Rational design of peptide-based implants for corneal bioengineering.

Regeneration of damaged cornea can save vision for millions of patients. Most of these patients are waiting for transplantation of a donor cornea or suitable substitute to restore vision. Although donor cornea transplantation is the most clinically accepted treatment, shortage of donor cornea results in almost 69 out of every 70 patients untreated with the waiting list for transplantation drastically increasing every year according to a prepandemic estimation. Therefore, corneal replacements are coming up as a cutting-edge alternative strategy. In view of the peptides, especially collagen-like peptides and peptide amphiphiles with bioactive functional motifs demonstrate promising avenue for the corneal tissue engineering and promoting regeneration, by their hierarchical self-assembling propensity to acquire desired nano- to macroscale 3D architecture. Here, we analyze rational peptide designing, self-assembly, and strategies of peptide/peptide-based nanoscale building blocks to create the extracellular matrix mimetic implants for functional regeneration of the cornea.

Research priorities to reduce the impact of COVID-19 in low- and middle-income countries.

Background: The COVID-19 pandemic has caused disruptions to the functioning of societies and their health systems. Prior to the pandemic, health systems in low- and middle-income countries (LMIC) were particularly stretched and vulnerable. The International Society of Global Health (ISoGH) sought to systematically identify priorities for health research that would have the potential to reduce the impact of the COVID-19 pandemic in LMICs. Methods: The Child Health and Nutrition Research Initiative (CHNRI) method was used to identify COVID-19-related research priorities. All ISoGH members were invited to participate. Seventy-nine experts in clinical, translational, and population research contributed 192 research questions for consideration. Fifty-two experts then scored those questions based on five pre-defined criteria that were selected for this exercise: 1) feasibility and answerability; 2) potential for burden reduction; 3) potential for a paradigm shift; 4) potential for translation and implementation; and 5) impact on equity. Results: Among the top 10 research priorities, research questions related to vaccination were prominent: health care system access barriers to equitable uptake of COVID-19 vaccination (ranked 1st), determinants of vaccine hesitancy (4th), development and evaluation of effective interventions to decrease vaccine hesitancy (5th), and vaccination impacts on vulnerable population/s (6th). Health care delivery questions also ranked highly, including: effective strategies to manage COVID-19 globally and in LMICs (2nd) and integrating health care for COVID-19 with other essential health services in LMICs (3rd). Additionally, the assessment of COVID-19 patients' needs in rural areas of LMICs was ranked 7th, and studying the leading socioeconomic determinants and consequences of the COVID-19 pandemic in LMICs using multi-faceted approaches was ranked 8th. The remaining questions in the top 10 were: clarifying paediatric case-fatality rates (CFR) in LMICs and identifying effective strategies for community engagement against COVID-19 in different LMIC contexts. Interpretation: Health policy and systems research to inform COVID-19 vaccine uptake and equitable access to care are urgently needed, especially for rural, vulnerable, and/or marginalised populations. This research should occur in parallel with studies that will identify approaches to minimise vaccine hesitancy and effectively integrate care for COVID-19 with other essential health services in LMICs. ISoGH calls on the funders of health research in LMICs to consider the urgency and priority of this research during the COVID-19 pandemic and support studies that could make a positive difference for the populations of LMICs.

Innate Immune Invisible Ultrasmall Gold Nanoparticles-Framework for Synthesis and Evaluation.

Nanomedicine is seen as a potential central player in the delivery of personalized medicine. Biocompatibility issues of nanoparticles have largely been resolved over the past decade. Despite their tremendous progress, less than 1% of applied nanosystems can hit their intended target location, such as a solid tumor, and this remains an obstacle to their full ability and potential with a high translational value. Therefore, achieving immune-tolerable, blood-compatible, and biofriendly nanoparticles remains an unmet need. The translational success of nanoformulations from bench to bedside involves a thorough assessment of their design, compatibility beyond cytotoxicity such as immune toxicity, blood compatibility, and immune-mediated destruction/rejection/clearance profile. Here, we report a one-pot process-engineered synthesis of ultrasmall gold nanoparticles (uGNPs) suitable for better body and renal clearance delivery of their payloads. We have obtained uGNP sizes of as low as 3 nm and have engineered the synthesis to allow them to be accurately sized (almost nanometer by nanometer). The synthesized uGNPs are biocompatible and can easily be functionalized to carry drugs, peptides, antibodies, and other therapeutic molecules. We have performed in vitro cell viability assays, immunotoxicity assays, inflammatory cytokine analysis, a complement activation study, and blood coagulation studies with the uGNPs to confirm their safety. These can help to set up a long-term safety-benefit framework of experimentation to reveal whether any designed nanoparticles are immune-tolerable and can be used as payload carriers for next-generation vaccines, chemotherapeutic drugs, and theranostic agents with better body clearance ability and deep tissue penetration.

Collagen-based scaffolds with infused anti-VEGF release system as potential cornea substitute for high-risk keratoplasty: A preliminary in vitro evaluation.

Currently the only widely accepted corneal blindness treatment is human donor cornea transplantation. However, increasing shortage of donor corneas as well as high risk of rejection in some corneal diseases remain two major problems, which limit the success of corneal transplantation. Corneal neovascularization is considered as one of the main risk factors of graft failure. Different cell-free biosynthetic scaffolds fabricated from collagens or collagen-like peptides are being tested as donor cornea substitutes (DCS). Here, we report for the first-time composite biosynthetic DCS with integrated sustained release system of anti-VEGF drug, bevacizumab and their preliminary in vitro validation. We have tethered gold nanoparticles with bevacizumab and integrated into a collagen-based cell-free hydrogel scaffold. Developed grafts preserved good optical properties and were confirmed not toxic to human corneal epithelial cells. Bevacizumab has been shown to constantly releasing from the DCS up to 3 weeks and preserved its anti-angiogenic properties. These results provide background for further use of infused composite biosynthetic DCS with integrated nanosystem of bevacizumab sustained release in corneal disease accompanied by neovascularisation where conventional corneal transplantation might fail.

Novel mannopyranoside esters as sterol 14α-demethylase inhibitors: Synthesis, PASS predication, molecular docking, and pharmacokinetic studies.

Direct unimolar one-step valeroylation of methyl α-d-mannopyranoside (MDM) furnished mainly 6-O-valeroate. However, similar reaction catalyzed by DMAP resulted 3,6-di-O-valeroate (21%) and 6-O-valeroate (47%) indicating reactivity sequence as 6-OH>3-OH>2-OH,4-OH. To get potential antimicrobial agents, 6-O-valeroate was converted into four 2,3,4-di-O-acyl esters, and 3,6-di-O-valeroate was converted into 2,4-di-O-acetate. Direct tetra-O-valeroylation of MDM gave a mixture of 2,3,4,6-tetra-O-valeroate and 2,3,6-tri-O-valeroate indicating that the C2-OH is more reactive than the equatorial C4-OH. The activity spectra analysis along with in vitro antimicrobial evaluation clearly indicated that these novel MDM esters had better antifungal activities over antibacterial agents. In this connection, molecular docking indicated that these MDM esters acted as competitive inhibitors of sterol 14α-demethylase (CYP51), an essential enzyme for clinical target to cure several infectious diseases. Furthermore, pharmacokinetic studies revealed that these MDM esters may be worth considering as potent candidates for oral and topical administration. Structure activity relationship (SAR) affirmed that saturated valeric chain (C5) in combination with caprylic (C8) chains was more promising CYP51 inhibitor over conventional antifungal antibiotics.

Proteins inform survival-based differences in patients with glioblastoma.

BACKGROUND: Improving the care of patients with glioblastoma (GB) requires accurate and reliable predictors of patient prognosis. Unfortunately, while protein markers are an effective readout of cellular function, proteomics has been underutilized in GB prognostic marker discovery. METHODS: For this study, GB patients were prospectively recruited and proteomics discovery using liquid chromatography-mass spectrometry analysis (LC-MS/MS) was performed for 27 patients including 13 short-term survivors (STS) (≤10 months) and 14 long-term survivors (LTS) (≥18 months). RESULTS: Proteomics discovery identified 11 941 peptides in 2495 unique proteins, with 469 proteins exhibiting significant dysregulation when comparing STS to LTS. We verified the differential abundance of 67 out of these 469 proteins in a small previously published independent dataset. Proteins involved in axon guidance were upregulated in STS compared to LTS, while those involved in p53 signaling were upregulated in LTS. We also assessed the correlation between LS MS/MS data with RNAseq data from the same discovery patients and found a low correlation between protein abundance and mRNA expression. Finally, using LC-MS/MS on a set of 18 samples from 6 patients, we quantified the intratumoral heterogeneity of more than 2256 proteins in the multisample dataset. CONCLUSIONS: These proteomic datasets and noted protein variations present a beneficial resource for better predicting patient outcome and investigating potential therapeutic targets.

Anti-Dengue ED3 Long-Term Immune Response With T-Cell Memory Generated Using Solubility Controlling Peptide Tags.

Recombinant proteins are an attractive choice as a safe alternative to traditional live attenuated vaccines. However, most small-size proteins are poorly immunogenic, and adjuvants, whose mode of action remain to be fully clarified, are needed for increasing their immunogenicity. Here, we report the effects of short solubility controlling peptide tags (SCP-tags) on the immunogenicity of DENV3 envelope protein domain 3 (3ED3; 103 residues, 11.46 kDa) in ICR and Swiss albino model mice. The attachment of a 4-Ile SCP-tag (C4I-tag) increased the hydrodynamic radius of 3ED3 from 2.2 ± 0.09 to 111 ± 146 nm as assessed by dynamic light scattering in phosphate buffered saline at 37°C, indicating that the C4I-tag oligomerized 3ED3. Immunization at 30 µg/dose showed that the untagged 3ED3 was not or poorly immunogenic, whereas the C4I-tag increased its immunogenicity by up to 39-fold as assessed by the IgG level measured using ELISA. Moreover, the increased antibody level was sustained for over 6 months after immunization and a high number of effector and central memory T cells were generated. These observations provide solid and quantitative evidence for the hypothesis that subvisible aggregates with hydrodynamic radii of 100 nm can increase immunogenicity and that SCP-tag can establish a long-term, target-specific immune response in a way adequate for the development of a peptide/protein-based DENV vaccine.

Assessing the Potential Impact of the Declaration of Istanbul 2008 on Internet Reporting of Human Organ Transplantation-Related Crimes Using Interrupted Time Series Analysis and Meta-Analysis Approaches.

BACKGROUND: A severe shortage in donor organs is the major driver for organ transplantation-related crimes. The Declaration of Istanbul 2008 (DOI) was created to stop such crimes. We investigated the impact of DOI on Internet reporting of transplantation-related crimes. METHODS: We conducted Google Advanced Searches to collect data on "kidney trade," "kidney sale," "organ trafficking," and "transplant tourism" in 15 original participant and 10 nonparticipant countries, 6 years prior through 8 years after the promulgation of DOI. The data were normalized for population and transformed to a logarithmic scale. Interrupted time series analysis (ITSA) was applied to estimate the changes in slopes of the outcome variables before and after DOI, and then the overall intervention impact was calculated by meta-analysis. RESULTS: The combined results indicated that the overall impact of DOI on the reporting of "organ trafficking" and "transplant tourism" was statistically negative (reporting reduced significantly) as intended but on "kidney sale" and "kidney trade" was statistically positive (reporting increased significantly), and the increase was higher in the nonparticipant countries compared to the participant countries. The rate of reporting on "transplant tourism" declined in the participant countries more pronouncedly than in the nonparticipant countries. CONCLUSIONS: DOI has a positive impact on the reporting of "organ trafficking" and "transplant tourism" but not on the reporting of "kidney sale" and "kidney trade." The increased reporting of "kidney sale" and "kidney trade" can be indicative of an impact of DOI on public awareness and increased reporting of the residual transplantation-related crimes.

Hydrophobic surface residues can stabilize a protein through improved water-protein interactions.

Protein stabilization is difficult to rationalize, but the detailed thermodynamic and structural analysis of a series of carefully designed mutants may provide experimental insights into the mechanisms underlying stabilization. Here, we report a systematic structural and thermodynamic analysis of bovine pancreatic trypsin inhibitor (BPTI) variants that are significantly stabilized through a single amino acid substitution at residue 38, which is located in a loop mostly exposed on the protein surface. Differential scanning calorimetry indicated that the BPTI-[5,55]Gly14 variants with a single mutation at position 38 were stabilized in an enthalpy-driven manner and that the magnitude of the stabilization increased as the hydrophobicity of residue 38 increased. This increase in the thermal stability of BPTI was unexpected because a hydrophobic residue on a protein surface is usually destabilizing. To identify the structural determinants of this stabilization, we determined the crystal structures of six BPTI-[5,55]Gly14 variants (Gly14 Gly38 , Gly14 Ala38 , Gly14 Val38 , Gly14 Leu38 , Gly14 Ile38 , and Gly14 Lys38 ) at high resolutions and showed that they retain essentially the same structure as the wild-type BPTI. A more detailed examination of their structures indicated that the extent of thermal stabilization correlated with both improved local packing and increased hydration around the substitution sites. In particular, the number of water molecules near residue 38 increased upon mutation to a hydrophobic residue suggesting that improved hydration contributed to the enthalpy-driven stabilization. Increasing a protein's thermal stability by the placement of a hydrophobic amino acid on the protein surface is a novel and unexpected phenomenon, and its exact nature is worth further examination, as it may provide a generic method for stabilizing proteins in an enthalpy-driven manner. DATABASE: The coordinates and structure factors of Gly14 Gly38 , Gly14 Ile38 , Gly14 Leu38 , and Gly14 Lys38 variants of BPTI-[5,55] are deposited in the Protein Data Bank under the PDB entry codes 5XX3, 5XX5, 5XX2, and 5XX4, respectively. We previously reported the structures of Gly14 Ala38 (2ZJX) and Gly14 Val38 (2ZVX).

A hetero-micro-seeding strategy for readily crystallizing closely related protein variants.

Protein crystallization remains difficult to rationalize and screening for optimal crystallization conditions is a tedious and time consuming procedure. Here, we report a hetero-micro-seeding strategy for producing high resolution crystals of closely related protein variants, where micro crystals from a readily crystallized variant are used as seeds to develop crystals of other variants less amenable to crystallization. We applied this strategy to Bovine Pancreatic Trypsin Inhibitor (BPTI) variants, which would not crystallize using standard crystallization practice. Out of six variants in our analysis, only one called BPTI-[5,55]A14G formed well behaving crystals; and the remaining five (A14GA38G, A14GA38V, A14GA38L, A14GA38I, and A14GA38K) could be crystallized only using micro-seeds from the BPTI-[5,55]A14G crystal. All hetero-seeded crystals diffracted at high resolution with minimum mosaicity, retaining the same space group and cell dimension. Moreover, hetero-micro-seeding did not introduce any biases into the mutant's structure toward the seed structure, as demonstrated by A14GA38I structures solved using micro-seeds from A14GA38G, A14GA38L and A14GA38I. Though hetero-micro-seeding is a simple and almost naïve strategy, this is the first direct demonstration of its workability. We believe that hetero-micro-seeding, which is contrasting with the popular idea that crystallization requires highly purified proteins, could contribute a new tool for rapidly solving protein structures in mutational analysis studies.

The Quest for Anti-inflammatory and Anti-infective Biomaterials in Clinical Translation.

Biomaterials are now being used or evaluated clinically as implants to supplement the severe shortage of available human donor organs. To date, however, such implants have mainly been developed as scaffolds to promote the regeneration of failing organs due to old age or congenital malformations. In the real world, however, infection or immunological issues often compromise patients. For example, bacterial and viral infections can result in uncontrolled immunopathological damage and lead to organ failure. Hence, there is a need for biomaterials and implants that not only promote regeneration but also address issues that are specific to compromised patients, such as infection and inflammation. Different strategies are needed to address the regeneration of organs that have been damaged by infection or inflammation for successful clinical translation. Therefore, the real quest is for multifunctional biomaterials with combined properties that can combat infections, modulate inflammation, and promote regeneration at the same time. These strategies will necessitate the inclusion of methodologies for management of the cellular and signaling components elicited within the local microenvironment. In the development of such biomaterials, strategies range from the inclusion of materials that have intrinsic anti-inflammatory properties, such as the synthetic lipid polymer, 2-methacryloyloxyethyl phosphorylcholine (MPC), to silver nanoparticles that have antibacterial properties, to inclusion of nano- and micro-particles in biomaterials composites that deliver active drugs. In this present review, we present examples of both kinds of materials in each group along with their pros and cons. Thus, as a promising next generation strategy to aid or replace tissue/organ transplantation, an integrated smart programmable platform is needed for regenerative medicine applications to create and/or restore normal function at the cell and tissue levels. Therefore, now it is of utmost importance to develop integrative biomaterials based on multifunctional biopolymers and nanosystem for their practical and successful clinical translation.

Unusual Reversible Oligomerization of Unfolded Dengue Envelope Protein Domain 3 at High Temperatures and Its Abolition by a Point Mutation.

We report differential scanning calorimetry (DSC) experiments between 10 and 120 °C of Dengue 4 envelope protein domain 3 (DEN4 ED3), a small 107-residue monomeric globular protein domain. The thermal unfolding of DEN4 ED3 was fully reversible and exhibited two peculiar endothermic peaks. AUC (analytical ultracentrifugation) experiments at 25 °C indicated that DEN4 ED3 was monomeric. Detailed thermodynamic analysis indicated that the two endothermic peaks separated with an increasing protein concentration, and global fitting of the DSC curves strongly suggested the presence of unfolded tetramers at temperatures around 80-90 °C, which dissociated to unfolded monomers at even higher temperatures. To further characterize this rare thermal unfolding process, we designed and constructed a DEN4 ED3 variant that would unfold according to a two-state model, typical of globular proteins. We thus substituted Val 380, the most buried residue at the dimeric interface in the protein crystal, with less hydrophobic amino acids (Ala, Ser, Thr, Asn, and Lys). All variants showed a single heat absorption peak, typical of small globular proteins. In particular, the DSC thermogram of DEN4 V380K indicated a two-state reversible thermal unfolding independent of protein concentration, indicating that the high-temperature oligomeric state was successfully abolished by a single mutation. These observations confirmed the standard view that small monomeric globular proteins undergo a two-state unfolding. However, the reversible formation of unfolded oligomers at high temperatures is a truly new phenomenon, which was fully inhibited by an accurately designed single mutation.

Cathelicidin LL-37 and HSV-1 Corneal Infection: Peptide Versus Gene Therapy.

PURPOSE: To evaluate the potential utility of collagen-based corneal implants with anti-Herpes Simplex Virus (HSV)-1 activity achieved through sustained release of LL-37, from incorporated nanoparticles, as compared with cell-based delivery from model human corneal epithelial cells (HCECs) transfected to produce endogenous LL-37. METHODS: We tested the ability of collagen-phosphorylcholine implants to tolerate the adverse microenvironment of herpetic murine corneas. Then, we investigated the efficacy of LL-37 peptides delivered through nanoparticles incorporated within the corneal implants to block HSV-1 viral activity. In addition, LL-37 complementary DNA (cDNA) was transferred into HCECs to confer viral resistance, and their response to HSV-1 infection was examined. RESULTS: Our implants remained in herpetic murine corneas 7 days longer than allografts. LL-37 released from the implants blocked HSV-1 infection of HCECs by interfering with viral binding. However, in pre-infected HCECs, LL-37 delayed but could not prevent viral spreading nor clear viruses from the infected cells. HCECs transfected with the LL-37 expressed and secreted the peptide. Secreted LL-37 inhibited viral binding in vitro but was insufficient to protect cells completely from HSV-1 infection. Nevertheless, secreted LL-37 reduced both the incidence of plaque formation and plaque size. CONCLUSION: LL-37 released from composite nanoparticle-hydrogel corneal implants and HCEC-produced peptide, both showed anti-HSV-1 activity by blocking binding. However, while both slowed down virus spread, neither was able on its own to completely inhibit the viruses. TRANSLATIONAL RELEVANCE: LL-37 releasing hydrogels may have potential utility as corneal substitutes for grafting in HSV-1 infected corneas, possibly in combination with LL-37 producing therapeutic cells.

Probing the physical determinants of thermal expansion of folded proteins.

The magnitude and sign of the volume change upon protein unfolding are strongly dependent on temperature. This temperature dependence reflects differences in the thermal expansivity of the folded and unfolded states. The factors that determine protein molar expansivities and the large differences in thermal expansivity for proteins of similar molar volume are not well understood. Model compound studies have suggested that a major contribution is made by differences in the molar volume of water molecules as they transfer from the protein surface to the bulk upon heating. The expansion of internal solvent-excluded voids upon heating is another possible contributing factor. Here, the contribution from hydration density to the molar thermal expansivity of a protein was examined by comparing bovine pancreatic trypsin inhibitor and variants with alanine substitutions at or near the protein-water interface. Variants of two of these proteins with an additional mutation that unfolded them under native conditions were also examined. A modest decrease in thermal expansivity was observed in both the folded and unfolded states for the alanine variants compared with the parent protein, revealing that large changes can be made to the external polarity of a protein without causing large ensuing changes in thermal expansivity. This modest effect is not surprising, given the small molar volume of the alanine residue. Contributions of the expansion of the internal void volume were probed by measuring the thermal expansion for cavity-containing variants of a highly stable form of staphylococcal nuclease. Significantly larger (2-3-fold) molar expansivities were found for these cavity-containing proteins relative to the reference protein. Taken together, these results suggest that a key determinant of the thermal expansivities of folded proteins lies in the expansion of internal solvent-excluded voids.

Regulatory control of human cytosolic branched-chain aminotransferase by oxidation and S-glutathionylation and its interactions with redox sensitive neuronal proteins.

Redox regulation of proteins through oxidation and S-thiolation are important regulatory processes, acting in both a protective and adaptive role in the cell. In the current study, we investigated the sensitivity of the neuronal human cytosolic branched-chain aminotransferase (hBCATc) protein to oxidation and S-thiolation, with particular attention focused on functionality and modulation of its CXXC motif. Thiol specific reagents showed significant redox cycling between the reactive thiols and the TNB anion, and using NEM, four of the six reactive thiols are critical to the functionality of hBCATc. Site-directed mutagenesis studies supported these findings where a reduced kcat (ranging from 50-70% of hBCATc) for C335S, C338S, C335/8S, and C221S, respectively, followed by a modest effect on C242S was observed. However, only the thiols of the CXXC motif (C335 and C338) were directly involved in the reversible redox regulation of hBCATc through oxidation (with a loss of 40-45% BCAT activity on air oxidation alone). Concurrent with these findings, under air oxidation, the X-ray crystallography structure of hBCATc showed a disulphide bond between C335 and C338. Further oxidation of the other four thiols was not evident until levels of hydrogen peroxide were elevated. S-thiolation experiments of hBCATc exposed to GSH provided evidence for significant recycling between GSH and the thiols of hBCATc, which implied that under reducing conditions GSH was operating as a thiol donor with minimal S-glutathionylation. Western blot analysis of WT hBCATc and mutant proteins showed that as the ratio of GSH:GSSG decreased significant S-glutathionylation occurred (with a further loss of 20% BCAT activity), preferentially at the thiols of the CXXC motif, suggesting a shift in function toward a more protective role for GSH. Furthermore, the extent of S-glutathionylation increased in response to oxidative stress induced by hydrogen peroxide potentially through a C335 sulfenic acid intermediate. Deglutathionylation of hBCATc-SSG using the GSH/glutaredoxin system provides evidence that this protein may play an important role in cellular redox regulation. Moreover, redox associations between hBCATc and several neuronal proteins were identified using targeted proteomics. Thus, our data provides strong evidence that the reactive thiol groups, in particular the thiols of the CXXC motif, play an integral role in redox regulation and that hBCATc has redox mediated associations with several neuronal proteins involved in G-protein cell signaling, indicating a novel role for hBCATc in cellular redox control.

Structural determinants for branched-chain aminotransferase isozyme-specific inhibition by the anticonvulsant drug gabapentin.

This study presents the first three-dimensional structures of human cytosolic branched-chain aminotransferase (hBCATc) isozyme complexed with the neuroactive drug gabapentin, the hBCATc Michaelis complex with the substrate analog, 4-methylvalerate, and the mitochondrial isozyme (hBCATm) complexed with gabapentin. The branched-chain aminotransferases (BCAT) reversibly catalyze transamination of the essential branched-chain amino acids (leucine, isoleucine, valine) to alpha-ketoglutarate to form the respective branched-chain alpha-keto acids and glutamate. The cytosolic isozyme is the predominant BCAT found in the nervous system, and only hBCATc is inhibited by gabapentin. Pre-steady state kinetics show that 1.3 mm gabapentin can completely inhibit the binding of leucine to reduced hBCATc, whereas 65.4 mm gabapentin is required to inhibit leucine binding to hBCATm. Structural analysis shows that the bulky gabapentin is enclosed in the active-site cavity by the shift of a flexible loop that enlarges the active-site cavity. The specificity of gabapentin for the cytosolic isozyme is ascribed at least in part to the location of the interdomain loop and the relative orientation between the small and large domain which is different from these relationships in the mitochondrial isozyme. Both isozymes contain a CXXC center and form a disulfide bond under oxidizing conditions. The structure of reduced hBCATc was obtained by soaking the oxidized hBCATc crystals with dithiothreitol. The close similarity in active-site structures between cytosolic enzyme complexes in the oxidized and reduced states is consistent with the small effect of oxidation on pre-steady state kinetics of the hBCATc first half-reaction. However, these kinetic data do not explain the inactivation of hBCATm by oxidation of the CXXC center. The structural data suggest that there is a larger effect of oxidation on the interdomain loop and residues surrounding the CXXC center in hBCATm than in hBCATc.