Supraphysiological levels of GDF11 induce striated muscle atrophy

Growth and differentiation factor (GDF) 11 is a member of the transforming growth factor β superfamily recently identified as a potential therapeutic for age‐related cardiac and skeletal muscle decrements, despite high homology to myostatin (Mstn), a potent negative regulator of muscle mass. Though several reports have refuted these data, the in vivo effects of GDF11 on skeletal muscle mass have not been addressed. Using in vitro myoblast culture assays, we first demonstrate that GDF11 and Mstn have similar activities/potencies on activating p‐SMAD2/3 and induce comparable levels of differentiated myotube atrophy. We further demonstrate that adeno‐associated virus‐mediated systemic overexpression of GDF11 in C57BL/6 mice results in substantial atrophy of skeletal and cardiac muscle, inducing a cachexic phenotype not seen in mice expressing similar levels of Mstn. Greater cardiac expression of Tgfbr1 may explain this GDF11‐specific cardiac phenotype. These data indicate that bioactive GDF11 at supraphysiological levels cause wasting of both skeletal and cardiac muscle. Rather than a therapeutic agent, GDF11 should be viewed as a potential deleterious biomarker in muscle wasting diseases.

In vitro and in vivo activity of cabozantinib (XL184), an inhibitor of RET, MET, and VEGFR2, in a model of medullary thyroid cancer

BACKGROUND

A limited number of approved therapeutic options are available to metastatic medullary thyroid cancer (MTC) patients, and the response to conventional chemotherapy and/or radiotherapy strategies is inadequate. Sporadic and inherited mutations in the tyrosine kinase RET result in oncogenic activation that is associated with the pathogenesis of MTC. Cabozantinib is a potent inhibitor of MET, RET, and vascular endothelial factor receptor 2 (VEGFR2), as well as other tyrosine kinases that have been implicated in tumor development and progression. The object of this study was to determine the in vitro biochemical and cellular inhibitory profile of cabozantinib against RET, and in vivo antitumor efficacy using a xenograft model of MTC.

METHODS

Cabozantinib was evaluated in biochemical and cell-based assays that determined the potency of the compound against wild type and activating mutant forms of RET. Additionally, the pharmacodynamic modulation of RET and MET and in vivo antitumor activity of cabozantinib was examined in a MTC tumor model following subchronic oral administration.

RESULTS

In biochemical assays, cabozantinib inhibited multiple forms of oncogenic RET kinase activity, including M918T and Y791F mutants. Additionally, it inhibited proliferation of TT tumor cells that harbor a C634W activating mutation of RET that is most often associated with MEN2A and familial MTC. In these same cells grown as xenograft tumors in nude mice, oral administration of cabozantinib resulted in dose-dependent tumor growth inhibition that correlated with a reduction in circulating plasma calcitonin levels. Moreover, immunohistochemical analyses of tumors revealed that cabozantinib reduced levels of phosphorylated MET and RET, and decreased tumor cellularity, proliferation, and vascularization.

CONCLUSIONS

Cabozantinib is a potent inhibitor of RET and prevalent mutationally activated forms of RET known to be associated with MTC, and effectively inhibits the growth of a MTC tumor cell model in vitro and in vivo.

The design, synthesis, and biological evaluation of potent receptor tyrosine kinase inhibitors

Variously substituted indolin-2-ones were synthesized and evaluated for activity against KDR, Flt-1, FGFR-1 and PDGFR. Extension at the 5-position of the oxindole ring with ethyl piperidine (compound 7i) proved to be the most beneficial for attaining both biochemical and cellular potencies. Further optimization of 7i to balance biochemical and cellular potencies with favorable ADME/ PK properties led to the identification of 8h, a compound with a clean CYP profile, acceptable pharmacokinetic and toxicity profiles, and robust efficacy in multiple xenograft tumor models.

Novel Carboxamide-Based Allosteric MEK Inhibitors: Discovery and Optimization Efforts toward XL518 (GDC-0973)

The ERK/MAP kinase cascade is a key mechanism subject to dysregulation in cancer and is constitutively activated or highly upregulated in many tumor types. Mutations associated with upstream pathway components RAS and Raf occur frequently and contribute to the oncogenic phenotype through activation of MEK and then ERK. Inhibitors of MEK have been shown to effectively block upregulated ERK/MAPK signaling in a range of cancer cell lines and have further demonstrated early evidence of efficacy in the clinic for the treatment of cancer. Guided by structural insight, a strategy aimed at the identification of an optimal diphenylamine-based MEK inhibitor with an improved metabolism and safety profile versus PD-0325901 led to the discovery of development candidate 1-({3,4-difluoro-2-[(2-fluoro-4-iodophenyl)amino]phenyl}carbonyl)-3-[(2S)-piperidin-2-yl]azetidin-3-ol (XL518, GDC-0973) (1). XL518 exhibits robust in vitro and in vivo potency and efficacy in preclinical models with sustained duration of action and is currently in early stage clinical trials.

Intermittent administration of MEK inhibitor GDC-0973 plus PI3K inhibitor GDC-0941 triggers robust apoptosis and tumor growth inhibition

Combinations of MAP/ERK kinase (MEK) and phosphoinositide 3-kinase (PI3K) inhibitors have shown promise in preclinical cancer models, leading to the initiation of clinical trials cotargeting these two key cancer signaling pathways. GDC-0973, a novel selective MEK inhibitor, and GDC-0941, a class I PI3K inhibitor, are in early stage clinical trials as both single agents and in combination. The discovery of these selective inhibitors has allowed investigation into the precise effects of combining inhibitors of two major signaling branches downstream of RAS. Here, we investigated multiple biomarkers in the mitogen-activated protein kinase (MAPK) and PI3K pathway to search for points of convergence that explain the increased apoptosis seen in combination. Using washout studies in vitro and alternate dosing schedules in mice, we showed that intermittent inhibition of the PI3K and MAPK pathway is sufficient for efficacy in BRAF and KRAS mutant cancer cells. The combination of GDC-0973 with the PI3K inhibitor GDC-0941 resulted in combination efficacy in vitro and in vivo via induction of biomarkers associated with apoptosis, including Bcl-2 family proapoptotic regulators. Therefore, these data suggest that continuous exposure of MEK and PI3K inhibitors in combination is not required for efficacy in preclinical cancer models and that sustained effects on downstream apoptosis biomarkers can be observed in response to intermittent dosing.

Cabozantinib (XL184), a novel MET and VEGFR2 inhibitor, simultaneously suppresses metastasis, angiogenesis, and tumor growth

The signaling pathway of the receptor tyrosine kinase MET and its ligand hepatocyte growth factor (HGF) is important for cell growth, survival, and motility and is functionally linked to the signaling pathway of VEGF, which is widely recognized as a key effector in angiogenesis and cancer progression. Dysregulation of the MET/VEGF axis is found in a number of human malignancies and has been associated with tumorigenesis. Cabozantinib (XL184) is a small-molecule kinase inhibitor with potent activity toward MET and VEGF receptor 2 (VEGFR2), as well as a number of other receptor tyrosine kinases that have also been implicated in tumor pathobiology, including RET, KIT, AXL, and FLT3. Treatment with cabozantinib inhibited MET and VEGFR2 phosphorylation in vitro and in tumor models in vivo and led to significant reductions in cell invasion in vitro. In mouse models, cabozantinib dramatically altered tumor pathology, resulting in decreased tumor and endothelial cell proliferation coupled with increased apoptosis and dose-dependent inhibition of tumor growth in breast, lung, and glioma tumor models. Importantly, treatment with cabozantinib did not increase lung tumor burden in an experimental model of metastasis, which has been observed with inhibitors of VEGF signaling that do not target MET. Collectively, these data suggest that cabozantinib is a promising agent for inhibiting tumor angiogenesis and metastasis in cancers with dysregulated MET and VEGFR signaling.

Inhibition of tumor cell growth, invasion, and metastasis by EXEL-2880 (XL880, GSK1363089), a novel inhibitor of HGF and VEGF receptor tyrosine kinases

The Met receptor tyrosine kinase and its ligand, hepatocyte growth factor (HGF), are overexpressed and/or activated in a wide variety of human malignancies. Vascular endothelial growth factor (VEGF) receptors are expressed on the surface of vascular endothelial cells and cooperate with Met to induce tumor invasion and vascularization. EXEL-2880 (XL880, GSK1363089) is a small-molecule kinase inhibitor that targets members of the HGF and VEGF receptor tyrosine kinase families, with additional inhibitory activity toward KIT, Flt-3, platelet-derived growth factor receptor beta, and Tie-2. Binding of EXEL-2880 to Met and VEGF receptor 2 (KDR) is characterized by a very slow off-rate, consistent with X-ray crystallographic data showing that the inhibitor is deeply bound in the Met kinase active site cleft. EXEL-2880 inhibits cellular HGF-induced Met phosphorylation and VEGF-induced extracellular signal-regulated kinase phosphorylation and prevents both HGF-induced responses of tumor cells and HGF/VEGF-induced responses of endothelial cells. In addition, EXEL-2880 prevents anchorage-independent proliferation of tumor cells under both normoxic and hypoxic conditions. In vivo, these effects produce significant dose-dependent inhibition of tumor burden in an experimental model of lung metastasis. Collectively, these data indicate that EXEL-2880 may prevent tumor growth through a direct effect on tumor cell proliferation and by inhibition of invasion and angiogenesis mediated by HGF and VEGF receptors.

Properties of p-cresol methylhydroxylase flavoprotein overproduced by Escherichia coli

The alpha(2)beta(2) flavocytochrome p-cresol methylhydroxylase (PCMH) from Pseudomonas putida is composed of a flavoprotein homodimer (alpha(2) or PchF(2); M(r) = 119 kDa) with a cytochrome monomer (beta, PchC; M(r) = 9.3 kDa) bound to each PchF subunit. Escherichia coli BL21(DE3) has been transformed with a vector for expression of the pchF gene, and PchF is overproduced by this strain as the homodimer. During purification, it was recognized that some PchF had FAD bound, while the remainder was FAD-free. However, unlike PchF obtained from PCMH purified from P. putida, FAD was bound noncovalently. The FAD was conveniently removed from purified E. coli-expressed PchF by hydroxyapatite chromatography. Fluorescence quenching titration indicated that the affinity of apo-PchF for FAD was sufficiently high to prevent the determination of the dissociation constant. It was found that p-cresol was virtually incapable of reducing PchF with noncovalently bound FAD (PchF(NC)), whereas 4-hydroxybenzyl alcohol, the intermediate product of p-cresol oxidation by PCMH, reduced PchF(NC) fairly quickly. In contrast, p-cresol rapidly reduced PchF with covalently bound FAD (PchF(C)), but, unlike intact PCMH, which consumed 4 electron equiv/mol when titrated with p-cresol (2 electrons from p-cresol and 2 from 4-hydroxybenzyl alcohol), PchF(C) accepted only 2 electron equiv/mol. This is explained by extremely slow release of 4-hydroxybenzyl alcohol from reduced PchF(C). 4-Hydroxybenzyl alcohol rapidly reduced PchF(C), producing 4-hydroxybenzaldehyde. It was demonstrated that p-cresol has a charge-transfer interaction with FAD when bound to oxidized PchF(NC), whereas 4-bromophenol (a substrate analogue) and 4-hydroxybenzaldehyde have charge-transfer interactions with FAD when bound to either PchF(C) or PchF(NC). This is the first example of a "wild-type" flavoprotein, which normally has covalently bound flavin, to bind flavin noncovalently in a stable, redox-active manner.

Alternative routes for entry of HgX2 into the active site of mercuric ion reductase depend on the nature of the X ligands

Wild-type mercuric ion reductase (CCCC enzyme) possesses four cysteines in each of its Hg(II) binding sites, a redox-active pair and a C-terminal pair. Mutation of the C-terminal cysteines to alanines (CCAA enzyme) leads to a loss of steady-state mercuric ion reductase activity using Hg(SR)2 substrates. However, CCCC and CCAA enzymes exhibit an equally high rate of binding and turnover using HgBr2 as substrate under pre-steady-state conditions [Engst and Miller (1998) Biochemistry 37, 11496-11507.]. Since the ligands in these HgX2 substrates differ both in size and in affinity for Hg(II), one or both of these properties may contribute to their different reactivities with CCAA enzyme. To further explore the importance of these two properties, we have examined the pre-steady-state reactions of CCCC and CCAA with Hg(CN)2, which has small, high-affinity ligands, and with Hg(Cys)2, which has bulky, high-affinity ligands. The results indicate that HgX2 substrates with small ligands can rapidly access the redox-active cysteines in the absence of the C-terminal cysteines, but those with large ligands require the C-terminal cysteines for rapid access. In addition, it is concluded that the C-terminal cysteines play a critical role in removing the high-affinity ligands before Hg(II) reaches the redox-active cysteines in the inner active site, since direct access of HgX2 substrates with high-affinity ligands leads to formation of an inhibited complex. Consistent with the results, both a narrow channel leading directly to the redox-active cysteines and a wider channel leading to the redox-active cysteines via initial contact with the C-terminal cysteines can be identified in the structure of the enzyme from Bacillus sp. RC607.

Mechanism of activation of acyl-CoA substrates by medium chain acyl-CoA dehydrogenase: interaction of the thioester carbonyl with the flavin adenine dinucleotide ribityl side chain

The flavin adenine dinucleotide (FAD) cofactor of pig kidney medium-chain specific acyl-coenzyme A (CoA) dehydrogenase (MCADH) has been replaced by ribityl-3'-deoxy-FAD and ribityl-2'-deoxy-FAD. 3'-Deoxy-FAD-MCADH has properties very similar to those of native MCADH, indicating that the FAD-ribityl side-chain 3'-OH group does not play any particular role in cofactor binding or catalysis. 2'-Deoxy-FAD-MCADH was characterized using the natural substrate C8CoA as well as various substrate and transition-state analogues. Substrate dehydrogenation in 2'-deoxy-FAD-MCADH is approximately 1.5 x 10(7)-fold slower than that of native MCADH, indicating that disruption of the hydrogen bond between 2'-OH and substrate thioester carbonyl leads to a substantial transition-state destabilization equivalent to approximately 38 kJ mol-1. The alphaC-H microscopic pKa of the substrate analogue 3S-C8CoA, which undergoes alpha-deprotonation on binding to MCADH, is lowered from approximately 16 in the free state to approximately 11 (+/-0.5) when bound to 2'-deoxy-FAD-MCADH. This compares with a decrease of the same pKa to approximately 5 in the complex with unmodified hwtMCADH, which corresponds to a pK shift of approximately 11 pK units, i.e., approximately 65 kJ mol-1 [Vock, P., Engst, S., Eder, M., and Ghisla, S. (1998) Biochemistry 37, 1848-1860]. The difference of this effect of approximately 6 pK units ( approximately 35 kJ mol-1) between MCADH and 2'-deoxy-FAD-MCADH is taken as the level of stabilization of the substrate carbanionic species caused by the interaction with the FAD-2'-OH. This energetic parameter derived from the kinetic experiments (stabilization of transition state) is in agreement with those obtained from static experiments (lowering of alphaC-H microscopic pKa of analogue, i.e., stabilization of anionic transition-state analogue). The contributions of the two single H-bonds involved in substrate activation (Glu376amide-N-H and ribityl-2'-OH) thus appear to behave additively toward the total effect. The crystal structures of native pMCADH and of 2'-deoxy-FAD-MCADH complexed with octanoyl-CoA/octenoyl-CoA show unambiguously that the FAD cofactor and the substrate/product bind in an identical fashion, implying that the observed effects are mainly due to (the absence of) the FAD-ribityl-2'-OH hydrogen bond. The large energy associated with the 2'-OH hydrogen bond interaction is interpreted as resulting from the changes in charge and the increased hydrophobicity induced by binding of lipophilic substrate. This is the first example demonstrating the direct involvement of a flavin cofactor side chain in catalysis.

Rapid reduction of Hg(II) by mercuric ion reductase does not require the conserved C-terminal cysteine pair using HgBr2 as the substrate

Conditions are described under which the nonphysiological substrate mercuric bromide (HgBr2) is rapidly turned over, both by the wild type (CCCC) and by an active site double mutant (CCAA) of mercuric reductase in which the C-terminal cysteines 557' and 558' are replaced by alanine and only the redox-active pair Cys135 and Cys140 are available for catalysis. A maximum rate of turnover kcatapp of approximately 18 s-1 (at 3 degreesC) for both enzymes is observed, and at high [HgBr2]/[enzyme] ratios, inhibition is found. The UV-vis spectral changes during turnover are closely similar in both enzymes, indicating that catalysis follows the same enzymatic mechanism. Single-turnover analysis of the mutant enzyme shows that after binding of HgBr2, two further rapid events ensue, followed by reduction of the metal ion (kobs approximately 23.5 s-1). It is shown that under multiple-turnover conditions, completion of the catalytic cycle must occur via an ordered mechanism where rapid binding of a new molecule of HgBr2 to EH2.NADP+ precedes exchange of the pyridine nucleotide. Binding of HgBr2 to the active site triple mutant C135A/C557A/C558A (ACAA) is ca. 100-fold slower compared to that of the CCAA mutant and results in no detectable turnover. It is concluded that in the reducible enzyme.Hg(II) complex, the metal ion is coordinated to Cys135 and Cys140 and that for efficient catalysis both residues are required. Furthermore, the data imply that binding to EH2.NADPH occurs via initial rate-limiting attack of Cys135, followed by reaction with Cys140.

Substrate activation by acyl-CoA dehydrogenases: transition-state stabilization and pKs of involved functional groups

The mechanism by which acyl-CoA dehydrogenases initiate catalysis was studied by using p-substituted phenylacetyl-CoAs (substituents-NO2, -CN, and CH3CO-), 3S-C8-, and 3'-dephospho-3S-C8CoA. These analogues lack a beta C-H and cannot undergo alpha,beta-dehydrogenation. Instead they deprotonate at alpha C-H at pH > or = 14 to form delocalized carbanions having strong absorbancies in the near UV-visible spectrum. The pKas of the corresponding phenylacetone analogues were determined as approximately 13.6 (-NO2), approximately 14.5 (-CN), and approximately 14.6 (CH3CO-). Upon binding to human wild-type medium-chain acyl-CoA dehydrogenase (MCADH), all analogues undergo alpha C-H deprotonation. While the extent of deprotonation varies, the anionic products from charge-transfer complexes with the oxidized flavin. From the pH dependence of the dissociation constants (Kd) of p-NO2-phenylacetyl-CoA (4NPA-CoA), 3S-C8-CoA, and 3'-dephospho-3S-C8CoA, four pKas at approximately 5, approximately 6, approximately 7.3, and approximately 8 were identified. They were assigned to the following ionizations: (a) pKa approximately 5, ligand (L-H) in the MCADH approximately ligand complex; (b) pKa approximately 6, Glu376-COOH in uncomplexed MCADH; (c) pKa approximately 7.3, Glu99-COOH in uncomplexed MCADH (Glu99 is a residue that flanks the bottom of the active-center cavity; this pK is absent in the mutant Glu99Gly-MCADH); and (d) pK approximately 8, Glu99-COOH in the MCADH approximately 4NPA-CoA complex. The pKa approximately 6 (b) is not significantly affected in the MCADH approximately 4NPA-CoA complex, but it is increased by > or = 1 pK unit in that with 3S-C8CoA and further in the presence of C8-CoA, the best substrate. The alpha C-H pKas of 4NPA-CoA, of 3S-C8-CoA, and of 3'-dephospho-3S-C8CoA in the complex with MCADH are approximately 5, approximately 5, and approximately 6. Compared to those of the free species these pKa values are therefore lowered by 8 to > or = 11 pH units (50 to > or = 65 kJ mol-1) and are close to the pKa of Glu376-COOH in the complex with substrate/ligand. This effect is ascribed mainly to the hydrogen-bond interactions of the thioester carbonyl group with the ribityl-2'-OH of FAD and Glu376-NH. It is concluded that the pKa shifts induced with normal substrates such as n-octanoyl-CoA are still higher and of the order of 9-13 pK units. With 4NPA-CoA and MCADH, alpha C-H abstraction is fast (kapp approximately 55 s-1 at pH 7.5 and 25 degrees C, deuterium isotope effect approximately 1.34). However, it does not proceed to completion since it constitutes an approach to equilibrium with a finite rate for reprotonation in the pH range 6-9.5. The extent of deprotonation and the respective rates are pH-dependent and reflect apparent pKas of approximately 5 and approximately 7.3, which correspond to those determined in static experiments.

Biochemical characterization of purified, human recombinant Lys304-->Glu medium-chain acyl-CoA dehydrogenase containing the common disease-causing mutation and comparison with the normal enzyme

Recombinant, normal human medium-chain acyl-CoA dehydrogenase (MCADH) and the common, human disease-causing K304E mutant ([Glu304]MCADH) protein were expressed in Escherichia coli using an optimized system, and the enzymes were purified to apparent homogeneity. The crucial factor leading to the production of active [Glu304]MCADH protein is the expression in E. coli cells at reduced temperature (28 degrees C). Expression in the same system at 37 degrees C results in very low amounts of active mutant protein. Several catalytic and physicochemical parameters of these two proteins have been determined and were compared to those of purified pig kidney MCADH. Although [Glu304]MCADH has approximately the same rate of substrate reduction with dodecanoyl-CoA and the same V(max) as human MCADH with the best substrate for the latter, octanoyl-CoA, the K(m) in the mutant MCADH is fourfold higher, which generates a correspondingly lower catalytic efficiency. Importantly, V(max) obtained using the natural acceptor, electron transfer flavoprotein, is only a third that for human MCADH. The V(max)/K(m) versus chain-length profile of the mutant shows a maximum with dodecanoyl-CoA which differs markedly from that of human MCADH, which has maximal efficiency with octanoyl-CoA. The substrate specificity of the mutant is broader with a less pronounced activity peak resembling long-chain acyl-CoA dehydrogenase. The purified mutant enzyme exhibits a reduced thermal stability compared to human wild-type MCADH. The major difference between the two proteins expressed in E. coli is the more pronounced lability of the K304E mutant in crude extracts, which suggests a higher susceptibility to attack by endogenous proteases. Differences between tetrameric [Glu304]MCADH which survives the first step(s) of purification and corresponding MCADH are minor. The overall differences in properties of [Glu304]MCADH together with its impaired folding and tetramer assembly may contribute to the generation of the abnormalities observed in patients homozygous for the K304E mutation.

Molecular characterization of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency: identification of a lys329 to glu mutation in the MCAD gene, and expression of inactive mutant enzyme protein in E. coli

A series of experiments has established the molecular defect in the medium-chain acyl-coenzyme A (CoA) dehydrogenase (MCAD) gene in a family with MCAD deficiency. Demonstration of intra-mitochondrial mature MCAD indistinguishable in size (42.5-kDa) from control MCAD, and of mRNA with the correct size of 2.4 kb, indicated a point-mutation in the coding region of the MCAD gene to be disease-causing. Consequently, cloning and DNA sequencing of polymerase chain reaction (PCR) amplified complementary DNA (cDNA) from messenger RNA of fibroblasts from the patient and family members were performed. All clones sequenced from the patient exhibited a single base substitution from adenine (A) to guanine (G) at position 985 in the MCAD cDNA as the only consistent base-variation compared with control cDNA. In contrast, the parents contained cDNA with the normal and the mutated sequence, revealing their obligate carrier status. Allelic homozygosity in the patient and heterozygosity for the mutation in the parents were established by a modified PCR reaction, introducing a cleavage site for the restriction endonuclease NcoI into amplified genomic DNA containing G985. The same assay consistently revealed A985 in genomic DNA from 26 control individuals. The A to G mutation was introduced into an E. coli expression vector producing mutant MCAD, which was demonstrated to be inactive, probably because of the inability to form active tetrameric MCAD. All the experiments are consistent with the contention that the G985 mutation, resulting in a lysine to glutamate shift at position 329 in the MCAD polypeptide chain, is the genetic cause of MCAD deficiency in this family. We found the same mutation in homozygous form in 11 out of 12 other patients with verified MCAD deficiency.

Characterization of wild-type and an active site mutant of human medium chain acyl-CoA dehydrogenase after expression in Escherichia coli

The cDNA of human medium chain acyl-CoA dehydrogenase (MCADH) was modified by in vitro mutagenesis, and the sequence encoding the mature form of MCADH was introduced into an inducible expression plasmid. We observed synthesis of the protein in Escherichia coli cells transformed with this plasmid with measurable MCADH enzyme activity in cell extracts. Glutamic acid 376, which has been proposed by Powell and Thorpe (Powell, P. J., and Thorpe, J. (1988) Biochemistry 27, 8022-8028) as an essential residue and the proton-abstracting base at the active site of the enzyme, was mutated to glutamine. After expression in bacteria of this plasmid, the corresponding extracts show no detectable MCADH activity, although mutant MCADH-protein production was detected by protein immunoblots. The mature enzyme and the Gln376 mutant were purified to apparent homogeneity. The wild-type enzyme is a yellow protein due to the content of stoichiometric FAD and had a specific activity which is 50% of MCADH purified from pig kidney. The Gln376 mutant is devoid of activity (less than 0.02% that of wild type, expressed enzyme) and is green because of bound CoA persulfide. Properties of the mutant enzyme suggest that the Glu376----Gln change specifically affects substrate binding. These results prove that Glu376 plays an important role in the initial step of dehydrogenation catalysis.