Frequency of endoscopic surveillance for Barrett's esophagus is influenced by health insurance status: results from a population-based analysis.

Factors that influence the frequency of surveillance endoscopy for nondysplastic Barrett's esophagus are not well understood. The objective of this study is to assess factors which influence the frequency of endoscopic surveillance for Barrett's esophagus, including health insurance/third-party payer status. Cases of nondysplastic Barrett's esophagus undergoing esophagogastroduodenoscopy with biopsy were identified using longitudinal data from the Healthcare Utilization Project database in 2005-2006 and followed through 2011. The threshold for appropriate surveillance utilization was defined as two to four surveillance esophagogastroduodenoscopies over a standardized 5-year period. Patients' insurance status was designated as either Medicare, Medicaid, private, or noninsured. 36,676 cases of nondysplastic Barrett's esophagus were identified. Among these, 4,632 patients (12.6%) underwent between two and four surveillance esophagogastroduodenoscopies in 5 years of follow-up versus 31,975 patients (87.3%) who underwent fewer than two esophagogastroduodenoscopies during follow-up. Multivariate analysis found that Barrett's patients insured through Medicaid (OR 1.273; 95% CI = 1.065-1.522) or without insurance (OR = 2.453; 95% CI = 1.67-3.603) were at increased likelihood of being under-surveilled. This study identified a difference in frequency of surveillance esophagogastroduodenoscopy for Barrett's esophagus by payer status. Patients without health insurance and those whose primary insurance was Medicaid were at increased odds for under-surveillance. These data suggest that a more robust system for tracking and ensuring longitudinal follow-up of patients with Barrett's esophagus, with attention to the uninsured and underinsured population, may be needed to ensure optimal surveillance.

Association of active smoking on 30-day wound events and additional morbidity and mortality following inguinal hernia repair with mesh: an analysis of the ACHQC database.

BACKGROUND: To date, there is limited data on the association of active smoking and 30-day wound events following inguinal hernia repair (IHR) with mesh. We aimed to determine if active smoking at the time of IHR with mesh was associated with worse 30-days wound events and additional morbidity outcomes using the Abdominal Core Health Quality Collaborative (ACHQC) database. METHODS: All adult patients undergoing elective, IHR with mesh who had 30-day follow-up data available were identified within the ACHQC database. Smokers were defined as having used nicotine within the 30 days prior to surgery. A 1:1 propensity score matched analysis was performed comparing smokers to non-smokers, controlling for factors previously shown to be associated with postoperative wound events. The effect of smoking on 30-day wound events and additional morbidity outcomes following IHR with mesh was investigated using Chi-square or Fisher's exact test for categorical data and Wilcoxon ranked test for continuous data. RESULTS: A total of 17,543 patients met inclusion criteria; 1855 (11%) were active smokers at the time of minimally invasive IHR with mesh. A total of 3694 patients were used for the matched analysis. There were no statistically significant differences between the non-smokers and smokers with respect to the incidence of surgical site infection (p = 0.10), surgical site occurrences (p = 0.22), or surgical site occurrences requiring procedural intervention (p = 0.64). Non-smokers were significantly more likely to be readmitted to the hospital and had significantly less improvement in all pain domains following IHR with mesh. CONCLUSIONS: Active smoking at the time of IHR with mesh is not associated with worse 30-day wound or additional morbidity and mortality outcomes. Based on these results, preoperative smoking cessation for all patients undergoing IHR may not reduce 30-day morbidity.

The role of an equine nutritionist in equine health, performance and wellbeing: Ideas stemming from an equine science society symposium workshop.

In the United States, there is little clarity on the qualifications and availability of equine nutritionists. Currently, no regulatory body exists for formal credentialing outside of veterinary medicine. Most equine nutritionists are not veterinarians but do have advanced scientific degrees (Master of Science and/or Doctor of Philosophy) in the field of Animal Science. However, not all reporting to be equine nutritionists have formal education in the field of equine nutrition. To discuss this, a workshop was held at the 2023 Equine Science Society (ESS) meeting. The purpose of this discussion was to share ideas among equine nutrition professionals about how best to provide support for the inclusion of the specialty as part of a horse's health team, alongside the veterinarian, farrier and other equine health specialists. In human, small animal and livestock practices, the importance of nutrition as part of an overall health, production (livestock) and well-being plan has been documented. However, surveys of veterinarians, the top source of information for horse owners, reveal a lack of confidence in the area of nutrition after graduating from veterinary school and a lack of available continuing education opportunities to learn more. Further, it appears that many horse owners may unknowingly be obtaining nutrition information from unverified sources (such as the internet). The discussion included formal and informal education of equine nutritionists, as well as avenues to open lines of communication with the veterinary community to provide nutrition resources for horse owners, managers and veterinarians.

Outcomes of complex abdominal wall reconstruction in patients with connective tissue disorders: a single center experience.

INTRODUCTION: Individuals diagnosed with connective tissue disorders (CTD) are known to be predisposed to incisional hernia formation. However, there is a scarcity of data on outcomes for these patients undergoing hernia repair. We sought to describe our outcomes in performing abdominal wall reconstructions in these complex patients. METHODS: Adult patients with CTD undergoing open, elective, posterior component separation with permanent synthetic mesh at our institution from January 2018 to October 2022 were queried from a prospectively collected database in the Abdominal Core Health Quality Collaborative. We evaluated 30-day wound morbidity, perioperative complications, long-term hernia recurrence, and patient-reported quality of life. RESULTS: Twelve patients were identified. Connective tissue disorders included Marfan's n = 7 (58.3%), Loeys-Dietz syndrome n = 2 (16.7%), Systemic Lupus Erythematosus n = 2 (16.7%), and Scleroderma n = 1 (8.3%). Prior incisions included three midline laparotomies and nine thoracoabdominal, mean hernia width measured 14 cm, and 9 were recurrent hernias. Surgical site occurrences (SSOs) were observed in 25% of cases, and 16.7% necessitated procedural intervention. All twelve patients were available for long-term follow-up, with a mean of 34 (12-62) months. There were no instances of reoperation or mesh excision related to the TAR procedure. One patient developed a recurrence after having his mesh violated for repair of a new visceral aneurysm. Mean HerQLes scores at 1 year were 70 and 89 at ≥ 2 years; Mean scaled PROMIS scores were 30.7 at 1 year and 36.3 at ≥ 2 years. CONCLUSION: Ventral hernia repair with TAR is feasible in patients with connective tissue disorder and can be a suitable alternative in patients with large complex hernias.

Comparing rates of bowel injury for laparoscopic and robotic ventral hernia repair: a retrospective analysis of the abdominal core health quality collaborative.

PURPOSE: Bowel injury during laparoscopic and robotic ventral hernia repair is a rare but potentially serious complication. We sought to compare bowel injury rates during minimally invasive approaches to ventral hernia repair using a national hernia registry. METHODS: Patients undergoing elective laparoscopic and robotic ventral hernia repair (including cases converted-to-open) between 2013 and 2021 were retrospectively identified in the Abdominal Core Health Quality Collaborative registry. The primary outcome was bowel injury, which included partial- and full-thickness injuries and re-operations for missed enterotomies. Statistical analysis was performed using multivariate logistic regression. RESULTS: Overall, 10,660 patients were included (4116 laparoscopic, 6544 robotic). The laparoscopic group included more incisional hernias (68% vs 62%, p < 0.001) and similar rates of recurrent hernias (23% vs 22%, p = 0.26). A total of 109 bowel injuries were identified, with more occurring in the laparoscopic group (55 [1.3%] laparoscopic vs. 54 [0.8%] robotic; p = 0.01). Specifically, there were more full-thickness and missed enterotomies in the laparoscopic group (29 laparoscopic vs. 20 robotic; p = 0.012). Bowel injury resulted in higher rates of wound morbidity and major post-operative complications including sepsis, re-admission, and re-operation. Following adjustment for recurrent and incisional hernias, prior mesh, patient age, and hernia width, bowel injury during laparoscopic repair remained significantly more likely than bowel injury during robotic repair (OR 1.669 [95% C.I.: 1.141-2.440]; p = 0.008). CONCLUSION: In a large registry, laparoscopic ventral hernia repair is associated with an increased risk of bowel injury compared to repairs utilizing the robotic platform. Knowing the limitations of retrospective research, large national registries are well suited to explore rare outcomes which cannot be feasibly assessed with randomized controlled trials.

Three-dimensional structure of an antigen-antibody complex at 2.8 A resolution.

The 2.8 A resolution three-dimensional structure of a complex between an antigen (lysozyme) and the Fab fragment from a monoclonal antibody against lysozyme has been determined and refined by x-ray crystallographic techniques. No conformational changes can be observed in the tertiary structure of lysozyme compared with that determined in native crystalline forms. The quaternary structure of Fab is that of an extended conformation. The antibody combining site is a rather flat surface with protuberances and depressions formed by its amino acid side chains. The antigen-antibody interface is tightly packed, with 16 lysozyme and 17 antibody residues making close contacts. The antigen contacting residues belong to two stretches of the lysozyme polypeptide chain: residues 18 to 27 and 116 to 129. All the complementarity-determining regions and two residues outside hypervariable positions of the antibody make contact with the antigen. Most of these contacts (10 residues out of 17) are made by the heavy chain, and in particular by its third complementarity-determining region. Antigen variability and antibody specificity and affinity are discussed on the basis of the determined structure.

Visualization of dioxygen bound to copper during enzyme catalysis.

X-ray crystal structures of three species related to the oxidative half of the reaction of the copper-containing quinoprotein amine oxidase from Escherichia coli have been determined. Crystals were freeze-trapped either anaerobically or aerobically after exposure to substrate, and structures were determined to resolutions between 2.1 and 2.4 angstroms. The oxidation state of the quinone cofactor was investigated by single-crystal spectrophotometry. The structures reveal the site of bound dioxygen and the proton transfer pathways involved in oxygen reduction. The quinone cofactor is regenerated from the iminoquinone intermediate by hydrolysis involving Asp383, the catalytic base in the reductive half-reaction. Product aldehyde inhibits the hydrolysis, making release of product the rate-determining step of the reaction in the crystal.

The predicted structure of immunoglobulin D1.3 and its comparison with the crystal structure.

Predictions of the structures of the antigen-binding domains of an antibody, recorded before its experimental structure determination and tested subsequently, were based on comparative analysis of known antibody structures or on conformational energy calculations. The framework, the relative positions of the hypervariable regions, and the folds of four of the hypervariable loops were predicted correctly. This portion includes all residues in contact with the antigen, in this case hen egg white lysozyme, implying that the main chain conformation of the antibody combining site does not change upon ligation. The conformations of three residues in each of the other two hypervariable loops are different in the predicted models and the experimental structure.

Early repair of ventral incisional hernia may improve quality of life after surgery for abdominal malignancy: a prospective observational cohort study.

PURPOSE: Recent work has shown that over 40% of patients undergoing surgery for abdominal malignancy develop ventral incisional hernias (VIH) within 2 years. We hypothesized that early repair of VIH for cancer survivors could improve long-term quality of life (QoL). METHODS: All patients presenting with a history of surgery for abdominal malignancy and a VIH were prospectively enrolled. QoL was assessed at baseline and 3-, 6-, 12-, 18-, and 24-month follow-up using abdominal wall-specific (HerQLes) and cancer-specific (FACT-G) instruments. At the study's conclusion, patients were divided into 2 groups-those that underwent VIH repair during the study's course (Repair Group) and those that did not (Control Group). Categorical variables were analyzed using Pearson's Chi-square and continuous variables with Wilcoxon rank sum test. RESULTS: Eighty-four patients were enrolled. Overall, 46 patients (55%) underwent VIH repair, with 36 repairs (78%) occurring within 3 months of initial evaluation. Sixty-six (79%) had complete 1-year follow-up data, and 30 (36%) had 2-year data, with a median follow-up duration of 15.6 months. At baseline, both groups were similar with respect to demographics, cancer stage, and HerQLes/FACT-G scores. Compared to the Controls, the Repair Group showed greater improvements over baseline HerQLes Summary Scores at the 3-, 6-, 12-, and 18-month time points (median increase, 37 vs. 26 points), and in FACT-G total scores at the 3-, 6-, and 12-month time points (median increase, 6 vs. 4 points). CONCLUSIONS: Repair of VIH after surgery for abdominal malignancy may improve abdominal wall-specific and cancer-specific QoL, making post-resection abdominal wall reconstruction an important aspect of cancer survivorship.

Copper amine oxidase: cunning cofactor and controversial copper.

Copper amine oxidases have a complex reaction cycle that converts a primary amine and molecular oxygen into the aldehyde, ammonia and hydrogen peroxide. Coupling structural studies of freeze-trapped reaction intermediates in crystals with kinetic and spectroscopic experiments in solution has generated a detailed molecular picture of catalysis. Although dioxygen has been directly observed bound to the copper at a late stage in the reaction cycle, whether copper is the initial binding site remains controversial.

Blood pressure as a predictor of cardiovascular events in the elderly: the William Hale Research Program.

This study evaluates the association between blood pressure (BP) and the risk of developing cardiovascular disease (CVD) events in the elderly. The Morton Plant Mease Foundation has followed 4,008 elderly patients >64 years of age for at least 5 years. Systolic and diastolic blood pressure (SBP and DBP) was divided into categories. Cardiovascular disease events were classified as myocardial infarction, stroke, and CVD-related deaths reported from the National Death Index. Cox proportional hazard ratios were used to assess the relationship between BP and CVD events and controlled for weight, gender, smoker, and alcohol use. Ages <75 and >or=75 years were assessed separately. After 11.1 years of follow-up, elevated SBP (P=<0.0001) is strongly associated with developing a future CVD event; the relationship is linear and graded and holds for ages above and below 75 years. The frequency of CVD events was lowest in the SBP <120 mm Hg group. In subjects <75 years of age, DBP elevations were not a significant risk factor for CVD events. (relative risk (RR): DBP 70 to <80 mm Hg=0.92; DBP 80 to <90 mm Hg=0.88; DBP >or=90 mm Hg=1.02.) With subjects >or=75 years of age, a DBP between 80 and 90 is associated with the lowest significant risk for CVD (RR: DBP 70 to <80 mm Hg=0.74; DBP 80 to <90 mm Hg=0.59; DBP >or=90=0.71). In conclusion, these findings support the Joint National Committee on Hypertension recommendations for SBP in the elderly. Further studies are warranted to identify optimal DBP for the elderly at various ages.

Direct and indirect readout in mutant Met repressor-operator complexes.

BACKGROUND: The methionine repressor, MetJ, represses the transcription of genes involved in methionine biosynthesis by binding to arrays of two to five adjacent copies of an eight base-pair 'metbox' sequence. Naturally occurring operators differ from the consensus sequence to a greater extent as the number of metboxes increases. MetJ, while accommodating this sequence variation in natural operators, is very sensitive to particular base changes, even where bases are not directly contacted in the crystal structure of a complex formed between the repressor and consensus operator. RESULTS: Here we report the high-resolution structure of a MetJ mutant, Q44K, bound to the consensus operator sequence (Q44Kwt19) and two related sequences containing mutations at sites believed to be important for indirect readout at non-contacted bases. The overall structure of the Q44Kwt19 complex is very similar to the wild-type complex, but there are small variations in sugar-phosphate backbone conformation and direct contacts to the DNA bases. The mutant complexes show a mixture of direct and indirect readout of sequence variations, with differences in direct contacts and DNA conformation. CONCLUSIONS: Comparison of the wild-type and mutant repressor-operator complexes shows that the repressor makes sufficiently strong interactions with the sugar-phosphate backbone to accommodate some variation in operator sequence with minor changes in direct bases contacts. The reduction in repressor affinity for the two mutant repressor complexes can be partially attributed to a loss in direct contacts to the DNA. In one case, however, the replacement of a flexible TA base-step leads to an unfavourable DNA conformation that reduces the stability of the repressor-operator complex.

Electrostatic activation of Escherichia coli methionine repressor.

BACKGROUND: The three-dimensional structure of the Escherichia coli methionine repressor (met repressor) is relatively unperturbed by the binding of its corepressor, S-adenosylmethionine (SAM), and of operator DNA. The positively charged corepressor binds to sites on the repressor remote from the DNA-binding site, and despite the lack of induced structural change is able to raise the affinity for operator DNA by a factor of up to 1000. Neutral corepressor analogues also bind to the repressor, but do not increase operator affinity. These observations suggest that the corepressor effect may be electrostatic. RESULTS: Using the program DELPHI, we have calculated electrostatic potentials for the repressor and its complexes, and have obtained results consistent with an electrostatic model for repressor activation. The positive potential originating from the corepressor is propagated through the repressor-operator complex, and is significant at DNA phosphate groups buried in the protein-DNA interface. The rank order of calculated electrostatic interaction energies for complexes with SAM, and two closely-related analogues, is in agreement with experimental measurements of the corresponding repressor-operator affinities. CONCLUSION: Long-range (> 10 A) electrostatic interactions between bound corepressor and operator phosphate groups in the repressor-operator complex may be sufficient to explain repressor activation Met repressor could, therefore, be an electrostatically triggered genetic switch.

Antibody fragment Fv4155 bound to two closely related steroid hormones: the structural basis of fine specificity.

BACKGROUND: The concentration of steroid glucuronides in serial samples of early morning urine (EMU) can be used to predict the fertile period in the female menstrual cycle. The monoclonal antibody 4155 has been used as a convenient means of measuring the concentration of steroid glucuronides in EMU, as it specifically recognises the steroid hormone estrone beta-D-glucuronide (E3G), with very high affinity, and the closely related hormone estriol 3-(beta-d-glucuronide) (EI3G), with reduced affinity. Although 4115 binds these hormones with different affinities, EI3G differs from E3G only in the addition of a hydroxyl group and reduction of an adjacent carbonyl. To investigate the structural basis of this fine binding specificity, we have determined the crystal structures of the variable fragment (Fv) of 4155 in complex with each of these hormones. RESULTS: Two crystal forms of the Fv4155-EI3G complex, at resolutions of 2.1 A and 2.5 A, and one form of the Fv4155-E3G complex, at 2.1 A resolution were solved and refined. The crystal structures show the E3G or EI3G antigen lying in an extended cleft, running form the centre of the antibody combining site down one side of the variable domain interface, and formed almost entirely from residues in the heavy chain. The binding cleft lies primarily between the heavy chain complementarity determining regions (CDRs), rather than in the interface between the heavy and light chains. In both complexes the binding of the glucuronic sugar, and rings A and B of the steroid, is specified by the shape of the narrow cleft. Analysis of the Fv structure reveals that five of the six CDR regions can be assigned to one of the predefined canonical structural classes. CONCLUSIONS: The difference in the binding affinity of Fv4155 for the two steroid hormones is accounted for by a subtle combination of a less favoured hydrogen-bond geometry, and a minor rearrangement of the water molecule network around the binding site. The rearrangement of water molecules results from the burial of the additional hydroxyl group of the EI3G in a hydrophobic environment.

Crystal structure of a quinoenzyme: copper amine oxidase of Escherichia coli at 2 A resolution.

BACKGROUND: Copper amine oxidases are a ubiquitous and novel group of quinoenzymes that catalyze the oxidative deamination of primary amines to the corresponding aldehydes, with concomitant reduction of molecular oxygen to hydrogen peroxide. The enzymes are dimers of identical 70-90 kDa subunits, each of which contains a single copper ion and a covalently bound cofactor formed by the post-translational modification of a tyrosine side chain to 2,4,5-trihydroxyphenylalanine quinone (TPQ). RESULTS: The crystal structure of amine oxidase from Escherichia coli has been determined in both an active and an inactive form. The only structural differences are in the active site, where differences in copper coordination geometry and in the position and interactions of the redox cofactor, TPQ, are observed. Each subunit of the mushroom-shaped dimer comprises four domains: a 440 amino acid C-terminal beta sandwich domain, which contains the active site and provides the dimer interface, and three smaller peripheral alpha/beta domains (D1-D3), each of about 100 amino acids. D2 and D3 show remarkable structural and sequence similarity to each other and are conserved throughout the quinoenzyme family. In contrast, D1 is absent from some amine oxidases. The active sites are well buried from solvent and lie some 35 A apart, connected by a pair of beta hairpin arms. CONCLUSIONS: The crystal structure of E. coli copper amine oxidase reveals a number of unexpected features and provides a basis for investigating the intriguing similarities and differences in catalytic mechanism of members of this enzyme family. In addition to the three conserved histidines that bind the copper, our studies identify a number of other conserved residues close to the active site, including a candidate for the catalytic base and a fourth conserved histidine which is involved in an interesting intersubunit interaction.

Evaluation of long-term surgical site occurrences in ventral hernia repair: implications of preoperative site independent MRSA infection.

PURPOSE: Previous work demonstrated that prior MRSA infection [MRSA(+)] is associated with 30-day surgical site infection (SSI) following ventral hernia repair (VHR). We aimed to determine the impact of MRSA(+) on long-term wound outcomes after VHR. PARTICIPANTS: A retrospective cohort study was performed at a tertiary center between July 11, 2005, and May 18, 2012, of patients undergoing elective VHR with class I wounds. Patients with documented preoperative MRSA infection at any site (urinary, bloodstream, SSI, etc.) were considered MRSA(+). Primary outcome was 2-year surgical site occurrence (SSO), defined as SSI, cellulitis, necrosis, nonhealing wound, seroma, hematoma, dehiscence, or fistula. SSOs were subdivided into those that required procedural intervention (SSOPI) and those that did not. RESULTS: Among 632 patients, 46 % were female with average age 53 ± 13 years. There were 368 SSOs in 193 patients (31 %); an SSOPI occurred in 9.8 % (n = 62). The most common SSOs were cellulitis (91/632), seroma (91/632), and serous drainage (58/632). The rate of 2-year SSO was higher with MRSA(+) compared to those without (46 vs. 29 %, p = 0.023), attributed to increased soft tissue necrosis, purulent drainage, serous drainage, cellulitis, and fistula. In multivariable analysis, MRSA(+) was not associated with 2-year SSO (HR 1.5, 95 % CI 0.91-2.55, p = 0.113); factors associated with SSO included obesity, immunosuppression, mesh repair, and operative times. CONCLUSIONS: This study is the first to evaluate long-term SSOs and SSOPIs after VHR, highlighting the importance of long-term follow-up. Though not independently associated with SSOs, MRSA(+) may be a marker of hernia complexity.

Norepinephrine transporter expression is inversely associated with glycaemic indices: a pilot study in metabolically diverse persons with overweight and obesity.

OBJECTIVE: The objective of this study was to examine the cross-sectional relationship between the expression of norepinephrine transporter (NET), the protein responsible for neuronal uptake-1, and indices of glycaemia and hyperinsulinaemia, in overweight and obese individuals. METHODS: Thirteen non-medicated, non-smoking subjects, aged 58 ± 1 years (mean ± standard error of the mean), body mass index (BMI) 31.4 ± 1.0 kg m-2, with wide-ranging plasma glucose and haemoglobin A1c (HbA1c, range 5.1% to 6.5%) participated. They underwent forearm vein biopsy to access sympathetic nerves for the quantification of NET by Western blot, oral glucose tolerance test (OGTT), euglycaemic hyperinsulinaemic clamp, echocardiography and assessments of whole-body norepinephrine kinetics and muscle sympathetic nerve activity. RESULTS: Norepinephrine transporter expression was inversely associated with fasting plasma glucose (r = -0.62, P = 0.02), glucose area under the curve during OGTT (AUC0-120, r = -0.65, P = 0.02) and HbA1c (r = -0.67, P = 0.01), and positively associated with steady-state glucose utilization during euglycaemic clamp (r = 0.58, P = 0.04). Moreover, NET expression was inversely related to left ventricular posterior wall dimensions (r = -0.64, P = 0.02) and heart rate (r = -0.55, P = 0.05). Indices of hyperinsulinaemia were not associated with NET expression. In stepwise linear regression analysis adjusted for age, body mass index and blood pressure, HbA1c was an independent inverse predictor of NET expression, explaining 45% of its variance. CONCLUSIONS: Hyperglycaemia is associated with reduced peripheral NET expression. Further studies are required to identify the direction of causality.

Crystal structure of a free radical enzyme, galactose oxidase.

The crystal structure of the copper-containing enzyme, galactose oxidase, has been solved by multiple isomorphous replacement and refined to a resolution of 1.7 A. The X-ray structure reveals a unique polypeptide fold. The protein can be divided into three domains, all of which consist almost entirely of beta-strands. The structure of the second domain is particularly striking, 28 beta-strands arranged in a pseudo 7-fold symmetry. The copper site is on the surface of the protein and extremely rich in aromatic side-chains. The copper ion has two histidines, two tyrosines, and one external ligand in distorted square pyramidal coordination. The presence of pyrroloquinoline quinone as a covalently bound cofactor in GOase has been excluded. Instead, an unexpected covalent linkage between Tyr272 and Cys228 has been observed, whose functional role may relate to the presence of a tyrosine free radical at Tyr272. The tyrosine free radical could be stabilized by delocalization to Cys228 and stacking interactions with Trp290. A structural model for substrate binding is proposed that offers an explanation for the substrate specificity of the enzyme and many of the spectroscopic and enzymological data. Although the model lacks direct confirmation at present, it should provide a stimulus for further spectroscopic and crystallographic studies.

Methylphenylmercury: a novel heavy atom reagent for protein crystallography.

Methylphenylmercury reacts with two normally inaccessible cysteine residues in crystals of carbonmonoxyhaemoglobin, but not with the third, normally reactive one. It may, therefore, be useful in the preparation of new heavy atom derivatives for protein crystallography.

The active site of the junction-resolving enzyme T7 endonuclease I.

Endonuclease I is a junction-resolving enzyme encoded by bacteriophage T7, that selectively binds and cleaves four-way DNA junctions. We have recently solved the structure of this dimeric enzyme at atomic resolution, and identified the probable catalytic residues. The putative active site comprises the side-chains of three acidic amino acids (Glu20, Asp55 and Glu65) together with a lysine residue (Lys67), and shares strong similarities with a number of type II restriction enzymes. However, it differs from a typical restriction enzyme as the proposed catalytic residues in both active sites are contributed by both polypeptides of the dimer. Mutagenesis experiments confirm the importance of all the proposed active site residues. We have carried out in vitro complementation experiments using heterodimers formed from mutants in different active site residues, showing that Glu20 is located on a different monomer from the remaining amino acid residues comprising the active site. These experiments confirm that the helix-exchanged architecture of the enzyme creates a mixed active site in solution. Such a composite active site structure should result in unilateral cleavage by the complemented heterodimer; this has been confirmed by the use of a cruciform substrate. Based upon analogy with closely similar restriction enzyme active sites and our mutagenesis experiments, we propose a two-metal ion mechanism for the hydrolytic cleavage of DNA junctions.