low denaturant concentration, resulting in low refolding yields at those conditions. However, optimum folding conditions exist at which the refolding yield is a maximum, in agreement with some experimental observations.

Re-activation of Clostridium symbiosum glutamate dehydrogenase from subunits denatured by urea

In a study of the re-activation of urea-denatured clostridial glutamate dehydrogenase (GDH) the maximum re-activation achieved without any added ligands was about 6%, but with NAD+ and 2-oxoglutarate in combination about 70%. NAD+ alone was also effective but 2-oxoglutarate was not, in striking contrast with the opposite pattern for protection of this enzyme against unfolding in urea [Aghajanian, Martin and Engel (1995) Biochem. J. 311,905-910]. The extent of re-activation was not increased by raising the incubation temperature to 37 degrees C and was independent of the time of enzyme denaturation. CD and fluorimetric studies showed that dilution of denatured enzyme into potassium phosphate buffer led to rapid (half-time <3-5 s)formation of 'structured' intermediates with secondary structure similar to that of native enzyme. These intermediate molecules were inactive, behaved as monomers on a size-exclusion column, and were unable to associate to give the native hexameric structure. Addition of NAD+ facilitated isomerization of these 'structured' monomers into a form(s) capable of re-activation. A side effect in the refolding process was non-specific aggregation, depending on final enzyme concentration. The hexamer fraction from re-activated samples, however, showed the same specific activity as native enzyme. The portion of the enzyme that is not lost through aggregation thus appears to regain the native structure fully. Detailed time-course studies showed that re-activation follows second-order kinetics, suggesting that formation of a dimer may be the rate-limiting step. The possible mechanism for the unfolding and refolding processes of clostridial GDH and effects of coenzyme and substrate on these are discussed in relation to the known crystal structure.

Tetrameric and octameric lactate dehydrogenase from the hyperthermophilic bacterium Thermotoga maritima. Structure and stability of the two active forms

Lactate dehydrogenase from the hyperthermophilic bacterium Thermotoga maritima has been functionally expressed in Escherichia coli. As shown by gel-permeation chromatography, dynamic light scattering, and ultracentrifugation, the recombinant protein forms homotetrameric and homooctameric assemblies with identical spectral properties and a common subunit molecular mass (35 kDa). Dynamic light scattering and sedimentation equilibrium experiments proved that both species are monodisperse, thus excluding their interconversion in the given ranges of concentration (0.02-50 mg/ml) and temperature (20-80 degrees C). Rechromatography confirms this finding: the octamer does not dissociate at low enzyme concentrations, nor do tetramers dimerize at the given upper limit of concentration. Renaturation of pure tetramers or octamers after preceding guanidine denaturation leads to redistribution of the two species; increased temperature favors octamer formation. Thermal analysis and denaturation by chaotropic agents do not allow the free energies of stabilization of the two forms to be quantified, because heat coagulation and kinetic partitioning between reconstitution and aggregation causes irreversible side reactions. Guanidine denaturation of the octamer leads to a highly cooperative dissociation to tetramers which subsequently dissociate and unfold to yield metastable dimers and, finally, fully unfolded monomers. Evidently, there is no tight coupling of the two tetramers within the stable octameric quaternary structure. Electron microscopy clearly corroborates this conclusion: image processing shows that the dumb-bell-shaped octamer is made up of two tetramers connected via surface contacts without significant changes in the dimensions of the constituent parts.

Refolding of denatured and denatured/reduced lysozyme at high concentrations

Refolding of proteins at high concentrations often results in aggregation. To gain insight into the molecular aspects of refolding and to improve the yield of active protein, we have studied the refolding of lysozyme either from its denatured state or from its denatured/reduced state. Refolding of denatured lysozyme, even at 1 mg/ml, yields fully active enzyme without aggregation. However, refolding of denatured/reduced lysozyme into buffer that lacks thiol/disulfide reagents leads to aggregation. Thiol/disulfide redox reagents such as cysteine/cystine and reduced/oxidized glutathione facilitate the renaturation, with the yield depending on their absolute concentrations. We have obtained an approximately 70% renaturation yield upon refolding of lysozyme at 150 microgram/ml. The cysteine/cystine redox system is more efficient compared with the glutathione redox system. When lysozyme is refolded in the absence of redox reagents, a transient intermediate that has regained a significant amount of secondary structure is formed. The tryptophans in this intermediate are as exposed to water as in the fully unfolded protein. It shows increased exposure of hydrophobic surfaces compared with the native or completely unfolded enzyme. This aggregation-prone intermediate folds to active enzyme upon addition of oxidized glutathione before the aggregation process starts. These properties of the intermediate in the refolding pathway of lysozyme are similar to those proposed for the molten globule.

A kinetic study of the competition between renaturation and aggregation during the refolding of denatured-reduced egg white lysozyme

The recovery of proteins following denaturation is optimal at low protein concentrations. The decrease in yield at high concentrations has been explained by the kinetic competition of folding and "wrong aggregation". In the present study, the renaturation-reoxidation of hen and turkey egg white lysozyme was used as a model system to analyze the committed step in aggregate formation. The yield of renatured protein for both enzymes decreased with increasing concentration in the folding process. In addition, the yield decreased with increasing concentrations of the enzyme in the denatured state (i.e., prior to its dilution in the renaturation buffer). The kinetics of renaturation of turkey lysozyme were shown to be very similar to those of hen lysozyme, with a half-time of about 4.5 min at 20 degrees C. The rate of formation of molecular species that lead to formation of aggregates (and therefore fail to renature) was shown to be rapid. Most of the reaction occurred in less than 5 s after the transfer to renaturation buffer, and after 1 min, the reaction was essentially completed. Yet, by observing the effects of the delayed addition of denatured hen lysozyme to refolding turkey lysozyme, it was shown that folding intermediates become resistant to aggregation only much more slowly, with kinetics indistinguishable from those observed for the appearance of native molecules. The interactions leading to the formation of aggregates were nonspecific and do not involve disulfide bonds. These observations are discussed in terms of possible kinetic and structural aspects of the folding pathway.

A physical-chemical and immunological comparison shows that native and renatured Escherichia coli tryptophan synthase beta 2 subunits are identical

An acid-denaturation of the beta 2 subunit of Escherichia coli tryptophan synthase has been recently described. In the present study, renaturation yield of acid-denaturated beta 2, and the influence of temperature, protein concentration and presence of ligands are investigated. It is also demonstrated that 3 forms of the protein are obtained at the end of the renaturation process: one is fully active, and is identical to native beta 2, as indicated by some of its chemical and physical properties, as well as by its immunological reactivity towards monoclonal antibodies specific for the native protein. A second form is composed of high molecular weight insoluble and inactive aggregates. A third form consists of low molecular weight soluble and inactive aggregates. The results obtained for the immunochemical reactivity of these small aggregates indicate that they are formed with partly correctly folded beta monomers assembled by specific but incorrect quaternary interactions. The capacity of monoclonal antibodies to detect such incorrect structures and to characterize renatured proteins is particularly emphasized.

Effect of chemical modifications on freeze denaturation of lactate dehydrogenase

Lactate dehydrogenase (LDH) was chemically ethyl-acetimidated (EA-), dimethyladipimidated (DMA-), carbamylated, acetylated, acetoacetylated, or succinylated in order to alter the ionic charges on the epsilon-amino group of lysine residues. Acetylation, acetoacetylation, and succinylation, which change the positive charge at the lysine side chains to a negative one, inactivated the enzymic activity, but the rest of the modifications exerted no such inactivating effects. The active modified enzymes were subjected to freeze denaturation study, using the enzymic activity as an indication of the degree of the denaturation. The active enzymes were diluted with deionized water and stored in a freezer (-23 degrees C) for 1-3 days. Enzymic activity was assayed immediately after thawing. All the modified enzymes retained their activity even after the 3-day frozen storage, while the control or native enzyme lost its activity within 1 day of storage. Furthermore, the modified LDHs freeze-stored in 0.2 M monosodium glutamate (MSG) or 0.2 M lysine-hydrochloride (Lys-HCl) retained their activity. The cryoprotective effects exerted by the modifications and by 0.2 M MSG seemed to be synergistic, whereas those exerted by the modifications and by 0.2 M Lys-HCl did not. The mechanisms of cryoprotection and freeze denaturation are discussed in relationship with the cryoprotective effect exerted by already known cryoprotectants, such as sucrose or dimethyl sulfoxide.

Aggregation and thermo-inactivation of glutamine synthetase from an extreme thermophile, Bacillus caldolyticus

The extreme thermophile, Bacillus caldolyticus, contains two regulatory isoforms of glutamine synthetase (glutamate-ammonia ligase, EC 6.3.1.2), E-I and E-II, produced as separate gene products. Light scattering and electron microscopy data indicate that these thermophilic enzymes aggregate to higher molecular weight species in two stages: initial polymerization of native dodecamers, followed by 'melting' of the aggregated species to produce amorphous denatured protein. The initial stages of the aggregation occurred at temperatures below those for time-dependent denaturation, especially for E-II. In contrast, mesophilic (B. subtilis) enzyme showed no evidence of temperature-dependent aggregation. Thus, aggregation may be a stabilizing mechanism for the thermophilic systems. Bound metal ions and substrates caused dramatic increases in the temperatures at which aggregation and loss of activity occurred for thermophilic enzymes. Certain combinations of ligands (e.g., MnATP + L-glutamate) acted synergistically, so that these complexes denatured only above 90 degrees C. Various models were considered for heat-driven aggregation followed by denaturation, plus ligand stabilization. Taken together, the data are most consistent with unfolding of subunits within the dodecameric unit, rather than unfolding to monomers prior to aggregation.

Influence of cofactor pyridoxal 5'-phosphate on reversible high-pressure denaturation of isolated beta 2 dimer of tryptophan synthase bienzyme complex from Escherichia coli

High hydrostatic pressure has been shown to cause reversible dissociation of the isolated apo beta 2 dimer of tryptophan synthase from Escherichia coli into enzymatically inactive monomers [Seifert, T., Bartholmes, P., & Jaenicke, R. (1982) Biophys. Chem. 15, 1-8]. Addition of the coenzyme pyridoxal 5'-phosphate affects the structural stability, as well as the kinetics of dissociation and deactivation. The apo beta 2 dimer is deactivated faster than the holoenzyme by a factor of 10. The midpoints of the corresponding equilibrium transition curves are observed at 690 and 870 bar, respectively. As shown by hybridization of native and chemically modified beta chains, the loss of enzymatic activity is accompanied by subunit dissociation. An additional deactivating effect is produced by the pressure-induced release of the cofactor from the holoenzyme. Renaturation after decompression has been monitored by circular dichroism and intrinsic fluorescence emission. Alterations of the dichroic absorption at 222 nm reflect the recovery of the native secondary structure, while tryptophan fluorescence represents a specific probe for the native tertiary structure in the immediate neighborhood of the active center of the enzyme. By application of both methods to monitor the reconstitution of the apo beta 2 dimer, two first-order processes may be separated along the time scale. The faster phase (k1 = 1.2 X 10(-2) s-1) yields a "structured monomer" with 85% native secondary structure and the tryptophan side chain buried in its native hydrophobic environment. As indicated by sodium borohydride reduction, this intermediate is able to interact with the coenzyme pyridoxal 5'-phosphate in the correct way; however, it does not show enzymatic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of lactate dehydrogenase activity: reversible and isoenzyme-specific inhibition of the tetramerization process by peptides

The transition of inactive lactic dehydrogenase (LDH) subunits to functional tetramers is controlled by 2 naturally-occurring peptides. One of these peptides inhibits the folding/association/activation process of H-LDH and the other peptide, the reconstitution of M-LDH, NADH and NAD+ are isoenzyme-specific antagonists of the 2 inhibitory peptides.

Isoenzyme specific inhibition of the reactivation of in vitro dissociated lactic dehydrogenase isoenzymes by two different peptides isolated from human liver

The catalytic activity of the LDH-isoenzymes depends on their tetrameric structure. Low pH or other denaturants leads to dissociation into monomers and to the loss of the specific activity. After removal of the denaturing conditions reassociation and reactivation occur spontaneously. Neither NADH nor NAD+ shows a significant effect on the reactivation. We have isolated two different peptides which isoenzyme specifically inhibit the reactivation of dissociated LDH. Inhibition was abolished by treating with proteases. Additionally, NAD+ and NADH were found to be antagonists of the inhibitors. The heart-type enzyme-inhibitor system is especially susceptible for NADH whereas NAD+ affects the inhibition only slightly. The muscle-type system shows the opposite behavior, e.g., the completely inhibited system can be fully reactivated by NAD+ but not by NADH. These findings together with first kinetic studies suggest a possible specific regulatory function of these peptides.

Pressure-induced structural changes of pig heart lactic dehydrogenase

Lactic dehydrogenase from pig heart can be reversibly dissociated at hydrostatic pressures above 1000 bar. The breakdown of the native quaternary structure occurs at lower pressures compared to the isoenzyme from pig skeletal muscle. As shown by hybridization experiments of the two isoenzymes the final product of dissociation is the homogeneous monomer. Fluorescence emission spectra of the monomeric enzyme at elevated pressure are characterized by a decrease in fluorescence intensity without any red shift, indicating that no significant unfolding occurs upon high-pressure dissociation. The spectral changes are comparable to those observed after acid dissociation. The amount and rate of deactivation depend on pressure and on the conditions of the solvent. The presence of various anions (C1-, SO2-/4, HPO2-/4) has no effect on the stability of the enzyme towards pressure. High-pressure denaturation (as monitored by intrinsic protein fluorescence), and deactivation (measured immediately after decompression) run parallel; the pressure dependence of their first-order rate constants is characterized by an activation volume delta V not equal to De = - 140 +/- 10 cm3/mol. As taken from the yield of reconstitution, dissociation, denaturation and deactivation are found to be fully reversible provided the pressure does not exceed a limiting value (p = 1000 bar in Tris, pH 7.6; 24 h incubation at 20 degrees C). After extended incubation beyond the limiting pressure of 1000 bar, "irreversible high-pressure denaturation" occurs which is accompanied by partial aggregation after decompression. The coenzyme, NAD+, stabilizes the native tetramer shifting the dissociation equilibrium to higher pressures. The overall dissociation-association reaction can be quantitatively described by a consecutive dissociation/unfolding mechanism N in equilibrium 4 M' in equilibrium 4 M* (where N is the native tetramer, and M' and M* two different conformations of the monomer). The reaction volume of the dissociation reaction N in equilibrium 4 M' is found to be delta V Diss = - 360 +/- 30 cm3/mol; as indicated by the pressure dependence of the yield of reconstitution, the reaction volume of the equilibrium M' in equilibrium M* is also negative.

Peptides isolated from human liver with specific inhibitory effects on reassociation/reactivation of in vitro dissociated lactic dehydrogenase (LDH-M4 and -H4) isozymes

Two different peptides have been purified from human liver, similar to those previously reported (Schoenenberger, G.A., and Wacker, W.E.C. (1966) Biochemistry 5, 1375--1379) to be present in human urine, which may serve as metabolic regulators of lactate dehydrogenase (EC 1.1.1.27) isoenzymes (LDH-M4 = muscle type; LDH-H4 = heart type). By trichloroacetic acid precipitation, ultrafiltration, Sephadex G-25 and Bio-Gel P-2 columns, affinity chromatography on immobilized LDH-isozymes and HPLC two peptides which differed with respect to molecular weight, retention on the affinity columns and amino acid composition were isolated. No effect was observed when native, tetrameric lactate dehydrogenase was incubated with these peptides. However, when lactate dehydrogenase was dissociated to monomers at low pH and allowed to reassociate by adjusting the pH to 7.5 complete inhibition of the reactivation occurred when the inhibitors were incubated together with respective reassociating monomeric isozymes. The two peptides showed no cross-specificity, i.e. each peptide exhibited inhibitory activity only on one of the two isozymes LDH-M4 or LDH-H4. From the amino acid analyses, gel filtrations and PAGE + SDS, molecular weights of 1800 for the M4 and approximately 2700 for the H4 inhibitor were calculated. An apparent Ki of approximately 3 X 10(-5) mM for the H4 and approximately 7 X 10(-5) mM for the H4 inhibitor was estimated. The interaction of the inhibitors with the enzyme system showed strong cooperativity with Hill coefficients of 2.9 (LDH-M4-specific) and 2.4 (LDH-H4-specific). Mathematical modelling of the reassociation and reactivation of lactate dehydrogenase and its specific inhibition by the peptides led to the conclusion that the peptides react with monomers, dimers or a transition state during the tetramerisation process. kappa 1 for the dimerisation step of M4 = 2.0 X 10(5) M-1 . S-1 and of H4 = 8.2 X 10(4) M-1 . S-1; kappa 2 for the tetramerisation step of M4 = 2.8 X 10(5) M-1 . S-1 and of H4 = 1.2 X 10(5) . M-1 S-1, were calculated, the second step still being the faster one (Rudolf, R. and Jaenicke, R. (1976) Eur. J Biochem. 63, 409--417).

Factors affecting the quaternary structure of the allosteric L-lactate dehydrogenase from Lactobacillus casei and Lactobacillus curvatus as investigated by hybridization and ultracentrifugation

The allosteric L-lactate dehydrogenases of Lactobacillus curvatus and Lactobacillus casei exist in the tetrameric from (molecular weight about 145 000) at pH 5.0--5.5 even in the absence of the effectors Mn2+ and Fru(1,6)P2 (fructose 1,6-bisphosphate), but undergo reversible dissociation to monomers (molecular weight about 35 000) at higher pH values or in the presence of urea. In the range between pH 5.5 (tetrameric state) and pH 7.4 (monomeric state) the L. curvatus L-lactate dehydrogenase exists in a dissociation-association equilibrium comprising tetramers, dimers and monomers as indicated by the Sc20,w values and the results of hybridization experiments. The simultaneous addition of both effectors [Mn2+ and Fru(1,6)P2] at pH 7.4, however, resulted in the stabilization of the tetrameric form. The addition of Fru(1,6)P2 alone at pH 7.4 had almost no influence on the quarternary structure, whereas the addition of Mn2+, as well as that of NADH, largely prevented dissociation. The L-lactate dehydrogenase of L. casei showed similar properties, although the enzyme dissociates only at about pH greater than or equal to 7.8. As in the case of the L. curvatus enzyme, Fru(1,6)P2 has no influence on the pH-dependent dissociation of the L. casei enzyme, whereas Mn2+ stabilizes the tetrameric structure. Reconstitution of a mixture of the two dissociated enzymes results in the formation of all statistically possible, enzymatically active hybrids. No hybridization between the allosteric enzymes from L. casei and L. curvatus and the non-allosteric ones of Lactobacillus plantarum and Lactobacillus acidophilus was observed.

Kinetics of reconstitution of porcein muscle lactic dehydrogenase after reversible high pressure dissociation

Porcine muscle lactic dehydrogenase can be reversibly dissociated into monomers at high hydrostatic pressure. The rate of dissociation depends on the conditions of the solvent (Schade et al., 1980, Biochemistry, in press). Maximum yields of reactivation are achieved after dissociation by 20 min incubation in 0.2 M Tris/HCl buffer or 0.2 M KCl at pH 7.6, in the presence of 10 mM dithioerythritol and 1 mM EDTA, provided that both dissociation and reassociation are performed under anaerobic conditions. At enzyme concentrations of the order of 1 microM reactivation amounts to greater than or equal to 95%, the product of reactivation being indistinguishable from the enzyme in its initial native state. Based on the long-term stability of the enzyme under the optimum given conditions of reactivation, the kinetics of reconstitution after pressure release were investigated over a wide range of enzyme concentrations (1 nM less than c less than 1 microM). The weakly sigmoidal kinetics may be described by an irreversible uni-bimolecular reaction scheme, corresponding to a sequential transconformation-association process. Assuming the protomers to be enzymatically inactive, the kinetic profiles may be fitted by one set of kinetic constants: kuni=1.5 X 10(-2) s-1 and kbi=7 X 10(3) s-1 M-1, the association step belonging to either dimer or tetramer formation.

Folding and association of oligomeric enzymes

The spontaneous structure formation of oligomeric enzymes consists of the consecutive 'folding' and association of the constituent polypeptide chains. Whether catalytic function is an intrinsic property of the folded monomers may be determined using kinetic reconstitution experiments. It is shown that full activity requires association; the correct assembly of subunits depends on their proper folding. The native structure is determined as the kinetically accessible state of lowest free energy.

[Lactate dehydrogenase. An example of the development of modern enzymology]

Lactate dehydrogenase is the prototype of an oligomeric, intracellular enzyme. Its primary, secondary, tertiary, and quaternary structures are now well known. This fact, especially with regard to the occurrence of isoenzymes, has effected a great progress in the understanding of the structure-function relationship of enzymes. The historical development of modern enzymology is described, together with discussions of the various methods and problems of enzymology.

Refolding and reactivation of liver alcohol dehydrogenase after dissociation and denaturation in 6M guanidine hydrochloride

Horse-liver alcohol dehydrogenase has been dissociated and denatured by 6 M guanidinium hydrochloride. Removal of the denaturant under optimum conditions of the solvent leads to partial reactivation. The concentrations of the enzyme, as well as the coenzyme (NAD+), and Zn2+, affect the reactivation significantly, since high concentrations promote the formation of inactive aggregation products. Analyzing the kinetics of reactivation and reassociation, conditions far from equilibrium of dissociation-association provide maximum yields (approximately 70%). The sigmoidal kinetic traces suggest a superposition of first-order transconformation and second-order association reactions; the latter are corroborated by the concentration dependence of the reactivation reaction. The coenzyme, NAD+, has no influence on the kinetics of reactivation. Addition of Zn2+ leads to a significant decrease of the rate and yield of reactivation. The process of renaturation, as reflected by the regain of native fluorescence shows complex kinetics: rapid relaxations are followed by slower first-order and second-order processes which parallel reactivation.

Oxidation of sulfhydryl groups in lactate dehydrogenase under high hydrostatic pressure

Lactate dehydrogenase from rabbit skeletal muscle in the presence of substrate exhibits irreversible deactivation at hydrostatic pressures beyond 1 kbar [Schmid, G., Lüdemann, H.-D. & Jaenicke, R. (1975) Biophys. Chem. 3, 90-98]. In the absence of substrate and coenzyme the lability towards pressure is enhanced. The pH dependence of the effect and its inhibition by SH-protecting agents suggest the oxidation of sulfhydryl groups to be involved in the mechanism of deactivation. Partial deactivation observed at pH 5.5-7.0 becomes complete in the range of the intrinsic pK of cysteine; addition of dithiothreitol and/or EDTA protects the enzyme from complete deactivation, and leads to the residual enzymatic activity observed at pH 7.0. Incubation of the enzyme with dithiothreitol after pressure deactivation at pH 8.5 causes partial reactivation. From pressure-dependent measurements of the kinetics of deactivation an activation volume of deltaVnot equal to = -285 +/- 30 cm3 . mol-1 is calculated, which exceeds numerical data reported for typical reactions in organic chemistry. Therefore, the assumption can be made that the oxidation of sulfhydryl groups is connected with structural changes in the enzyme in the rate-determining step of the deactivation. The proposed mechanism may contribute to the toxicity of oxygen towards bacteria under high hydrostatic pressure.