Hydrodynamic properties of D-beta-hydroxybutyrate dehydrogenase, a lipid-requiring enzyme

Metabolism of hyperpolarized 13 C-acetoacetate to β-hydroxybutyrate detects real-time mitochondrial redox state and dysfunction in heart tissue

Mitochondrial dysfunction is considered to be an important component of many metabolic diseases yet there is no reliable imaging biomarker for monitoring mitochondrial damage in vivo. A large prior literature on inter-conversion of β-hydroxybutyrate and acetoacetate indicates that the process is mitochondrial and that the ratio reflects a specifically mitochondrial redox state. Therefore, the conversion of [1,3-¹³ C]acetoacetate to [1,3-¹³ C]β-hydroxybutyrate is expected to be sensitive to the abnormal redox state present in dysfunctional mitochondria. In this study, we examined the conversion of hyperpolarized (HP) ¹³ C-acetoacetate (AcAc) to ¹³ C-β-hydroxybutyrate (β-HB) as a potential imaging biomarker for mitochondrial redox and dysfunction in perfused rat hearts. Conversion of HP-AcAc to β-HB was investigated using ¹³ C magnetic resonance spectroscopy in Langendorff-perfused rat hearts in four groups: control, global ischemic reperfusion, low-flow ischemic, and rotenone (mitochondrial complex-I inhibitor)-treated hearts. We observed that more β-HB was produced from AcAc in ischemic hearts and the hearts exposed to complex I inhibitor rotenone compared with controls, consistent with the accumulation of excess mitochondrial NADH. The increase in β-HB, as detected by ¹³ C MRS, was validated by a direct measure of tissue β-HB by ¹ H nuclear magnetic resonance in tissue extracts. The redox ratio, NAD⁺ /NADH, measured by enzyme assays of homogenized tissue, also paralleled production of β-HB from AcAc. Transmission electron microscopy of tissues provided direct evidence for abnormal mitochondrial structure in each ischemic tissue model. The results suggest that conversion of HP-AcAc to HP-β-HB detected by ¹³ C-MRS may serve as a useful diagnostic marker of mitochondrial redox and dysfunction in heart tissue in vivo.

Inactivation of D-3-hydroxybutyrate dehydrogenase by fumaroyl bis(methyl phosphate)

Fumaroyl bis(methyl phosphate) reacts with the NADH-dependent enzyme, D-3-hydroxybutyrate dehydrogenase, leading to irreversible inactivation. The bifunctional reagent cross-links the subunits of the enzyme. The inactivation is subject to saturation and protection by substrate, consistent with the reaction occurring at the active site. The stoichiometry of inactivation indicates two active sites undergo reaction with each equivalent of reagent. These results indicate that the dimeric enzyme has contiguous active sites. The reagent is likely to react with an active site lysine, consistent with previous suggestions.

Monoclonal antibodies for structure-function studies of (R)-3-hydroxybutyrate dehydrogenase, a lipid-dependent membrane-bound enzyme

Monoclonal antibodies (mAbs) have been used to study structure-function relationships of (R)-3-hydroxybutyrate dehydrogenase (BDH) (EC 1.1.1.30), a lipid-requiring mitochondrial membrane enzyme with an absolute and specific requirement for phosphatidylcholine (PC) for enzymic activity. The purified enzyme (apoBDH, devoid of phospholipid and thereby inactive) can be re-activated with preformed phospholipid vesicles containing PC or by short-chain soluble PC. Five of six mAbs cross-react with BDH from bovine heart and rat liver, including two mAbs to conformational epitopes. One mAb was found to be specific for the C-terminal sequence of BDH and served to: (1) map endopeptidase cleavage and epitope sites on BDH; and (2) demonstrate that the C-terminus is essential for the activity of BDH. Carboxypeptidase cleavage of only a few (< or = 14) C-terminal amino acids from apoBDH (as detected by the loss of C-terminal epitope for mAb 3-10A) prevents activation by either bilayer or soluble PC. Further, for BDH in bilayers containing PC, the C-terminus is protected from carboxy-peptidase cleavage, whereas in bilayers devoid of PC the C-terminus is cleaved, and subsequent activation by PC is precluded. We conclude that: (1) the C-terminus of BDH is essential for enzymic activity, consistent with the prediction, from primary sequence analysis, that the PC-binding site is in the C-terminal domain of BDH; and (2) the allosteric activation of BDH by PC in bilayers protects the C-terminus from carboxypeptidase cleavage, indicative of a PC-induced conformational change in the enzyme.

Noncooperative vs. cooperative reactivation of D-beta-hydroxybutyrate dehydrogenase: multiple equilibria for lecithin binding are determined by the physical state (soluble vs. bilayer) and composition of the phospholipids

D-beta-Hydroxybutyrate dehydrogenase (BDH) is a lecithin-requiring mitochondrial enzyme that catalyzes the interconversion of beta-hydroxybutyrate and acetoacetate. The purified soluble enzyme devoid of lipid (i.e., the apodehydrogenase) can be reactivated with soluble lecithin or by insertion into phospholipid vesicles containing lecithin. Lipid activation curves have a sigmoidal shape, and two models have been proposed to explain them. We have previously reported that the kinetics of reactivation with short-chain lecithins in the soluble state is consistent with a model in which the enzyme enzyme contains two identical, noninteracting lecithin binding sites, both of which must be occupied to activate the enzyme [noncooperative mechanism; Cortese, J.D., Vidal, J.C., Churchill, P., McIntyre, J.O., & Fleischer, S. (1982) Biochemistry 21, 3899-3908]. More recently a kinetic model involving cooperative interactions between lecithin binding sites was proposed for the reactivation of the membrane-bound enzyme [Sandermann, H., Jr., McIntyre, J.O., & Fleischer, S. (1986) J. Biol. Chem. 261, 6201-6208]. This study reinvestigates the basis for the different conclusions in these two studies. The previous study with soluble lecithins was limited to about 34% of maximal activation compared with mitochondrial phospholipid, due to inactivation of the enzyme at the critical micellar concentration. We could now extend this study to 91% activation by increasing the ethanol concentration. This experimental evidence confirms that the soluble system follows a noncooperative equation. We provide a new kinetic approach to test the cooperative model. A velocity equation is derived for a Hill-type cooperative ligand binding system interacting with a mixture of ligands. This equation predicts a proportionality between an overall weighted cooperative dissociation constant [Kcoop(w)] and a dissociation constant for a single lecithin (PC) species from interacting sites (KPC), regulated by the PC molar fraction (XPC): 1/Kcoop(w) = XPC/KPC. The equation was applied to the data of Sandermann et al. [Sandermann, H., Jr., McIntyre, J.O., & Fleischer, S. (1986) J. Biol. Chem. 261, 6201-6208] as well as to newly obtained data. The results obtained over a wide range of PC molar fractions and different mixtures of bilayer phospholipids fit this equation, confirming the cooperative behavior. We conclude that BDH has a different mode of reactivation depending on the nature of the lipid environment. With soluble lecithin, the activation is noncooperative, whereas in the bilayer, mixtures of phospholipids give cooperative behavior that fits a Hill equation.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipid-protein interactions in biomembranes studied through the phospholipid specificity of D-beta-hydroxybutyrate dehydrogenase

Since the biological membranes are fundamental units in the living cells, the studies of lipid-protein interactions are crucial for the understanding of their structure, functions and properties. Beside hydrophobic interactions between fatty acids chain of phospholipids and intrinsic membrane proteins, the interactions between charged groups of the protein with the polar heads of phospholipids generally confer the specificity which may be absolute or preferential. This paper reports essential results obtained these last few years with D-beta-hydroxybutyrate dehydrogenase (BDH) from inner mitochondrial membrane, one of the most interesting and best documented examples of a lipid-requiring enzyme. This is a review of the molecular basis--knowledge and strategy of study--of the lipid specificity for membrane protein functions.

Kinetic aspects of the role of phospholipids in D-beta-hydroxybutyrate dehydrogenase activity

Interactions of phospholipids with D-beta-hydroxybutyrate dehydrogenase (BDH), a lecithin-requiring enzyme, have been studied by a kinetic approach. The process of reactivation of BDH by phospholipids, which follows a second-order mechanism, reveals that (1) at least 2 mol of lecithins is essential for the reactivation of the enzyme, and (2) the enzyme contains two dependent binding sites for lecithins. The graphic representation of the time course of reactivation shows a latent phase which decreases when there is an increase in the amount of phospholipids. A Scatchard plot treatment of the reactivation kinetic data reveals the presence of two classes of phospholipid binding sites, which exhibit high and low affinities related to the binding of four and two lecithin molecules, respectively. The effect of temperature on BDH activity and on the inactivation of the apoenzyme with N,N'-dicyclohexylcarbodiimide (a specific carboxyl reagent) or with phenylglyoxal (a specific arginine reagent) shows a break at 22-24 degrees C, indicating a slight structural change in the enzyme-active site around this temperature. In addition, the variations in enzyme kinetic parameters, according to the nature of phospholipids, are in agreement with conformational changes related to the nature and to the fluidity state of phospholipids. However, the apparent NAD+ binding constant does not depend on the phospholipid's fluidity.

Involvement of tyrosyl residues in the structure-function relationships of D-beta-hydroxybutyrate dehydrogenase: a phospholipid-requiring enzyme

The involvement of tyrosyl residues in the function of D-beta-hydroxybutyrate dehydrogenase, a lipid-requiring enzyme, has been investigated by using several tyrosyl modifying reagents, i.e., N-acetylimidazole, a hydrophilic reagent, and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole and tetranitromethane, two hydrophobic reagents. Modification of the tyrosyl residues highly inactivates the derived enzyme: Treatment of the enzyme with 7-chloro-4-nitro[14C]benzo-2-oxa-1,3-diazole leads to an absorbance at 380 nm and to an incorporation of about 1 mol of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole per polypeptide chain for complete inactivation. Inactivation by N-acetylimidazole induces a decrease in absorbance at 280 nm which can be reversed by hydroxylamine treatment. On the other hand, the ligands of the active site, such as methylmalonate, a pseudosubstrate, and NAD+ (or NADH), do not protect the enzyme against inactivation. In contrast, the presence of phospholipids strongly protects the enzyme against hydrophobic reagents. Finally, previous modification of the enzyme with N-acetylimidazole does not affect the incorporation of 7-chloro-4-nitro[14C]benzo-2-oxa-1,3-diazole while modification with tetranitromethane does. These results indicate the existence of two classes of tyrosyl residues which are essential for enzymatic activity, and demonstrate their location outside of the active site. One of these residues appears to be located close to the enzyme-phospholipid interacting sites. These essential residues may also be essential for maintenance of the correct active conformation.

The insertion of D-beta-hydroxybutyrate apodehydrogenase into phospholipid monolayers and phospholipid vesicles

The strong interaction of D-beta-hydroxybutyrate dehydrogenase with phospholipid monomolecular films is demonstrated by the surface pressure increase of a film compressed up to 33 mN/m. Although the D-beta-hydroxybutyrate apodehydrogenase was able to penetrate many phospholipid monolayers, it interacted preferentially with negatively charged monolayers such as those made from diphosphatidylglycerol. The weakest interaction was found with phosphatidylcholine, which is the reactivating phospholipid for the enzyme. These interactions were dependent on the phospholipid chain length, ionic strength, and pH. At basic pH the apoenzyme lost its specificity for negatively charged phospholipids, suggesting the deprotonation of a cationic amino acid residue of the enzyme polypeptide chain. The charge effects are in agreement with results obtained using phospholipid vesicles. Beside the electrostatic interactions, the influence of phospholipid chain length and the ionic strength indicate that D-beta-hydroxybutyrate apodehydrogenase penetrates into the hydrophobic part of the lipid interface.

Photoaffinity labeling of D-(-)-beta-hydroxybutyrate dehydrogenase by (arylazido)-beta-alanyl-substituted nicotinamide adenine dinucleotide

(Arylazido)-beta-alanyl-substituted nicotinamide adenine dinucleotide (N3-NAD) is a photosensitive analogue of NAD capable of photoinduced nitrene generation and insertion into a nearby molecule. In the dark, N3-NAD can replace NAD as a cosubstrate for the mitochondrial D-(-)-beta-hydroxybutyrate dehydrogenase (BDH). With purified, phospholipid-reconstituted BDH and NAD as the variable substrate, the apparent Km and Vmax values were respectively 0.25 mM and 62.5 mumol min-1 (mg of protein)-1. With N3-NAD as the variable substrate, these values were respectively 0.59 mM and 5 mumol min-1 (mg of protein)-1. Photoirradiation of BDH in the presence of N3-NAD resulted in irreversible inhibition of the enzyme and incorporation into the protein of radioactivity from tritiated N3-NAD. Photoirradiation of BDH plus or minus NAD in the absence or presence of (arylazido)-beta-alanine caused little or no inhibition. The photoinhibition of BDH in the presence of N3-NAD was prevented nearly completely by addition of NADH, NAD plus beta-hydroxybutyrate, or NAD plus 2-methylmalonate and partially by addition of NAD. Moreover, the presence of NADH prevented, and prior partial modification of BDH at the NAD(H)-protectable site by N-ethylmaleimide decreased, the incorporation of radioactivity into BDH from photoirradiated [3H]N3-NAD. The above results suggest that N3-NAD can be used for photoaffinity labeling of BDH at the active site.

The essential cationic charge of phospholipid polar head in the reactivation of D-beta-hydroxybutyrate apodehydrogenase revealed by cationic surfactants

Attempts to reactivate purified D-beta-hydroxybutyrate apodehydrogenase, a lecithin-requiring enzyme, have been carried out using neutral, anionic, cationic and zwitterionic surfactants. Cationic and zwitterionic compounds exclusively are able to partially replace phosphatidylcholine, the reactivating phospholipid. The extent of reactivation depends on the steric hindrance of the polar head and on the hydrophobic tail length. A molecule bearing a positive charge and an aliphatic chain is the sole structure absolutely required for activity. However the presence of a negative charge is important for enzyme binding to amphiphilic structures and for the efficiency of reactivation.

Site--site interaction in the lipid activation of beta-hydroxybutyrate dehydrogenase

Kinetic data for the activation of the beta-hydroxybutyrate dehydrogenase by long-chain lecithins [(1979) Biochemistry 18, 2420-2429; (1983) J. Biol. Chem. 258, 208-214] are analyzed. A previous kinetic model [(1982) Biochemistry 21, 3899-3908] is shown not to apply. Instead, the use of a two-site Adair equation points to a strongly cooperative interaction between the lecithin binding sites (delta G, -2.8 kcal/mol).

Modification of arginines in D-beta-hydroxybutyrate dehydrogenase

D-beta-Hydroxybutyrate dehydrogenase (D-3-hydroxybutyrate: NAD+ oxidoreductase, EC 1.1.1.30) is a lipid-dependent enzyme which has an absolute and specific requirement for phosphatidylcholine for function. Chemical derivatization studies using 1,2-cyclohexanedione, an arginine-specific reagent, have been carried out on the purified enzyme devoid of lipid as well as on the enzyme reactivated with phospholipid. Cyclohexanedione inactivated the active enzyme-phospholipid complex and the lipid-free enzyme was rendered inactivatable by phospholipid. From kinetic studies and by direct chemical derivatization studies with [1-14C]cyclohexanedione, we find that incorporation of a single cyclohexanedione molecule per enzyme monomer resulted in complete loss of enzymic activity. The presence of NADH or NAD+, cofactors for the enzyme, offered no protection for the rate of inactivation. The substrates beta-hydroxybutyrate and acetoacetate with or without coenzyme gave little or no protection. However, 2-methyl malonate, a competitive inhibitor for beta-hydroxybutyrate, strongly protected against inactivation. These studies indicate that: (1) a single arginine serves a vital role and is essential for function; (2) the arginine is located in the proximity of the substrate binding site.

6-phospho-D-gluconate dehydrogenase from Pseudomonas fluorescens. Properties and subunit structure

1. The 6-phospho-D-gluconate dehydrogenase (decarboxylating) (EC 1.1.1.44) from Pseudomonas fluorescens, a B-side stereospecific enzyme, is active with both NAD+ and NADP+, having a specific activity of the homogeneous enzyme of 121 mumols NADH and 23 mumols NADPH, respectively, formed min-1 mg protein-1. The pI of the native enzyme is 4.62, the pH optimum is about 8.2. 2. The molecular weight of the native enzyme has been determined to be 126000 by sedimentation equilibrium studies. The molecular weight of the polypeptide chains composing the enzyme has been found to be 32000 by dodecylsulfate/polyacrylamide gel electrophoresis and 31000 by sedimentation equilibrium studies in presence of 6 M guanidine hydrochloride. The native enzyme is composed of four polypeptide chains. 3. Reacting enzyme centrifugation studies gave at pH 8.2 a sedimentation coefficient s20, w of 8.04 S and a diffusion coefficient D20, w of 6.56 F, resulting in a molecular weight of 115000 for the catalytically active form. Thus, the enzyme is active as the tetramer. So far the enzyme from P. fluorescens is the sole 6-phospho-D-gluconate dehydrogenase (decarboxylating) composed of four polypeptide chains.

Reactivation of D-beta-hydroxybutyrate dehydrogenase with short-chain lecithins: stoichiometry and kinetic mechanism

D-beta-Hydroxybutyrate dehydrogenase (BDH), purified as soluble, lipid-free apoenzyme (inactive) from either beef heart or rat liver mitochondria, can be reactivated by short-chain lecithins in the monomeric state. The enzyme was reactivated with dihexanoyl- [PC(6:0)], diheptanoyl- [PC(7:0)], and dioctanoyllecithins [PC(8:0)]. The titration curves of enzyme activity as a function of the phospholipid concentration are consistent with a model in which the enzyme contains two identical, noninteracting lecithin binding sites. The simultaneous occupation of these sites (via an equilibrium random mechanism) is required to activate the apoenzyme. Similar results were obtained with both rat liver and beef heart apoenzymes. The maximal velocities obtained with the different lecithins were similar [110-140 mumol of NAD+ reduced min-1 (mg of protein)-1]. The KL values (the apparent dissociation constants of the lecithin-site complexes) were 1.2 X 10(-4) M [PC(8:0)], 1.5 X 10(-3) M [PC(7:0)], and 4.5 X 10(-3) M [PC(6:0)] at 37 degrees C. This was confirmed by using phospholipase A2 to compete with the dehydrogenase for the lecithin monomers. Comparison of the delta G degrees values for complex formation with the different lecithins shows an average contribution of approximately 2.4 kJ/mol (0.9RT) per CH2 group. The interaction of the apolar moiety of lecithin with the protein seems to be essential for effective binding of phosphatidylcholine to apoBDH. The delta G degrees values, when combined with the estimated delta H degrees values, suggest that the binding of lecithin to the apoenzyme is approximately 60% enthalpy and approximately 40% entropy driven.

The effects of anions on fumarate reductase isolated from the cytoplasmic membrane of Escherichia coli

A broad range of anions was shown to stimulate the maximal velocity of purified fumarate reductase isolated from the cytoplasmic membrane of Escherichia coli, while leaving the Km for fumarate unaffected. Reducing agents potentiate the effects of anions on the activity, but have no effect by themselves. Thermal stability, conformation as monitored by circular dichroism and susceptibility to the thiol reagent 5,5'-dithiobis-(2-nitrobenzoic acid) are also altered by anions. The apparent Km for succinate in the reverse reaction (succinate dehydrogenase activity) varies as a function of anion concentration, but the maximal velocity is not affected. The membrane-bound activity is not stimulated by anions and its properties closely resemble those of the purified enzyme in the presence of anions. Thus it appears that anions alter the physical and chemical properties of fumarate reductase, so that it more closely resembles the membrane-bound form.

Physical and enzymatic properties of myosin from porcine brain

Porcine brain myosin is a cytoplasmic protein similar to, but distinct from, its muscle counterpart. It has a high K+-ATPase activity at high ionic strength in EDTA and a low Mg+2-ATPase activity that is activated fivefold by either porcine brain or rabbit skeletal muscle actin. The molecule consists of three classes of subunits, with molecular weights of approximately 195,000 , 19,000, and 16,000. Brain myosin contains less glutamic acid, less lysine, and more threonine, serine, proline, and tyrosine than skeletal muscle myosin. The brain myosin extinction coefficient at 278 nm is 0.810 cm2/mg. Hydrodynamic studies yield an S020,w of 4.95S, a D020,w of 1.07 x 10(-7) cm2/s for brain myosin, and indicate that the molecules aggregate at high ionic strength. The molecular weight of the molecule, as calculated from extrapolation of D020,w/S20,w to zero concentration, is 444,000. The intrinsic viscosity of brain myosin is 0.191 ml/mg. These data are consistent with a highly asymmetric molecular species. Circular dichroism spectroscopy indicates that brain myosin is 58-60% alpha-helical in the presence of Ca+2 ions, and that removal of Ca+2 causes a small change in the spectrum.