Abstract P4-02-02: Increased tumor perfusion following treatment with trastuzumab as measured by contrast-enhanced ultrasound

Introduction: The primary purpose of this study is to determine if the order of dosing in standard-of-care (SOC) combination therapy for HER2+ breast cancer has an effect on the perfusion characteristics within the tumor. Improving the intratumoral delivery of cytotoxic systemic therapy is a significant challenge in advancing cancer treatment. Knowledge of the vascular changes following SOC treatments could enable optimizing their order and timing, potentially leading to significantly improved response. Currently, one SOC regimen for HER2+ breast cancer treatment is doxorubicin administered for 3-4 cycles prior to trastuzumab. Our goal is to quantitatively map the changes in perfusion in response to different combinations of trastuzumab plus doxorubicin treatment through imaging in a murine model of HER2+ breast cancer.

Experimental Design: BT474 breast cancer cells (1×107) were subcutaneously implanted into mice (n = 12) and randomly assigned into three treatment groups: two doses of trastuzumab (10 mg/kg) followed by doxorubicin (1.5 mg/kg), doxorubicin prior to trastuzumab (same total drug dosage as group 1), and saline. After tumors reached ~225 mm3, animals were imaged with contrast-enhanced ultrasound (CEUS) (VisualSonics Vevo 770, Definity microbubbles) before treatment (day 0), and on days 1, 3, 4, and 7. Treatment occurred on days 0, 3 and 4. Percent change (from baseline, day 0 scans) of the CEUS signal intensity quantified from the functional vasculature (surrogate for vessel perfusion) following contrast injection were measured for each animal for each day. Tumors were extracted on day 7, and sectioned, paraffin-embedded, and stained with CD31, alpha-SMA and H&E.

Results: Tumors treated with trastuzumab initially exhibited a significant increase in CEUS signal intensity (from the functional vasculature) on day 1 compared to tumors initially treated with doxorubicin (p < 0.01). Additionally, compared to the control tumors, tumors treated with trastuzumab prior to doxorubicin revealed a significant increase in perfusion (change in signal intensity of functional vasculature) of contrast agent on days 3 (p = 0.01), 4 (p = 0.001) and day 7 (p < 0.01). There were no significant differences in the doxorubicin treated first group and the controls on any of the days (p > 0.25). Qualitative differences were noted between control and treated groups for alpha-SMA, no apparent differences were noted in microvessel density.

Conclusion: Trastuzumab significantly improves a tumor's vascular perfusion in this HER2+ breast cancer model. Doxorubicin dosing prior to treatment with trastuzumab may potentially be hindering the intratumoral delivery of the subsequently delivered, targeted therapy. Improving the tumor's functional vasculature by altering the order of dosing of these combination therapies by giving trastuzumab prior to cytotoxic therapy has potential to enhance both the delivery and the effectiveness of these combination therapies. These data indicate a potential pathway to optimize therapeutic efficacy for individual HER2+ breast cancer patients.

Citation Format: Sorace AG, Barnes SL, Quarles CC, McIntyre JO, Yankeelov TE. Increased tumor perfusion following treatment with trastuzumab as measured by contrast-enhanced ultrasound [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P4-02-02.

FKBP binding characteristics of cardiac microsomes from diverse vertebrates

FK506 binding protein (FKBP) is a cytosolic receptor for the immunosuppressive drug FK-506. The common isoform, FKBP12, was found to be associated with the calcium release channel (ryanodine receptor 1) of different species of vertebrate skeletal muscle, whereas 12.6, a novel FKBP isoform was found to be associated with canine cardiac ryanodine receptor (ryanodine receptor 2). Until recently, canine cardiac sarcoplasmic reticulum was considered to be the prototype for studying heart RyR2 and its interactions with FKBP. In this study, cardiac microsomes were isolated from diverse vertebrates: human, rabbit, rat, mice, dog, chicken, frog, and fish and were analyzed for their ability to bind or exchange with FKBP isoforms 12 and 12.6. Our studies indicate that RyR2 from seven out of the eight animals contain both FKBP12 and 12.6. Dog is the exception. It can now be concluded that the association of FKBP isoforms with RyR2 is widely conserved in the hearts of different species of vertebrates.

(R)-3-hydroxybutyrate dehydrogenase: selective phosphatidylcholine binding by the C-terminal domain

(R)-3-Hydroxybutyrate dehydrogenase (BDH) is a lipid-requiring mitochondrial enzyme that has a specific requirement of phosphatidylcholine (PC) for function. The C-terminal domain (CTBDH) of human heart BDH (residues 195-297) has now been expressed in Escherichia coli as a chimera with a soluble protein, glutathione S-transferase (GST), yielding GST-CTBDH, a novel fusion protein that has been purified and shown to selectively bind to PC vesicles. Both recombinant human heart BDH (HH-Histag-BDH) and GST-CTBDH (but not GST) form well-defined protein-lipid complexes with either PC or phosphatidylethanolamine (PE)/diphosphatidylglycerol (DPG) vesicles (but not with digalactosyl diglyceride vesicles) as demonstrated by flotation in sucrose gradients. The protein-PC complexes are stable to 0.5 M NaCl, but complexes of either HH-Histag-BDH or GST-CTBDH with PE/DPG vesicles are dissociated by salt treatment. Thrombin cleavage of GST-CTBDH, either before or after reconstitution with PC vesicles, yields CTBDH (12 111 Da by MALDI mass spectrometry) which retains lipid binding without attached GST. The BDH activator, 1-palmitoyl-2-(1-pyrenyl)decanoyl-PC (pyrenyl-PC), at <2.5% of total phospholipid in vesicles, efficiently quenches a fraction (0.36 and 0.47, respectively) of the tryptophan fluorescence of both HH-Histag-BDH and GST-CTBDH with effective Stern-Volmer quenching constants, (K(Q))(eff), of 11 and 9.3 (%)(-)(1), respectively (half-maximal quenching at approximately 0.1% pyrenyl-PC). Maximal quenching by pyrenyl-PC obtains at approximately stoichiometric pyrenyl-PC to protein ratios, reflecting high-affinity interaction of pyrenyl-PC with both HH-Histag-BDH and GST-CTBDH. The analogous pyrenyl-PE effects a similar maximal quenching of tryptophan fluorescence for both proteins but with approximately 15-fold lower (K(Q))(eff) (half-maximal quenching at approximately 1.5% pyrenyl-PE) referable to nonspecific interaction of pyrenyl-PE with HH-Histag-BDH or GST-CTBDH. Thus, the 103-residue CTBDH constitutes a PC-selective lipid binding domain of the PC-requiring BDH.

Phosphatidylcholine activation of human heart (R)-3-hydroxybutyrate dehydrogenase mutants lacking active center sulfhydryls: site-directed mutagenesis of a new recombinant fusion protein

(R)-3-Hydroxybutyrate dehydrogenase (BDH) is a lipid-requiring mitochondrial enzyme with a specific requirement of phosphatidylcholine (PC) for function. A plasmid has been constructed to express human heart (HH) BDH in Escherichia coli as a hexahistidine-tagged fusion protein (HH-Histag-BDH). A rapid two-step affinity purification yields active HH-Histag-BDH (and six mutants) with high specific activity ( approximately 130 micromol of NAD(+) reduced.min(-1).mg(-1)). HH-Histag-BDH has no activity in the absence of phospholipid and exhibits a specific requirement of PC for function. The HH-Histag-BDH-PC complex (and HH-BDH derived therefrom by enterokinase cleavage) has apparent Michaelis constants (K(m) values) for NAD(+), NADH, (R)-3-hydroxybutyrate (HOB), and acetoacetate (AcAc) similar to those for bovine heart or rat liver BDH. A computed structural model of HH-BDH predicts the two active center sulfhydryls to be C69 (near the adenosine moiety of NAD) and C242. With both sulfhydryls derivatized, BDH has minimal activity, but site-directed mutagenesis of C69 and/or C242 now shows that neither of these cysteines is required for PC activation or catalysis (the double mutant, C69A/C242A, is highly active with essentially normal kinetic parameters). Six cysteine mutants each have an increased K(m)(NADH) (2-6-fold) but an unchanged K(m)(NAD)+. The C242S and C69A/C242S enzymes (but not the analogous C242A mutants nor the C69A or C69S mutants) exhibit approximately 10-fold increases in K(m)(HOB) and K(m)(AcAc), reflecting an altered substrate binding site. Thus, although C242 (in the C-terminal lipid binding domain of BDH) is close to the active site, it appears to be in a hydrophobic environment and only indirectly defines the substrate binding site at the catalytic center of BDH.

4-Hydroxy-2(E)-nonenal inhibits CNS mitochondrial respiration at multiple sites

A destructive cycle of oxidative stress and mitochondrial dysfunction is proposed in neurodegenerative disease. Lipid peroxidation, one outcome of oxidative challenge, can lead to the formation of 4-hydroxy-2(E)-nonenal (HNE), a lipophilic alkenal that forms stable adducts on mitochondrial proteins. In this study, we characterized the effects of HNE on brain mitochondrial respiration. We used whole rat brain mitochondria and concentrations of HNE comparable to those measured in patients with Alzheimer's disease. Our results showed that HNE inhibited respiration at multiple sites. Complex I-linked and complex II-linked state 3 respirations were inhibited by HNE with IC50 values of approximately 200 microM HNE. Respiration was apparently diminished owing to the inhibition of complex III activity. In addition, complex II activity was reduced slightly. The lipophilicity and adduction characteristics of HNE were responsible for the effects of HNE on respiration. The inhibition of respiration was not prevented by N-acetylcysteine or aminoguanidine. Studies using mitochondria isolated from porcine cerebral cortex also demonstrated an inhibition of complex I- and complex II-linked respiration. Thus, in neurodegenerative disease, oxidative stress may impair mitochondrial respiration through the production of HNE.

Wild type and mutant human heart (R)-3-hydroxybutyrate dehydrogenase expressed in insect cells

(R)-3-Hydroxybutyrate dehydrogenase (BDH) is a lipid-requiring mitochondrial enzyme with a specific requirement of phosphatidylcholine (PC) for function. PC is an allosteric activator that enhances NAD(H) binding to BDH. The enzyme serves as a paradigm to study specific lipid-protein interactions in membranes. Analysis of the primary sequence of BDH, as determined by molecular cloning, predicts that lipid binding and substrate specificity are contributed by the C-terminal third of the protein [Marks, A. R., McIntyre, J. O., Duncan, T. M., Erdjument-Bromage, H., Tempst, P., & Fleischer, S. (1992) J. Biol. Chem. 267, 15459-15463]. The mature form of human heart BDH has now been expressed in catalytically active form in insect cells (Sf9, Spodoptera frugiperda) transfected with BDH-cDNA in baculovirus. Endogenous PC in the insect cells fulfills the lipid requirement for the expressed BDH since enzymatic activity is lost upon digestion with phospholipase A2 and restored selectively by reconstitution with PC vesicles. The K(m)s for NAD+ and (R)-3-hydroxybutyrate (R-HOB) of expressed BDH are similar to those for bovine heart or rat liver BDH in mitochondria. Replacing Cys242 (the only cysteine in the C-terminal domain) with serine by site-directed mutagenesis resulted in a 10-fold increase in K(m) for R-HOB with no change in the K(m) for NAD+, indicating a role for Cys242 in substrate binding. Carboxypeptidase cleavage studies had indicated a requirement of the C-terminal for catalysis and a role in lipid binding [Adami, P., Duncan, T. M., McIntyre, J. O., Carter, C. E., Fu, C., Melin, M., Latruffe, N., & Fleischer, S. (1993) Biochem J. 292, 863-872]. We now show that deletion of twelve C-terminal amino acids to form a truncated BDH mutant results in loss of enzymic function. The expression in Sf 9 cells of the constitutively active full-length mature form of human heart BDH and the first expression and characterization of BDH mutants validate this system for structure-function studies of BDH.

Specific interaction of (R)-3-hydroxybutyrate dehydrogenase with membrane phosphatidylcholine as studied by ESR spectroscopy in oriented phospholipid multibilayers: coenzyme binding enhances the interaction with phosphatidylcholine

The interaction of phospholipid with (R)-3-hydroxybutyrate dehydrogenase, a phosphatidylcholine-requiring membrane enzyme, has been studied using ESR spectroscopy of spin-labeled lipids, both as ordered multibilayers and in lipid vesicle suspensions (liposomes). Partially oriented phospholipid multibilayers were prepared from lipid vesicles composed of a 1:1 mixture of phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Vesicles containing (R)-3-hydroxybutyrate dehydrogenase yielded active preparations of the enzyme in such multibilayers. With increasing protein/lipid ratio, the order of the multibilayers was disrupted as monitored by ESR spectroscopy with a spin-labeled analogue of PC, 5-doxyl-PC (5 mol %, 10% of total PC) as a probe. The outer peak separation of 5-doxyl-PC varied with the lipid/protein ratio. The lower the ratio, the larger was the separation, with higher activity enzyme being more effective in exerting this effect. When 5-doxylstearic acid was substituted for 5-doxyl-PC or when the enzyme was inactive, the 2A(zz) value stayed practically constant at its lower limit (about 54 G). Multilayers composed of 81% PE, 11% diphosphatidylglycerol (DPG), and 8% 5-doxyl-PC (no unlabeled PC present) gave similar results. With this lipid mixture, the maximal 2A(zz) value (about 61 G) was reached at lower protein/lipid ratios, although the enzymic activity of (R)-3-hydroxybutyrate dehydrogenase is reduced to 40% in this system. The outer peak separation also depended on the presence of the coenzyme, NAD+, and 2-methylmalonate. The latter enhances binding of NAD+ about 100-fold by forming a ternary complex. With this ternary complex, the 2A(zz) values were increased unless the maximal values had been reached already in the absence of coenzyme. In all these experiments only a single ESR spectral component was observed. Similar results were obtained for the enzyme in liposomes, although the effect was less pronounced apparently due to the higher mobility of the probe. It is concluded that PC is motionally restricted by (R)-3-hydroxybutyrate dehydrogenase and yet is in rapid exchange with the bulk lipid on the ESR time scale. PC is required for formation of tight and functional complexes with NAD [Rudy et al. (1989) Biochemistry 28, 5354-5366], and such complexes strengthen the interaction of the enzyme with PC.

Effect of selective thiol-group derivatization on enzyme kinetics of (R)-3-hydroxybutyrate dehydrogenase

(R)-3-Hydroxybutyrate dehydrogenase (BDH) is a phosphatidylcholine-requiring tetrameric enzyme with two thiol groups (SH-1 and SH-2) per protomer. By first protecting the more rapidly reacting thiol group (SH-1) with diamide [1,1'-azobis-(NN'-dimethylformamide), DM] to form DM(SH-1)BDH, SH-2 can be selectively derivatized by reaction with maleimide reagents such as 4-maleimido-2,2,6,6-tetramethyl-piperidine-N-oxyl (MSL), which gives DM(SH-1)MSL(SH-2)BDH. Reduction with dithiothreitol (DTT) regenerates SH-1, yielding MAL(SH-2)BDH (where MAL is the diamagnetic reduction product of MSL-BDH and DTT). The enzymic activity of DM(SH-1)BDH is decreased to approx. 4% relative to the native purified enzyme, and the apparent Km for substrate, KmBOH, is increased approx. 100-fold. Reduction of DM(SH-1)BDH with DTT regenerates SH-1 and restores normal enzymic function. Modification of SH-2 with piperidinylmaleimide [MAL(SH-2)BDH] diminishes enzymic activity to approx. 35% of its original value, but has no significant effect on apparent KmBOH. The doubly derivatized enzyme, DM(SH-1)MSL(SH-2)BDH, has lower enzymic activity [about half that for DM(SH-2)BDH] and a yet higher apparent KmBOH than DM(SH-1)BDH. Derivatization of SH-2 with different maleimide reagents results in diminished activity approximately proportional to the size of the maleimide substituent, suggesting that this inhibition is steric. Whereas modification of SH-1 results in marked changes in kinetic parameters (increased apparent Km and reduced apparent Vmax), derivatization of SH-2 has a lesser effect on enzymic function. Thus SH-1 is postulated to be closer to the active centre than is SH-2, although neither is involved in catalysis, since: (1) the activity of the derivatized enzyme is not abolished; and (2) activity can be enhanced by increasing substrate (and cofactor) concentrations.

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.

Target sizes of galactosyltransferase, sialyltransferase, and uridine diphosphatase in Golgi apparatus of rat liver

Target inactivation analysis was used to measure the functional size of uridine diphosphogalactose: N-acetylglucosamine beta(1,4)galactosyltransferase (galactosyltransferase), cytidine monophospho-N-acetyl-neuraminic acid: beta-galactoside alpha(2,6) sialytransferase (sialyltransferase), and uridine diphosphatase (UDPase) in Golgi membranes isolated from rat liver. The size of nucleoside diphosphatase (NDPase), an enzyme similar to UDPase but localized in rat liver endoplasmic reticulum, was also estimated by target inactivation analysis. The related enzymes, UDPase and NDPase, have target sizes of 96 +/- 4 and 77 +/- 3 kDa, while galactosyltransferase and sialyltransferase have target sizes of 97 +/- 10 and 130 +/- 20 kDa, respectively. The target inactivation sizes of galactosyltransferase and of sialyltransferase are about twice the monomer molecular weights of these enzymes obtained from sedimentation studies of the solubilized membranes as well as those predicted from previously reported cDNA sequences. We conclude from our studies that galactosyltransferase and sialyltransferase probably function as dimers in the Golgi membrane.

Molecular cloning and characterization of (R)-3-hydroxybutyrate dehydrogenase from human heart

The complete amino acid sequence of human heart (R)-3-hydroxybutyrate dehydrogenase (EC 1.1.1.30) has been deduced from the nucleotide sequence of cDNA clones. This mitochondrial enzyme has an absolute and specific requirement of phosphatidylcholine for enzymic activity (allosteric activator) and is an important prototype of lipid-requiring enzymes. Despite extensive studies, the primary sequence has not been available and is now reported. The mature form of the enzyme consists of 297 amino acids (predicted M(r) of 33,117), does not appear to contain any transmembrane helices, and is homologous with the family of short-chain alcohol dehydrogenases (SC-ADH) (Persson, B., Krook, M., and Jörnvall, H. (1991) Eur. J. Biochem. 200, 537-543) (30% residue identity with human 17 beta-hydroxysteroid dehydrogenase). The first two-thirds of the enzyme includes both putative coenzyme binding and active site conserved residues and exhibits a predicted secondary structure motif (alternating alpha-helices and beta-sheet) characteristic of SC-ADH. Bovine heart peptide sequences (174 residues in nine sequences determined by microsequencing) have extensive homology (89% identical residues) with the deduced human heart sequence. The C-terminal third (Asn-194 to Arg-297) shows little sequence homology with the SC-ADH and likely contains elements that determine the substrate specificity for the enzyme including the phospholipid (phosphatidylcholine) binding site(s). Northern blot analysis identifies a 1.3-kilobase mRNA encoding the enzyme in heart tissue.

Synthesis of spin-labeled 2-azido-ATP: evidence for distinct nucleotide-binding sites in calcium pump protein from sarcoplasmic reticulum

A spin-labeled and photoreactive derivative of ATP was synthesized with the spin label attached to the 2'- or 3'-position of the ribose moiety and an azido group to C2 of the adenine ring (SL-2N3-ATP). Irradiation of this compound at 350 nm generates a nitrene, which then reacts with nucleophiles in its vicinty. SL-2N3-ATP, in the presence of Ca2+, was hydrolyzed by the calcium pump protein (Ca2+-ATPase) of fast twitch skeletal muscle sarcoplasmic reticulum. The SL-2N3-ATP-enzyme complex in the absence of Ca2+ exhibited strongly immobilized ESR spectra. ESR spectra obtained after covalent incorporation of SL-2N3-ATP into Ca2+-ATPase and removal of freely tumbling SL-2N3-ATP exhibited motionally constrained species indicative of distinct and possibly adjacent ATP-binding sites. By contrast, with SL-ATP devoid of the azido group or with the corresponding 'non-cleavable' beta, gamma-methylene triphosphate analogue (SL-AMP-PCP), two distinct sites were not as well resolved in the ESR spectra due to spectral overlap with the signal from the freely tumbling fraction even with the enhanced spectral resolution provided by perdeuteration of the spin label. Thus, SL-2N3-ATP may have general application for ESR studies of ATP-dependent proteins under conditions in which non-covalent interactions are too weak for motionally restricted species to be resolved.

Coenzyme binding by 3-hydroxybutyrate dehydrogenase, a lipid-requiring enzyme: lecithin acts as an allosteric modulator to enhance the affinity for coenzyme

The role of phospholipid in the binding of coenzyme, NAD(H), to 3-hydroxybutyrate dehydrogenase, a lipid-requiring membrane enzyme, has been studied with the ultrafiltration binding method, which we optimized to quantitate weak ligand binding (KD in the range 10-100 microM). 3-Hydroxybutyrate dehydrogenase has a specific requirement of phosphatidylcholine (PC) for optimal function and is a tetramer quantitated both for the apodehydrogenase, which is devoid of phospholipid, and for the enzyme reconstituted into phospholipid vesicles in either the presence or absence of PC. We find that (i) the stoichiometry for NADH and NAD binding is 0.5 mol/mol of enzyme monomer (2 mol/mol of tetramer); (ii) the dissociation constant for NADH binding is essentially the same for the enzyme reconstituted into the mixture of mitochondrial phospholipids (MPL) (KD = 15 +/- 3 microM) or into dioleoyl-PC (KD = 12 +/- 3 microM); (iii) the binding of NAD+ to the enzyme-MPL complex is more than an order of magnitude weaker than NADH binding (KD approximately 200 microM versus 15 microM) but can be enhanced by formation of a ternary complex with either 2-methylmalonate (apparent KD = 1.1 +/- 0.2 microM) or sulfite to form the NAD-SO3- adduct (KD = 0.5 +/- 0.1 microM); (iv) the binding stoichiometry for NADH is the same (0.5 mol/mol) for binary (NADH alone) and ternary complexes (NADH plus monomethyl malonate); (v) binding of NAD+ and NADH together totals 0.5 mol of NAD(H)/mol of enzyme monomer, i.e., two nucleotide binding sites per enzyme tetramer; and (vi) the binding of nucleotide to the enzyme reconstituted with phospholipid devoid of PC is weak, being detected only for the NAD+ plus 2-methylmalonate ternary complex (apparent KD approximately 50 microM or approximately 50-fold weaker binding than that for the same complex in the presence of PC). The binding of NADH by equilibrium dialysis or of spin-labeled analogues of NAD+ by EPR spectroscopy gave complementary results, indicating that the ultrafiltration studies approximated equilibrium conditions. In addition to specific binding of NAD(H) to 3-hydroxybutyrate dehydrogenase, we find significant binding of NAD(H) to phospholipid vesicles. An important new finding is that the nucleotide binding site is present in 3-hydroxybutyrate dehydrogenase in the absence of activating phospholipid since (a) NAD+, as the ternary complex with 2-methylmalonate, binds to the enzyme reconstituted with phospholipid devoid of PC and (b) the apodehydrogenase, devoid of phospholipid, binds NADH or NAD-SO3- weakly (half-maximal binding at approximately 75 microM NAD-SO3- and somewhat weaker binding for NADH).(ABSTRACT TRUNCATED AT 400 WORDS)

Cooperativity in lipid activation of 3-hydroxybutyrate dehydrogenase: role of lecithin as an essential allosteric activator

3-Hydroxybutyrate dehydrogenase (BDH) is a lecithin-requiring mitochondrial enzyme which catalyzes the interconversion of 3-hydroxybutyrate and acetoacetate with NAD(H) as coenzyme. The purified enzyme devoid of lipid (i.e., the apodehydrogenase or apoBDH) can be reactivated with soluble lecithin or by insertion into phospholipid vesicles containing lecithin. Two different models have been proposed to explain the sigmoidal lipid activation curves. For both models, activation of BDH is assumed to require the binding of two lecithin molecules per functional unit. Activation of soluble enzyme (dimeric form) by short-chain (soluble) lecithin is consistent with a model in which lecithin binding is noncooperative, whereas activation of the membrane-bound enzyme (tetrameric form) indicates cooperativity between the lecithin binding sites. A new comprehensive model is presented in which lecithin is considered to be an essential allosteric activator that shifts the equilibrium between conformational states of the enzyme. Resonance energy transfer data, reflecting NADH binding to membrane-bound and soluble apoBDH, are consistent with such a lecithin-induced conformational change. Apparent dissociation constants for binding of NADH to BDH are approximately 10 microM and approximately 37 microM for BDH activated by bilayer and soluble lecithin, respectively. The maximal fluorescence resonance energy transfer (delta F max) increases with higher mole fraction of lecithin in the bilayer. The largest changes occur between mole fractions 0 and 0.13, thereby correlating with enzymic function. Essentially no binding of NADH is observed in the absence of lecithin.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of 3-hydroxybutyrate dehydrogenase from bovine heart and rat liver mitochondria

3-Hydroxybutyrate dehydrogenase is a lipid-requiring enzyme with an absolute requirement of phosphatidylcholine for enzymatic activity. Purification of the enzyme to homogeneity from bovine heart mitochondria was described more than a decade ago [H. G. Bock and S. Fleischer (1975) J. Biol. Chem. 250, 5774-5781]. We have modified the purification procedure so that it is faster, the yield has been improved, and the specific activity is greater by approximately 50%. The updated procedure has also been applied to isolate the enzyme from rat liver mitochondria. Characteristics of the enzyme from bovine heart and rat liver mitochondria have been compared and found to be similar with respect to: (1) purification characteristics; (2) amino acid composition; (3) pH optimum for enzymatic activity; (4) kinetic characteristics; (5) molecular weight as determined by sedimentation equilibrium in guanidine hydrochloride; (6) peptide maps; (7) immunological cross-reactivity. These studies show that 3-hydroxybutyrate dehydrogenase from bovine heart and rat liver mitochondria, though similar, are not identical.

Target size of the ryanodine receptor from junctional terminal cisternae of sarcoplasmic reticulum

Target inactivation analysis was carried out on the ryanodine receptor. This receptor recently has been implicated as the channel involved in the calcium release process in excitation-contraction coupling and was localized to the junctional terminal cisternae of sarcoplasmic reticulum from skeletal muscle [Fleischer, S., Ogunbunmi, E. M., Dixon, M. C., & Fleer, E.A.M. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 7256-7259]. Irradiation of the junctional terminal cisternae resulted in an exponential decrease in ryanodine binding with radiation dose, thereby consistent with target theory. The target molecular weight was found to be 138,000 +/- 21,000, i.e., smaller than the polypeptide that binds ryanodine. The calcium pump protein in the same membrane preparation served as an internal control to validate the methodology.

Distance estimate of the active center of D-beta-hydroxybutyrate dehydrogenase from the membrane surface

D-beta-Hydroxybutyrate dehydrogenase (EC 1.1.1.30) is a membrane-bound, lipid-requiring enzyme which has a reactive sulfhydryl in the vicinity of the active center. The spin-probe-spin-label technique has been used to estimate the distance of separation of the reactive sulfhydryl of D-beta-hydroxybutyrate dehydrogenase from the bilayer surface. The reactive sulfhydryl of the enzyme was derivatized with the maleimide spin-label reagent 4-maleimido-2,2,6,6-tetramethylpiperidinyl-1-oxy in the presence of the cofactor NAD+. The derivatized enzyme, inserted (inlaid orientation) into phospholipid vesicles, was titrated with spin probes, either Mn2+ or Gd3+, until the spin-label EPR spectrum was reduced in amplitude to its residual (limiting) value. From this limiting amplitude, the dipolar interaction coefficient was obtained, which is related to the reciprocal of the distance to the sixth power. The radial distances of closest approach of the paramagnetic Mn2+ and Gd3+ ions to the spin-label nitroxide on the enzyme were found to be 18 and 16 A, respectively. These calculated distances were in accord with those determined by comparison with a phosphatidylcholine calibration system having 2,2-dimethyloxazolidinyl-1-oxy spin-labels located at selected positions along the sn-2 fatty acyl chain. Since the distal nitroxide moiety of the maleimide spin-label (17 A from the bilayer surface) is 8 A from the sulfhydryl addition site, the two limiting distances of immersion of the reactive sulfhydryl within the bilayer are 9 and 25 A. The shorter distance is considered more compatible with facile access of the coenzyme to the active site of the enzyme.

The secondary structure of calcium pump protein in light sarcoplasmic reticulum and reconstituted in a single lipid component as determined by Raman spectroscopy

Raman spectra have been measured of the following samples: active calcium pump protein in light sarcoplasmic reticulum (SR) membranes, lipids extracted from light SR membranes, active calcium pump protein reconstituted in dielaidoylphosphatidylcholine (DEPC), and pure DEPC. The spectra of native SR lipids and of pure DEPC are different, and yet when these spectra are subtracted from the spectra of the respective protein-lipid complexes, the resulting amide I spectra of the calcium pump protein are the same, indicating that appropriate criteria have been chosen for subtraction of the spectrum of a lipid. This spectrum has been analyzed for secondary structure with the following results. The SR calcium pump protein contains 51 +/- 5% helix, in agreement with a prediction of secondary structure obtained from an analysis of the sequence, and 21 +/- 4% beta-strand. In addition, the presence of protein broadens and lowers the main melting transition of DEPC.

Site-site interaction in the phospholipid activation of D-beta-hydroxybutyrate dehydrogenase

D-beta-Hydroxybutyrate dehydrogenase is a lipid-requiring enzyme with absolute specificity for phosphatidylcholine (PC). The enzyme devoid of lipid, the apodehydrogenase, inserts spontaneously into phospholipid vesicles where it exists as a tetramer. We now find the lipid activation to be limited by the mole fraction of PC in the total phospholipid. These studies suggest that the concentration of the enzyme-PC complex, which is essential for enzymic activity, becomes diffusion limited at lower PC concentration. The lipid activation and the tryptophan fluorescence of purified D-beta-hydroxybutyrate dehydrogenase were studied in the presence of a constant "bilayer background" of approximately 100 nonactivating phospholipid molecules/enzyme monomer. Activation by PC was half-maximal at 20 PC molecules/enzyme monomer. This value was doubled when the amount of "background" phospholipid was doubled. Activation proceeded with positive cooperativity having a Hill coefficient of approximately 2.4. These data indicate interactions between at least three PC-binding sites. The quenching of tryptophan fluorescence by the phospholipid activator, 1-palmitoyl-2-(1-pyrenyl)-decanoyl-PC (2-pyrenyl-PC), gives a saturation curve with half-maximal quenching of 6 quencher molecules/enzyme monomer. This value is equivalent to an apparent phospholipid-protein dissociation constant in the two-dimensional membrane and corresponds to approximately 6 mol % of total phospholipid. In distinct contrast to the phospholipid activation curve, the fluorescence quenching saturation curve was hyperbolic and there was no specificity for PC. The fluorescence quenching by 2-pyrenyl-PC could be diminished by using a several-fold excess of PC or other phospholipids so as to reduce the mole fraction of quencher in the bilayer. It would appear that formation of enzyme-PC complex is a dynamic process consisting of at least two discernible steps: 1) a primary interaction, as measured by tryptophan quenching, which is hyperbolic and not specific for lecithin. This interaction is independent from and precedes 2) phospholipid activation of D-beta-hydroxybutyrate dehydrogenase, which is cooperative in nature and specific for lecithin.

Cyanylation of 3-hydroxybutyrate dehydrogenase. The "essential" sulfhydryl group is not involved in catalysis

3-Hydroxybutyrate dehydrogenase is a lipid-requiring enzyme with an absolute requirement of lecithin for function. The enzyme contains two sulfhydryl groups per monomer. Modification of the more reactive sulfhydryl group with N-ethylmaleimide resulted in inactivation of the enzyme and modification of coenzyme-binding characteristics [McIntyre, J. O., Fleer, E. A. M. and Fleischer, S. (1984) Biochemistry 23, 5135-5141]. The present study further investigates the function of the sulfhydryl groups by utilizing chemical derivatization techniques. The reactive sulfhydryl was derivatized first with 3,3'-dithiobis(6-nitrobenzoic acid) (Ellman's reagent) to form the S-(carboxynitrophenylthio) derivative which could then be replaced with cyanide to form the S-cyanylated enzyme. We find that derivatizing the essential sulfhydryl group leads to some loss of activity. The effect appears to be steric since a larger derivatizing group gives greater loss of activity. The normal enzyme is inhibited approximately 50% in excess substrate. Derivatization of the reactive sulfhydryl group results in loss of this substrate inhibition, the modified enzyme being at least three-fold more active at high substrate concentrations; the activity increases from 18% to 54% and from 1% to 4% of maximal activity for the S-cyanylated and S-(carboxynitrophenylthio) enzyme derivatives, respectively. Cyanylation results in complete loss of fluorescence energy transfer from tryptophan to NADH at low salt concentration but is normal in the presence of 100mM NaCl. However, the binding constant of the coenzyme is decreased only several-fold in the cyanylated enzyme as studied by fluorescence quenching. The cyanylated enzyme formed tight ternary complexes (spin-labeled NADH-monomethylmalonate) (spin-labeled NAD-sulfite) similar to that formed by the normal enzyme. The spin label is highly immobilized, but the hyperfine splitting values differ somewhat from the normal enzyme. We conclude that the reactive sulfhydryl is close to the active site of 3-hydroxybutyrate dehydrogenase but is not involved in the catalytic mechanism.

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides is a reliable internal standard for radiation-inactivation studies of membranes in the frozen state

The target size of four soluble enzymes (beta-galactosidase, pyruvate kinase, alcohol dehydrogenase, and glucose-6-phosphate dehydrogenase) in the presence or absence of subcellular membrane fractions has been determined by the radiation-inactivation method using samples in the frozen state. For each of the four enzymes, full activity was recovered after freezing and thawing in the absence of radiation. We found minimal (less than 20%) binding of the enzymes to either submitochondrial vesicles or sarcoplasmic reticulum vesicles. Under the conditions tested, beta-galactosidase, pyruvate kinase, and alcohol dehydrogenase exhibited target sizes which varied according to the experimental conditions, i.e., the buffer selected and also the presence or absence of membrane preparations. For these tetrameric enzymes, the target sizes were generally comparable to either a monomer or a dimer. By contrast, the target size of glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides was found to be essentially invariant when frozen in a variety of buffers and in the presence or absence of either cryoprotectant (sucrose or glycerol) or different membrane preparations. The target size from 19 separate determinations gave an average value of 104 +/- 16 kDa, which is comparable to the molecular weight of the enzyme (104 kDa). We conclude that glucose-6-phosphate dehydrogenase from L. mesenteroides is a reliable internal standard for radiation-inactivation studies of membrane preparations in the frozen state.

Phospholipid protection against proteolysis of D-beta-hydroxybutyrate dehydrogenase, a lecithin-requiring enzyme

D-beta-Hydroxybutyrate dehydrogenase is a lipid-requiring enzyme which is localized on the inner face of the mitochondrial inner membrane. The apodehydrogenase, i.e. the purified enzyme devoid of lipid, has been purified from beef heart mitochondria and as such is inactive. It can be reactivated by insertion into phospholipid vesicles containing lecithin. Proteolytic digestion with different proteases has been carried out to obtain insight into the orientation of the enzyme in the membrane and to assess the extent of immersion of the protein into the phospholipid bilayer. Digestion of the apodehydrogenase with either trypsin, chymotrypsin, Staphylococcus aureus protease, thermolysin, carboxypeptidases A and Y, or Pronase (from Streptomyces griseus) leads to loss of activity, as assayed with phospholipid. Limited digestion with carboxypeptidase results in complete inactivation. Of the proteases tested, only Pronase and chymotrypsin cleave and inactivate the enzyme inserted into phospholipid vesicles (enzyme-phospholipid complex). For the enzyme-phospholipid complex, the loss of activity with Pronase digestion follows a single exponential decay to less than 10% of the initial activity. With chymotrypsin digestion, the staining intensity of the original approximately 31,500-dalton polypeptide decreases more rapidly than the loss of enzymic activity. The enzyme-phospholipid complex, after limited cleavage with chymotrypsin, retains enzymic activity and resonance energy transfer from protein to bound NADH and an approximately 26,000-dalton polypeptide is observed. Phospholipid alters the cleavage pattern with both chymotrypsin and Pronase, and the rate of inactivation of the enzyme-phospholipid complex is slowed in the presence of NAD(H). Moreover, the rate of inactivation of the apodehydrogenase with chymotrypsin is diminished approximately 3-fold in the presence of NAD+. Digestion of submitochondrial vesicles with either trypsin, chymotrypsin, or Pronase rapidly inactivates D-beta-hydroxybutyrate dehydrogenase; the addition of NAD+ or NADH, together with dithiothreitol and increased salt (to 50 mM), decreases the rate of inactivation, and with trypsin, virtually eliminates inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)

Complex formation between nucleotides and D-beta-hydroxybutyrate dehydrogenase studied by fluorescence and EPR spectroscopy

D-beta-Hydroxybutyrate dehydrogenase (D-3-hydroxybutyrate:NAD+ oxidoreductase, EC 1.1.1.30) is a lipid-requiring enzyme which specifically requires phosphosphatidylcholine for enzymic activity. The phosphatidylcholine modifies the binding and orientation of the coenzyme, NAD(H), with respect to the enzyme. In the present study, two derivatives of NAD, spin-labeled either at N-6 or C-8 of the adenine ring, were found to be active as coenzyme. The binding affinity of NADH to the enzyme was opitimized by increasing the salt concentration and increasing the pH from 6 to 8, with the pK at 6.8. Monomethylmalonate, a substrate analogue, was found to enhance NADH binding (Kd is reduced from 4 to 1 microM). Sulfite strongly enhances the binding of NAD+ via the enzyme-catalyzed formation of an adduct of sulfite with the nucleotide; the Kd for binding of NAD-sulfite is in the micromolar range, whereas NAD+ binding is more than a magnitude weaker. The binding of spin-labeled NAD(H) was further characterized by EPR spectroscopy. Increased sensitivity and resolution were obtained with the use of NAD(H) analogues perdeuterated in the spin-label moiety. For these analogues bound to D-beta-hydroxybutyrate dehydrogenase in phospholipid vesicles, EPR studies showed the spin-label moiety to be constrained and revealed two distinct components. Increasing the viscosity of the medium by addition of glycerol affected the EPR spectral characteristics of only the component with the smaller resolved averaged hyperfine splitting. The stage is now set to study motional characteristics of the enzyme, using these spin-labeled probes which mimic the coenzyme.

The synthesis of 15N- and deuterium-substituted, spin-labeled analogues of NAD+ and their use in EPR studies of dehydrogenases

Two spin-labeled analogues of NAD+ were synthesized with an 15N and perdeuterated nitroxide radical, 4-amino-2,2,6,6-[2H17, 15N]tetramethylpiperidone-1-oxyl, which was attached to either the C-6 or C-8 position of the purine ring. The EPR spectra of these derivatives exhibit an approx. 6-fold increase in sensitivity compared with the corresponding 14N, protonated analogues due to a decrease in both the number of nuclear manifolds (from three to two) and the linewidth. The enhanced spectral resolution obtained with (2H17, 15N)spin-labeled-NAD+ analogues has facilitated simulation of the EPR lineshape of the nucleotide bound to lactate dehydrogenase (L-lactate:NAD+ oxidoreductase, EC 1.1.1.27). The spin-label moiety exhibits highly constrained motion indicative of a single environment. The motion of the spin label does not reflect the overall motion of the enzyme; rather, it is characteristic of some limited mobility relative to the lactate dehydrogenase. By contrast, the spin label on the membrane-bound enzyme, D-beta-hydroxybutyrate dehydrogenase (D-beta-hydroxybutyrate:NAD+ oxidoreductase, EC 1.1.1.30), is completely immobilized and exhibits two distinct spectral components for spin-labeled NAD+, which appear to differ in the polarity of the environment of the nitroxide.

Half-site reactivity of an essential thiol group of D-beta-hydroxybutyrate dehydrogenase

D-beta-Hydroxybutyrate dehydrogenase is a lipid-requiring enzyme, which is a tetramer both in the mitochondrial inner membrane and as the purified enzyme reconstituted with phospholipid. For the active enzyme-phospholipid complex in the absence of ligands, we previously found that reaction with N-ethylmaleimide (at 5 mol/mol of enzyme subunit) resulted in progressive loss of enzymic activity with an inactivation stoichiometry of 1 equiv of sulfhydryl derivatized per mole of enzyme and a maximum derivatization of 2 equiv [Latruffe, N., Brenner, S. C., & Fleischer, S. (1980) Biochemistry 19, 5285-5290]. We now find, in the presence of nucleotide or substrate, that the rate of inactivation is significantly reduced, which indicates that these ligands afford protection of the essential sulfhydryl. Further, in the presence of ligands, the inactivation stoichiometry is 0.5, consistent with half-of-the-site reactivity of the essential sulfhydryl. Thus, at a low ratio of N-ethylmaleimide to enzyme, nucleotide or substrate affords essentially complete protection of the nonessential sulfhydryl from derivatization. The binding characteristics of NADH to both the native and N-ethylmaleimide-derivatized enzyme have been compared by fluorescence spectroscopy. Quenching of intrinsic tryptophan fluorescence of the protein shows that the enzyme, derivatized with N-ethylmaleimide either in the absence or in the presence of NAD+, binds NADH but with a reduced Kd (approximately 50 microM as compared with approximately 20 microM for native enzyme). However, a critical change has occurred in that resonance energy transfer from protein to bound NADH, observed in the native enzyme, is abolished in the N-ethylmaleimide-derivatized enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversible modification of D-beta-hydroxybutyrate dehydrogenase by diamide

D-beta-Hydroxybutyrate dehydrogenase is a lipid-requiring enzyme with a specific requirement of lecithin for function. The purified enzyme devoid of lipid (apodehydrogenase) is inactive but can be reactivated by forming a complex with phospholipid containing lecithin. We find that, of the six half cysteines present in D-beta-hydroxybutyrate dehydrogenase, only two are in the reduced form and available for modification with N-ethylmaleimide, even after denaturation in sodium dodecyl sulfate. Diamide treatment of either the inactive apodehydrogenase or the active enzyme-phospholipid complex resulted in complete loss of enzymic activity, the apodehydrogenase being assayed after addition of phospholipid. The inactivation by diamide can be reversed by the addition of dithiothreitol with full recovery of activity. Derivatization using N-[14C]ethylmaleimide showed that diamide modified only one sulfhydryl per enzyme monomer. The other sulfhydryl appears not to be essential for function since full activity can be restored after this sulfhydryl had been covalently derivatized with N-ethylmaleimide. Protein cross-linking was not observed after diamide modification of D-beta-hydroxybutyrate dehydrogenase, indicating that a disulfide bridge was not formed between enzyme subunits. The diamide-modified enzyme retains the ability to bind coenzyme, NAD(H), as detected by quenching of the intrinsic fluorescence of the protein. However, resonance energy transfer from protein to bound NADH and enhancement of NADH fluorescence were not observed, indicating that diamide modification of the protein alters the nucleotide binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of diabetes on rat liver mitochondria: decreased unsaturation of phospholipid and D-beta-hydroxybutyrate dehydrogenase activity

Liver mitochondria and submitochondrial vesicles have been prepared from rats made diabetic by treatment with streptozotocin (diabetic membranes). The membranes were characterized in terms of phospholipid and fatty acid composition, electron transport functions, and D-beta-hydroxybutyrate dehydrogenase activity and compared with mitochondria and submitochondrial vesicles prepared from control animals (control membranes). No change in the phospholipid composition (44% lecithin, 35% phosphatidylethanolamine, and 21% diphosphatidylglycerol) was found, but a marked alteration in fatty acid composition of both the total phospholipid and lecithin occurred within 3 weeks after streptozotozin treatment and persisted thereafter. In lecithin, the 18:1/18:0 ratio decreases approximately 33% and the 20:4/18:2 ratio decreases approximately 55%. D-beta-hydroxybutyrate dehydrogenase is a lipid-requiring enzyme which has a specific requirement of lecithin for function. In diabetic membranes, there is a progressive decrease in D-beta-hydroxybutyrate dehydrogenase activity with time after streptozotocin treatment to about 40% of control value at 15 weeks. In contrast, succinate oxidase and succinate- or NADH-cytochrome c reductase activities remain essentially unaltered. Further, the Arrhenius plot characteristics differ for D-beta-hydroxybutyrate dehydrogenase in diabetic membranes as compared with control membranes, in that the break point of the biphasic plot increases from 20 +/- 1 degree C in controls to 29 +/- 1 degree C in samples from diabetic animals. The change occurs about 3 weeks after streptozotocin treatment and is correlatable with the increased saturation of the fatty acid moiety of the phospholipids. The observed changes in D-beta-hydroxybutyrate dehydrogenase function and phospholipid composition were prevented by administration of insulin to the diabetic animals and are therefore referable to insulin insufficiency.

Basis for decreased D-beta-hydroxybutyrate dehydrogenase activity in liver mitochondria from diabetic rats

Liver mitochondria from rats made diabetic with streptozotocin have a reduced level of D-beta-hydroxybutyrate dehydrogenase (BDH) activity and decreased ratios of oleic/stearic and arachidonic/linoleic acids in the phospholipids of the mitochondrial membrane. This altered activity and lipid environment result from insulin deprivation since maintenance of the diabetic rats on insulin leads to normal characteristics (J.C. Vidal, J.O. McIntyre, P.F. Churchill, and S. Fleischer (1983) Arch. Biochem, Biophys. 224, 643-658). In the present study, the basis for the reduced enzymatic activity of this lipid-requiring enzyme was analyzed using three approaches: (i) Purified D-beta-hydroxybutyrate, dehydrogenase was inserted into membranes from mitochondria, submitochondrial vesicles, and mitochondrial lipids extracted therefrom. The activation was the same and optimal irrespective of whether the preparations were derived from normal or diabetic rat liver. Therefore, the decreased activity does not appear to be referable to an altered lipid composition. (ii) BDH activity can be released from the mitochondria by phospholipase A2 digestion. The released activity was proportional to the endogenous activity in the submitochondrial vesicles from normal and diabetic membranes. (iii) The BDH activity in submitochondrial vesicles was titrated by inhibition with specific antiserum. Less enzyme was found in mitochondria from diabetic rats as compared with those from normal animals. Hence, the lowered enzymatic activity is due to decreased enzyme in the mitochondrial inner membrane and not to the modified lipid environment.

Target size of D-beta-hydroxybutyrate dehydrogenase. Functional and structural molecular weight based on radiation inactivation

D-beta-Hydroxybutyrate dehydrogenase is a lipid-requiring enzyme which is localized on the inner face of the mitochondrial inner membrane. The apoenzyme has been purified to homogeneity from beef heart; it is devoid of lipid and inactive. It can be functionally reconstituted with lecithin or phospholipid mixtures containing lecithin. The active form of the enzyme is the enzyme-phospholipid complex. Classical target analysis of radiation-inactivation data has now been used to determine the molecular size of the enzyme both in the native membrane (submitochondrial vesicles) and in the reconstituted enzyme inserted into phospholipid vesicles containing lecithin. For both forms of the enzyme, we find the same molecular size, approximately 110,00 daltons. This size is consistent with a tetramer. Radiation results in fragmentation of the polypeptide and the destruction of the polypeptide correlates with loss of enzymic function. A similar size is obtained when purified D-beta-hydroxybutyrate dehydrogenase is inserted into a nonactivating mixture of phospholipid (i.e. in the absence of lecithin). We conclude that: 1) the native enzyme in submitochondrial vesicles and the purified active enzyme in phospholipid vesicles are the same size, approximating a tetramer; 2) radiation of D-beta-hydroxybutyrate dehydrogenase results in loss of activity and fragmentation of the polypeptide; and 3) the role of lecithin in activation of D-beta-hydroxybutyrate dehydrogenase is unrelated to determining oligomeric size of the enzymes since both active and nonactive forms exhibit the same structural size.

Activation of D-beta-hydroxybutyrate apodehydrogenase using molecular species of mixed fatty acyl phospholipids

D-beta-Hydroxybutyrate apodehydrogenase is a lipid-requiring enzyme with a specific requirement of lecithin for enzymatic function. The purified enzyme which is devoid of lipid can be reactivated with lecithin or mixtures of natural phospholipid-containing lecithin. However, it is mitochondrial phospholipid which activates the enzyme optimally and with kinetic parameters similar to that of the native membrane-bound enzyme. Mitochondrial phospholipid consists of three classes of phospholipid (lecithin:phosphatidylethanolamine:diphosphatidylglycerol in a ratio of approximately 2:2:1 by phosphorus); each class consists of a multiplicity of different molecular species due to diversity in the fatty acyl substituents. In this study, we have synthesized defined molecular species of mixed fatty acyl phospholipids to evaluate whether multiplicity of phospholipid molecular species are essential for optimal reactivation. We find that: 1) ternary mixtures of single molecular species of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylpropan-1,3-diol in the liquid crystalline state mimic the optimal reactivation of the enzyme obtained with mitochondrial phospholipids; 2) although some negatively charged phospholipid appears necessary for optimizing the efficiency of activation, diphosphatidylglycerol can be replaced by phosphatidylpropan-1,3-diol, another negatively charged phospholipid; and 3) biphasic Arrhenius plots can be correlated with the liquid crystalline and gel states of the phospholipid.

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 orientation of D-beta-hydroxybutyrate dehydrogenase in the mitochondrial inner membrane

D-beta-Hydroxybutyrate dehydrogenase of beef heart mitochondria is a lipid-requiring enzyme, bound to the inner membrane. The orientation of this enzyme in the membrane has been studied by comparing the characteristics of the enzyme in mitochondria and 'inside-out' submitochondrial vesicles. We observe that the enzymic activity is (1) latent in intact mitochondria; (2) relatively stable to trypsin digestion in mitochondria but rapidly inactivated in submitochondrial vesicles by this treatment; and (3) released more rapidly from submitochondrial vesicles by phospholipase A2 digestion than from mitochondria. Conclusive evidence that D-beta-hydroxybutyrate dehydrogenase is localized on the matrix face of the mitochondrial inner membrane is provided by the correlation that the enzyme is released from submitochondrial vesicles before the membrane becomes leaky to cytochrome c. The arrangement of D-beta-hydroxybutyrate dehydrogenase in the membrane is discussed within a generalized classification of the orientation of proteins in membranes. The evidence indicates that D-beta-hydroxybutyrate dehydrogenase is an amphipathic molecule and as such is inlaid in the membrane, i.e. the enzyme is partially inserted into the hydrophobic milieu of the membrane, with the polar, functional end extending into the aqueous milieu.