Immobilized hybrids of glyceraldehyde-3-phosphate dehydrogenase.

Yeast glyceraldehyde-3-phosphate dehydrogenase (glyceraldehyde-3-phosphate:NAD+ oxidoreductase (phosphorylating), EC 1.2.1.12) immobilized on CNBr-activated Sepharose 4-B has been subjected to dissociation to obtain matrix-bound dimeric species of the enzyme. Hybridization was then performed using soluble glyceraldehyde-3-phosphate dehydrogenase isolated from rat skeletal muscle. Immobilized hybrid tetramers thus obtained were demonstrated to exhibit two distinct pH-optima of activity characteristic of the yeast and muscle enzymes, respectively. The results indicate that under appropriate conditions the activity of each of the dimers composing the immobilized hybrid tetramer can be studied separately.     

Rate-determining folding and association reactions on the reconstitution pathway of porcine skeletal muscle lactic dehydrogenase after denaturation by guanidine hydrochloride

Reactivation of tetrameric porcine skeletal muscle lactic dehydrogenase after dissociation and extensive unfolding of the monomers by 6 M guanidine hydrochloride (Gdn . HCl) is characterized by sigmoidal kinetics, indicating a complex mechanism involving rate-limiting folding and association steps. For analysis of the association reactions, chemical cross-linking with glutaraldehyde may be used [Hermann, R., Jaenicke, R., & Rudolph, R. (1981) Biochemistry 20, 2195-2201]. The data clearly show that the formation of a dimeric intermediate is determined by a first-order folding reaction of the monomers with k1 = (8.0 +/- 0.1) x 10(-4) s-1. The rate constant of the association of dimers to tetramers which represents the second rate-limiting step on the pathway of reconstitution after guanidine denaturation, was then determined by reactivation and cross-linking experiments after dissociation in 0.1 M H3PO4 containing 1 M Na2SO4. The rate constant for the dimer association (which is the only rate-limiting step after acid dissociation) was k2 = (3.0 +/- 0.5) x 10(4) M-1 s-1. On the basis of the given two rate constants, the complete reassociation pattern of porcine lactic dehydrogenase after dissociation and denaturation in 6 M Gdn . HCl can be described by the kinetic model (formula: see text).     

Misfolded forms of glyceraldehyde-3-phosphate dehydrogenase interact with GroEL and inhibit chaperonin-assisted folding of the wild-type enzyme

We studied the interaction of chaperonin GroEL with different misfolded forms of tetrameric phosphorylating glyceraldehyde-3-phosphate dehydrogenase (GAPDH): (1) GAPDH from rabbit muscles with all SH-groups modified by 5,5'-dithiobis(2-nitrobenzoate); (2) O-R-type dimers of mutant GAPDH from Bacillus stearothermophilus with amino acid substitutions Y283V, D282G, and Y283V/W84F, and (3) O-P-type dimers of mutant GAPDH from B. stearothermophilus with amino acid substitutions Y46G/S48G and Y46G/R52G. It was shown that chemically modified GAPDH and the O-R-type mutant dimers bound to GroEL with 1:1 stoichiometry and dissociation constants K(d) of 0.4 and 0.9 muM, respectively. A striking feature of the resulting complexes with GroEL was their stability in the presence of Mg-ATP. Chemically modified GAPDH and the O-R-type mutant dimers inhibited GroEL-assisted refolding of urea-denatured wild-type GAPDH from B. stearothermophilus but did not affect its spontaneous reactivation. In contrast to the O-R-dimers, the O-P-type mutant dimers neither bound nor affected GroEL-assisted refolding of the wild-type GAPDH. Thus, we suggest that interaction of GroEL with certain types of misfolded proteins can result in the formation of stable complexes and the impairment of chaperonin activity.     

Production of (S)-3-chlorolactaldehyde from (S)-alpha-chlorohydrin by boar spermatozoa and the inhibition of glyceraldehyde 3-phosphate dehydrogenase in vitro

The (S)-isomer of the male antifertility agent alpha-chlorohydrin was metabolized by mature boar spermatozoa in vitro to (S)-3-chlorolactaldehyde. This oxidative process, which did not occur when (R)-alpha-chlorohydrin was offered as a substrate, was catalysed by an NADP+-dependent dehydrogenase that converts glycerol to glyceraldehyde. (S)-3-chlorolactaldehyde, produced by this metabolic reaction or when added to suspensions of boar spermatozoa, was a specific inhibitor of glyceraldehyde 3-phosphate dehydrogenase as assessed by the accumulation of fructose 1,6-bisphosphate and the triosephosphates. When glycerol and (S)-alpha-chlorohydrin were added concomitantly to boar spermatozoa in vitro, the presence of glycerol decreased the degree of inhibition of glyceraldehyde 3-phosphate dehydrogenase. Extracts of glyceraldehyde 3-phosphate dehydrogenase that were obtained from boar spermatozoa incubated with (S)-alpha-chlorohydrin or (R,S)-3-chlorolactaldehyde showed significant reductions in their enzymic activity.     

Thermodynamic analysis of the emergence of new regulatory properties in a phosphoribulokinase-glyceraldehyde 3-phosphate dehydrogenase complex

Glyceraldehyde 3-phosphate dehydrogenase and phosphoribulokinase exist as stable enzymes and as part of a complex in Chlamydomonas reinhardtii. We show here that phosphoribulokinase exerts an imprinting on glyceraldehyde 3-phosphate dehydrogenase, which affects its catalysis by decreasing the energy barrier of the reactions with NADH or NADPH by 3.8 +/- 0.5 and 1.3 +/- 0.3 kJ.mol(-1). Phosphoribulokinase and glyceraldehyde 3-phosphate dehydrogenase within the complex are regulated by NADP(H) but not by NAD(H). The activities of the metastable phosphoribulokinase and glyceraldehyde 3-phosphate dehydrogenase released from the complex preincubated with NADP(H) are different from those of the metastable enzymes released from the untreated complex. NADP(H) increases phosphoribulokinase and NADPH-glyceraldehyde 3-phosphate dehydrogenase activities with a (~)K(0.5 (NADP)) of 0.68 +/- 0.16 mm and a (~)K(0.5 (NADPH)) of 2.93 +/- 0.87 mm and decreases NADH-dependent activity. 1 mm NADP increases the energy barrier of the NADH-glyceraldehyde 3-phosphate dehydrogenase-dependent reaction by 1.8 +/- 0.2 kJ.mol(-1) and decreases that of the reactions catalyzed by phosphoribulokinase and NADPH-glyceraldehyde 3-phosphate dehydrogenase by 3 +/- 0.2 and 1.2 +/- 0.3 kJ.mol(-1), respectively. These cofactors have no effect on the independent stable enzymes. Therefore, protein-protein interactions may give rise to new regulatory properties.     

Modification of glyceraldehyde 3-phosphate dehydrogenase activity by adsorption on phospholipid vesicles.

1. The adsorption of [14C]carboxymethylated glyceraldehyde 3-phosphate dehydrogenase to negatively charged liposomes of phsphatidic acid/phosphatidylcholine (3:7, w/w) was investigated. The apparent association constant at I/2 = 60, pH 7.6, was 0.4 X 10(6)M-1. Adsorption decreased as ionic strength and pH were increased. 2. In the presence of negatively charged liposomes, the Km value for glyceraldehyde 3-phosphate of glyceraldehyde 3-phosphate dehydrogenase was increased and Vmax. decreased. In the presence of positively charged liposomes, the Km value for glyceraldehyde 3-phosphate decreased and there was no significant change in Vmax. Addition of Triton X-100 abolished the effect of both positively and negatively charged liposomes on the kinetic properties of the enzyme.     

Kinetic evidence for interaction between aldolase and D-glyceraldehyde-3-phosphate dehydrogenase.

The possibility of interaction between purified rabbit muscle aldolase and D-glyceraldehyde-3-phosphate dehydrogenase was studied by rapid kinetic methods, by analyzing the kinetics of the consecutive reaction catalyzed by the coupled enzyme system. The Km of the intermediary product, glyceraldehyde 3-phosphate, produced by aldolase was determined in the coupled reaction for glyceraldehyde-3-phosphate dehydrogenase. Its value corresponds to that of the aldehyde (active) form of glyceraldehyde 3-phosphate, although in the given conditions the aldehyde leads to diol interconversion is faster than the enzymic reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. We suggest that above a certain concentration of the enzymes the glyceraldehyde 3-phosphate produced by aldolase gets direct access to glyceraldehyde-3-phosphate dehydrogenase without participating in the aldehyde leads to diol interconversion which otherwise would occur if the substrate were to mix with the bulk medium.     

D-glyceraldehyde-3-phosphate dehydrogenase subunit cooperativity studied using immobilized enzyme forms

Tetrameric D-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12) isolated from rabbit skeletal muscle was covalently bound to CNBr-activated Sepharose 4B via a single subunit. Catalytically active immobilized dimer and monomeric forms of the enzyme were prepared after urea-induced dissociation of the tetramer. A study of the coenzyme-binding properties of matrix-bound tetrameric, dimeric and monomeric species has shown that: (1) an immobilized tetramer binds NAD+ with negative cooperativity, the dissociation constants being 0.085 microM for the first two coenzyme molecules and 1.3 microM for the third and the fourth one; (2) coenzyme binding to the dimeric enzyme form also displays negative cooperativity with Kd values of 0.032 microM and 1.1 microM for the first and second sites, respectively; (3) the binding of NAD+ to a monomer can occur with a dissociation constant of 1.6 microM which is close to the Kd value for low-affinity coenzyme binding sites of the tetrameric or dimeric enzyme forms. In the presence of NAD+ an immobilized monomer acquires a stability which is not inferior to that of a holotetramer. The catalytic properties of monomeric and tetrameric enzyme forms were compared and found to be different under certain conditions. Thus, the monomers of rabbit muscle D-glyceraldehyde-3-phosphate dehydrogenase displayed a hyperbolic kinetic saturation curve for NAD+, whereas the tetramers exhibited an intermediary plateau region corresponding to half-saturating concentrations of NAD+. At coenzyme concentrations below half-saturating a monomer is more active than a tetramer. This difference disappears at saturating concentrations of NAD+. Immobilized monomeric and tetrameric forms of D-glyceraldehyde-3-phosphate dehydrogenase from baker's yeast were also used to investigate subunit interactions in catalysis. The rate constant of inactivation due to modification of essential arginine residues in the holoenzyme decreased in the presence of glyceraldehyde 3-phosphate, probably as a result of conformational changes accompanying catalysis. This effect was similar for monomeric and tetrameric enzyme forms at saturating substrate concentrations, but different for the two enzyme species under conditions in which about one-half of the active centers remained unsaturated. Taken together, the results indicate that association of D-glyceraldehyde-3-phosphate dehydrogenase monomers into a tetramer imposes some constraints on the functioning of the active centers.(ABSTRACT TRUNCATED AT 400 WORDS)     

The pentose phosphate pathway of glucose metabolism. Measurement of the non-oxidative reactions of the cycle

Methods for the quantitative determination of ribose 5-phosphate isomerase, ribulose 5-phosphate 3-epimerase, transketolase and transaldolase in tissue extracts are described. The determinations depend on the measurement of glyceraldehyde 3-phosphate by using the coupled system triose phosphate isomerase, α-glycero-phosphate dehydrogenase and NADH. By using additional purified enzymes transketolase, ribose 5-phosphate isomerase and ribulose 5-phosphate epimerase conditions could be arranged so that each enzyme in turn was made rate-limiting in the overall system. Transaldolase was measured with fructose 6-phosphate and erythrose 4-phosphate as substrates, and again glyceraldehyde 3-phosphate was measured by using the same coupled system. Measurements of the activities of the non-oxidative reactions of the pentose phosphate pathway were made in a variety of tissues and the values compared with those of the two oxidative steps catalysed by glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase.     

Microtubules bind glyceraldehyde 3-phosphate dehydrogenase and modulate its enzyme activity and quaternary structure

Glyceraldehyde 3-phosphate dehydrogenase, a tetramer of 140,000 Da, interacts with in vitro reconstituted microtubules. It results in a partial inhibition of the activity of the microtubule-bound enzyme. After cold depolymerization of the microtubule-glyceraldehyde 3-phosphate dehydrogenase complexes, a fraction of the enzyme is recovered in an active form in the disassembly supernatant; the other fraction devoid of activity, identified by polyacrylamide gel electrophoresis, remains associated with the undepolymerizable microtubule protein pellet. The inactivation of the microtubule-bound enzyme is related to the concentration of microtubule protein. Higher the concentration of microtubule protein, lower the fraction of inactivated enzyme; consequently, glyceraldehyde 3-phosphate dehydrogenase is able to copolymerize quantitatively with microtubule protein through one assembly-disassembly cycle, provided that the concentration of microtubule protein is high. Monomeric glyceraldehyde 3-phosphate dehydrogenase (molecular weight: 35,000) devoid of enzyme activity, prepared by reversible dissociation of the tetrameric enzyme, also binds to microtubules and is quantitatively recovered in the undepolymerizable microtubule protein fraction after cold treatment. These results indicate that interacting with microtubules, glyceraldehyde 3-phosphate dehydrogenase partly dissociates into inactive monomers, this process is regulated by the concentration of assembled microtubule protein, and active and inactive glyceraldehyde 3-phosphate dehydrogenase bound to microtubules have different fate at the step of microtubule disassembly. These data suggest that an association of glyceraldehyde 3-phosphate dehydrogenase to microtubules could play a role in modulating the activity of the glycolytic enzyme in intact cells.     

Dissociation and in vitro reconstitution of bovine liver uridine diphosphoglucose dehydrogenase. The paired subunit nature of the enzyme

Uridine diphosphoglucose dehydrogenase (EC 1.1.1.22: UDPglucose dehydrogenase) at pH 5.5-7.8 is a stable homohexamer of 305 +/- 7 kDa that does not undergo concentration-dependent dissociation at enzyme concentrations greater than 5 micrograms/mL. Chemical cross-linking of the native enzyme at varying glutaraldehyde concentrations yields dimers, tetramers, and hexamers; at greater than 2% (w/v) glutaraldehyde, plateau values of 21% monomers, 16% dimers, 5% tetramers, and 58% hexamers are obtained. Dissociation at acid pH (pH 2.3) or in 4-6 M guanidine hydrochloride leads to inactive monomers (Mr 52,000). Denaturation at increasing guanidine hydrochloride concentration reveals separable unfolding steps suggesting the typical domain structure of dehydrogenases holds for the present enzyme. At greater than 4 M guanidine hydrochloride complete randomization of the polypeptide chains is observed after 10-min denaturation. Reconstitution of the native hexamer after dissociation/denaturation has been monitored by reactivation and glutaraldehyde fixation. The kinetics may be described in terms of a sequential uni-bimolecular model, governed by rate-determining folding and association steps at the monomer level. Trimeric intermediates do not appear in significant amounts. Reactivation is found to parallel hexamer formation. Structural changes during reconstitution (monitored by circular dichroism) are characterized by complex kinetics, indicating the rapid formation of "structured monomers" (with most of the native secondary structure) followed by slow "reshuffling" prior to subunit association. The final product of reconstitution is indistinguishable from the initial native enzyme.     

The glyceraldehyde 3-phosphate and glycerate 3-phosphate shuttle and carbon dioxide assimilation in intact spinach chloroplasts

The regulation of CO(2) assimilation by intact spinach (Spinacia oleracea) chloroplasts by exogenous NADP-linked nonreversible d-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.9) was investigated. This dehydrogenase mediated a glyceraldehyde 3-phosphate/glycerate 3-phosphate shuttle for the indirect transfer of NADPH from chloroplast to the external medium. The rate of NADPH formation in the medium reflected glyceraldehyde 3-phosphate efflux from the chloroplast. Increasing enzyme concentrations stimulated NADP reduction and, in turn, CO(2) fixation. Pyrophosphate increased CO(2) fixation by apparently inhibiting glyceraldehyde 3-phosphate efflux. Increasing the glycerate 3-phosphate concentration above 0.1 mm stimulated glyceraldehyde 3-phosphate efflux but inhibited CO(2) fixation. Addition of up to 0.5 mm orthophosphate enhanced both glyceraldehyde 3-phosphate efflux and CO(2) fixation while each was inhibited by higher orthophosphate concentrations. The mechanism by which the extent of glyceraldehyde 3-phosphate efflux regulated the rate of CO(2) fixation in chloroplasts was discussed.     

Free radical scavengers in susceptible/resistant Biomphalaria alexandrina snails before and after infection

The activities of catalase (Cat), superoxide dismutase (SOD), glutathione peroxidase (GSHPx), glutathione transferase (GST), glucose-6-phosphate dehydrogenase (G6PD) and glyceraldehyde3-phosphate dehydrogenase (G3PD) were studied in tissue and hemolymph of susceptible (S) (EgBS(2)) and resistant (R) (EgBR(2)) Biomphalaria alexandrina snails. The results showed that CAT and GST were higher in the hemolymph of snails susceptible to Schistosoma mansoni than in that of snails resistant to infestation, while SOD and G3PD were lower in the susceptible snails. The role of these enzymes as free radical scavengers was traced 1 and 24 h after infection of the two snail lines with S. mansoni. Moreover, the activities of SOD and G3PD were also measured 2 and 4 weeks post infection. The results revealed that the overall enzymatic activities were higher in susceptible than in resistant snail tissues. After 1 h of infection, all enzymes were increased in R and S snails except GST and G6PD which decreased in S snails. After 24 h of infection, GST increased in S snails and G3PD decreased in both S and R snails while other enzymes reached normal levels.     

Striking conformational change suspected within the phosphoribulokinase dimer induced by interaction with GAPDH.

A multitechnique approach was used to study the [glyceraldehyde-3-phosphate dehydrogenase](2 x 4)-[phosphoribulokinase](2 x 2) multienzymatic complex of the alga Chlamydomonas reinhardtii. On the one hand, each component of the complex was compared with known atomic structures of related enzymes or of similar enzymes originating from different organisms. On the other hand, the overall low resolution architecture of the whole complex was studied using cryoelectron microscopy and image processing techniques. The dimers of phosphoribulokinase are suspected to undergo a dramatic change in activity during a cycle of binding and detaching from tetramers of glyceraldehyde-3-phosphate dehydrogenase. This is likely supported by strong structural differences between the modeled phosphoribulokinase dimers and the counterpart in the three-dimensional reconstruction volume of the whole complex obtained from cryoelectron microscope images.     

Mechanism of glyceraldehyde-3-phosphate transfer from aldolase to glyceraldehyde-3-phosphate dehydrogenase

The catalytic interaction of glyceraldehyde-3-phosphate dehydrogenase with glyceraldehyde 3-phosphate has been examined by transient-state kinetic methods. The results confirm previous reports that the apparent Km for oxidative phosphorylation of glyceraldehyde 3-phosphate decreases at least 50-fold when the substrate is generated in a coupled reaction system through the action of aldolase on fructose 1,6-bisphosphate, but lend no support to the proposal that glyceraldehyde 3-phosphate is directly transferred between the two enzymes without prior release to the reaction medium. A theoretical analysis is presented which shows that the kinetic behaviour of the coupled two-enzyme system is compatible in all respects tested with a free-diffusion mechanism for the transfer of glyceraldehyde 3-phosphate from the producing enzyme to the consuming one.     

Human platelet factor 4 subunit association/dissociation thermodynamics and kinetics

Platelet factor 4 (PF4) monomers (7800 daltons) form dimers and tetramers in varying molar ratios under certain solution conditions [Mayo, K. H., & Chen, M. J. (1989) Biochemistry 28, 9469]. The presence of a simplified aromatic region (one Tyr and two His) and resolved monomer, dimer, and tetramer Y60 3,5 ring proton resonances makes study of PF4 aggregate association/dissociation thermodynamics and kinetics possible. PF4 protein subunit association/dissociation equilibrium thermodynamic parameters have been derived by 1H NMR (500MHz) resonance line-fitting analysis of steady-state Y60 3,5 ring proton resonance monomer-dimer-tetramer populations as a function of temperature from 10 to 40 degrees C. Below 10 degrees C and above 40 degrees C, resonance broadening and overlap severely impaired analysis. Enthalpic and entropic contributions to dimer association Gibb's free energy [-5.1 kcal/mol (30 degrees C)] are +2.5 +/- 1 kcal/mol and +26 +/- 7 eu, respectively, and for tetramer association Gibb's free energy [-5.7 kcal/mol (30 degrees C)], they are -7.5 +/- 1 kcal/mol and -7 +/- 3 eu, respectively. These thermodynamic parameters are consistent with low dielectric medium electrostatic/hydrophobic interactions governing dimer formation and hydrogen bonding governing tetramer formation. Association/dissociation kinetic parameters, i.e., steady-state jump rates, have been derived from exchange-induced line-width increases and from 1H NMR (500 MHz) saturation-transfer and spin-lattice (Tl) relaxation experiments. From dissociation jump rates and equilibrium constants, association rate constants were estimated. For dimer and tetramer equilibria at 30 degrees C, unimolecular dissociation rate constants are 35 +/- 10 s-1 for dimer dissociation and 6 +/- 2 s-1 for tetramer dissociation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reactions of d-glyceraldehyde 3-phosphate dehydrogenase facilitated by oxidized nicotinamide–adenine dinucleotide

Transient kinetic methods have been used to study the influence of NAD(+) on the rate of elementary processes of the reversible oxidative phosphorylation of d-glyceraldehyde 3-phosphate catalysed by d-glyceraldehyde 3-phosphate dehydrogenase. In the pH range 5-8 NAD(+) is bound to the enzyme during the following elementary processes of the mechanism: phosphorolysis of the acyl-enzyme, its formation from 1,3-diphosphoglycerate and the enzyme and the formation and breakdown of the glyceraldehyde 3-phosphate-enzyme complex. The rates of these four elementary processes only equal or exceed the turnover rate of the enzyme when NAD(+) is bound and are as much as 10(4) times the rates in the absence of NAD(+). Autocatalysis of the reductive dephosphorylation of 1,3-diphosphoglycerate occurs when glyceraldehyde 3-phosphate release is rate determining because NAD(+) is a reaction product. An important feature of the enzyme mechanism is that the negative-free-energy change of a chemical reaction, acyl-enzyme formation, is linked in a simple way to the positive-free-energy change of a dissociation reaction, NAD(+) release.     

Effect of alpha-crystallin on thermal denaturation and aggregation of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase

The study of thermal denaturation of rabbit muscle glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in the presence of alpha-crystallin by differential scanning calorimetry (DSC) showed that the position of the maximum on the DSC profile (T(max)) was shifted toward lower temperatures with increasing alpha-crystallin concentration. The diminishing GAPDH stability in the presence of alpha-crystallin has been explained assuming that heating of GAPDH induces dissociation of the tetrameric form of the enzyme into dimers interacting with alpha-crystallin. The dissociation of the enzyme tetramer was shown by sedimentation velocity at 45 degrees C. Suppression of thermal aggregation of GAPDH by alpha-crystallin was studied by dynamic light scattering under the conditions wherein temperature was elevated at a constant rate. The construction of the light scattering intensity versus the hydrodynamic radius (R(h)) plots enabled estimating the hydrodynamic radius of the start aggregates (R(h,0)). When aggregation of GAPDH was studied in the presence of alpha-crystallin, the start aggregates of lesser size were observed.     

Kinetic studies of glyceraldehyde-3-phosphate dehydrogenase from rabbit muscle

Initial rate studies at pH 7.6 with three aldehydes, product inhibition patterns with NADH and dead-end inhibition with adenosine diphosphoribose show that the kinetic mechanism of glyceraldehyde-3-phosphate dehydrogenase from rabbit muscle cannot be ordered, and support an enzyme-substitution mechanism. Deviations from Michaelis-Menten behaviour are consistent with negative interactions in the binding of NAD+ and instability of the species E(NAD)3 and E(NAD)4. Inhibition with large concentrations of phosphate and arsenate indicates competition for a binding site for glyceraldehyde 3-phosphate, and is not found with glyceraldehyde as substrate.     

Immobilized flounder muscle glyceraldehyde 3-phosphate dehydrogenase

Summary Partially purified flounder muscle (Pseudopleuronectus americanus) glyceraldehyde 3-phosphate dehydrogenase was immobilized on cyanogen bromide-activated Sepharose. The catalytic properties of the immobilized preparation were studied to determine if immobilization alters the kinetic properties of the native holoenzyme. The results indicate that the pH activity profile of immobilized glyceraldehyde 3-phosphate dehydrogenase did not differ from that of the native enzyme. The Michaelis constants (Km) for NAD and glyceraldehyde 3-phosphate were somewhat altered. The enzyme stability toward various inactivation treatments in the presence and absence of NAD was characterized and compared to that of he native enzyme. When either form of the enzyme was incubated with urea at concentrations greater than 2m, inactivation occurred very rapidly. Incubation in 0.1% trypsin for 60 minutes decreased the activity of immobilized glyceraldehyde 3-phosphate dehydrogenase by 45% and of the native soluble enzyme by 70%. The immobilized enzyme also exhibited considerably more stability than the native soluble enzyme when exposed to a temperature of 50° or to 20 mm ATP. In all cases NAD either greatly reduced the rate of inactivation or completely protected the enzyme from inactivation.