Implementation of a pediatric trauma registry at a national referral center in Kenya: Utility and concern for sustainability

BACKGROUND

Pediatric trauma disproportionately affects low- and middle-income countries, particularly the pediatric trauma systems, are frequently limited. This study assessed the patterns of pediatric traumatic injuries and treatment at the only free-standing public children's hospital in East Africa as well as the implementation and sustainability of the trauma registry.

METHODS

A prospective pediatric trauma registry was established at Shoe4Africa Children's Hospital (S4A) in Eldoret, Kenya. All trauma patients over a six-month period were enrolled. Descriptive analyses were completed via SAS 9.4 to uncover patterns of demographics, trauma mechanisms and injuries, as well as outcomes. Implementation was assessed using the RE-AIM framework.

RESULTS

The 425 patients had a median age of 5.14 years (IQR 2.4, 8.7). Average time to care was 267.5 min (IQR 134.0, 625.0). The most common pediatric trauma mechanisms were falls (32.7 %) and burns (17.7 %), but when stratified by age group, toddlers had a higher risk of sustaining injuries from burns and poisonings. Over half (56.2 %) required an operation during the hospitalization. Overall, implementation of the registry was limited by the clinical burden and inadequate personnel. Sustainability of the registry was limited by finances.

CONCLUSIONS

This is the first study to describe the trauma epidemiology from a Kenyan public pediatric hospital. Maintenance of the trauma registry failed due to cost. Streamlining global surgery efforts through implementation science may allow easier development of trauma registries to then identify modifiable risk factors to prevent trauma and long-term outcomes to understand associated disability.

Bilateral Exchange: Enteral Nutrition Clinical Decision Making in Pediatric Surgery Patients

INTRODUCTION

Evidence-based medicine guides clinical decision-making; however, promoting enteral nutrition has historically followed a dogmatic approach in which patients graduate from clear liquids to full liquids to a regular diet after return of bowel function. Enhanced recovery after surgery has demonstrated that early enteral nutrition initiation is associated with shorter hospital stays. We aimed to understand postoperative pediatric nutrition practices in Kenya and the United States.

METHODS

We completed a prospective observational study of pediatric surgery fellows during clinical rounds in a pediatric referral center in Kenya (S4A) and one in the United States (Riley). Fellow-patient interactions were observed from postoperative day one to discharge or postoperative day 30, whichever happened first. Patient demographic, operative information, and daily observations including nutritional status were collected via REDCap.

RESULTS

We included 75 patients with 41 (54.7%) from Kenya; patients in Kenya were younger with 40% of patients in Kenya presenting as neonates. Median time to initiation and full enteral nutrition was shorter for the patients at Riley when compared to their counterparts at S4A. In the neonatal subgroup, patients at S4A initiated enteral nutrition sooner, but their hospital length of stays were not significantly different.

CONCLUSIONS

Studying current nutrition practices may guide early enteral nutrition protocols. Implementing these protocols, particularly in a setting where enteral nutrition alternatives are minimal, may provide evidence of success and overrule dogmatic nutrition advancement. Studying implementation of these protocols in resource-constrained areas, where patient length of stay is often related to socioeconomic factors, may identify additional benefits to patients.

Breast Health Awareness: Understanding Health-Seeking Behavior in Western Kenya

INTRODUCTION

In Kenya, patients with breast cancer predominantly present with late-stage disease and experience poor outcomes. To promote early-stage diagnosis, we implemented the Academic Model Providing Access to Healthcare (AMPATH) Breast and Cervical Cancer Control Program (ABCCCP) in Western Kenya.

OBJECTIVE

The aim of this study was to assess differences between patients presenting to health facilities and health fairs.

METHODS

This was an institutional Review and Ethics Commitee-approved retrospective cohort study of all individuals who underwent clinical breast examination (CBE) via local healthcare workers in Western Kenya. From 2017 to 2021, the program hosted health fairs, and trained healthcare providers at health facilities to complete CBEs. Results were analyzed using the Chi-square and Kruskal-Wallis tests, with an α < 0.05.

RESULTS

Over a 5-year period, the ABCCCP completed 61,812 CBEs with 75.9% (n = 46,902) performed at a health facility. Patients presenting to health fairs were older (44 vs. 38 years; p < 0.0001) and had higher risk factor rates including early menarche, family history of breast and ovarian cancer, and use of alcohol or smoking. Only 27.6% of patients with an abnormal CBE underwent core needle biopsy, and only 5.2% underwent repeat CBE over the 5-year period, of whom 90.3% presented to health facilities.

CONCLUSIONS

Successful uptake of CBE through the ABCCCP is the first step to introduce breast health awareness (BHA). Benefits of broad advertisements for health fairs in promoting BHA may be limited to a single event. Poor rates of repeat examinations and diagnostic testing of abnormal CBEs indicate additional resources should be allocated to educating patients, including about possible treatment trajectories for breast cancer.

Bilateral trauma case conferences: an approach to global surgery equity through a virtual education exchange

Purpose

With increased interest in international surgical experiences, many residency programs have integrated global surgery into their training curricula. For surgical trainees in low- and middle-income countries (LMICs), physical exchange can be costly, and laws in high-income countries (HICs) prevent LMIC trainees from practicing surgery while on visiting rotations. To enrich the educational experience of trainees in both settings, we established a monthly virtual trauma conference between surgery training programs.

Methods

General surgery teams from two public institutions, a public university with two surgical training programs in Kenya and a public university with two level I trauma centers in the United States, meet monthly to discuss complex and interesting trauma patients. A trainee from each institution presents a clinical case vignette and supplements the case with pertinent peer-reviewed literature. The attendees then answer a series of multiple-choice questions like those found on surgery board exams.

Results

Monthly case conferences began in September 2017 with an average of 24 trainees and consultant surgeons. Case discussions serve to stimulate dialogue on patient presentation and management, highlighting cost-conscious, high-quality care and the need to adapt practice patterns to meet resource constraints and provide culturally appropriate care.

Conclusion

Our 5-year experience with this virtual case conference has created a unique and robust surgical education experience for trainees and surgeons who have withstood the effects of the pandemic. These case conferences have not only strengthened the camaraderie between our departments, but also promoted equity in global surgery education and prioritized the learning of trainees from both settings.

Implementation of Multimodal Pain Protocol Associated With Opioid Use Reduction in Trauma Patients

INTRODUCTION

Many trauma centers have adopted multimodal pain protocols (MMPPs) to provide safe and effective pain control. The objective was to evaluate the association of a protocol on opioid use in trauma patients and patient-reported pain scores.

METHODS

This was a retrospective review of adult trauma patients admitted from 7/1-9/30/2018 to 7/1-9/30/2019 at an urban academic level 1 trauma center. The MMPP consisted of scheduled nonopioid medications implemented on July 1, 2019. Patients were stratified by level of care upon admission, intensive care unit (ICU) or floor, and by injury severity score (ISS) (ISS < 16 or ISS ≥ 16). Pain scores, opioid, and nonopioid analgesic medication use were compared for the hospital stay or first 30 d.

RESULTS

Seven hundred ninety eight patients were included with a mean age of 54 ± 22 y and 511 (64.0%) were men. Demographic and clinical characteristics between those in the pre-MMP (n = 404) and post-MMPP (n = 394) groups were not different. The average pain scores were not different between the two groups (3.7 versus 3.8, P = 0.44), but patients in the post-MMPP group received 36% less morphine milliequivalents (109.6 versus 70; P < 0.0001). The MMPP had the largest effect on patients admitted to the ICU regardless of injury severity. ICU patients with ISS ≥ 16 had the greatest reduction in morphine milliequivalents (174.6 versus 84.4; P < 0.0001). The use of nonopioid analgesics was significantly increased in all groups.

CONCLUSIONS

A MMPP is associated with a reduction of opioids and increase in nonopioid analgesics with no difference in patient-reported pain scores.

Demographics and medical school exposures to rural health influence future practice

BACKGROUND

One-fifth of the US population lives in rural areas. A record number of rural hospitals have closed, creating increased burdens on regional centers and delays in care. This study aimed to assess medical student perceptions of rural surgery and health care, and to elucidate influential factors for future practice.

METHODS

We administered a survey to medical schools throughout Indiana, Illinois, and Michigan. The survey was designed and evaluated by a survey content expert and piloted among a group of students. Student and faculty liaisons disseminated the survey between February and May 2021. Descriptive analysis of data was completed using Stata v.16.1 (StataCorp, LLC, College Station, TX).

RESULTS

The respondents included 700 medical students; 59.5% were female, with an equal distribution across medical school classes. More than 98% of students believe we "lack" or "are in great need of" rural health care providers, as well as rural surgeons; however, more than half of the students did not agree that the rural workforce is declining. Only 15.7% of students reported an interest in "pursuing a future career in a rural setting." Students with exposure to rural health care, coming from a rural hometown, or having a dependent had a positive association with interest in pursuing rural practice.

CONCLUSION

Although students are aware of the lack of rural surgeons and health care providers, there remains an educational deficit. Expanding exposure to rural health care and surgery while in medical school may increase the number of students interested in pursuing a career in a rural setting, potentially shrinking the rural workforce gap.

AMPATH surgical app: Low-cost simulator for the open appendectomy

BACKGROUND

Due to a shortage and maldistribution of surgeons within Kenya, doctors with limited formal surgical training often perform emergency surgical procedures such as appendectomy. This lack of training can compromise patient outcomes and complicate care delivery. Our aim was to develop a low-cost simulator and skills curriculum to effectively teach open appendectomy.

METHODS

Surgeons from 4 countries participated in semi-structured interviews to define the steps and technique of open appendectomy using cognitive task analysis. Using this input, our Academic Model Providing Access to Healthcare surgical team developed a curriculum, including a simulator and feedback mechanism. Surgeons and surgical trainees from Kenya and the United States tested the simulator prototype and provided feedback for its refinement based on clarity, utility, and realism.

RESULTS

Instructions for a self-constructed simulator were developed at the cost of 70 Kenyan shillings (0.64 US dollars). Fifteen surgeons and surgical residents gave feedback on the simulator and curriculum, and each was presented with an updated version based on feedback. Overall, the curriculum was clear, with each sub-step receiving a median score of ≥83.5 out of 100 for clarity; however, through iterative design, the utility of sub-steps on the simulator improved.

CONCLUSION

A comprehensive open appendectomy curriculum, including a low-cost appendectomy simulator model, was developed and refined using surgeon feedback. Such curricula may benefit trainees in low-resource settings who may otherwise have limited access to quality training material.

Uncovering the Overlap of Global and Domestic Rural Surgery for Medical Trainees

Introduction

Medical trainees who participate in global rotations demonstrate improved cultural sensitivity, increased involvement in humanitarian efforts, and ability to adapt to limited resources. The global coronavirus pandemic halted global rotations for medical trainees. Domestic rural surgery (DRS) may offer a unique alternative. We aimed to understand medical students’ perceptions of the similarities and differences between global surgery and DRS and how students’ priorities impact career choices.

Methods

An electronic survey was administered at eleven medical training institutions in Indiana, Illinois, and Michigan in spring 2021. Mixed methods analysis was performed for students who reported an interest in global surgery. Quantitative analysis was completed using Stata 16.1.

Results

Of the 697 medical student respondents, 202 were interested in global surgery. Of those, only 18.3% were also interested in DRS. Students interested in DRS had more rural exposures. Rural exposures associated with DRS interest were pre-clinical courses (P = 0.002), clinical rotations (P = 0.045), and rural health interest groups (P < 0.001). Students interested in DRS and those unsure were less likely to prioritize careers involving teaching or research, program prestige, perceived career advancement, and well-equipped facilities. The students who were unsure were willing to utilize DRS exposures.

Conclusions

Students interested in global surgery express a desire to practice in low-resource settings. Increased DRS exposures may help students to understand the overlap between global surgery and DRS when it comes to working with limited resources, achieving work-life balance and practice location.

Recognition and management of traumatic fetal injuries

Trauma during pregnancy is the leading non-obstetric cause of morbidity and mortality, and accounts for five per 1000 fetal deaths. Direct fetal injury due to trauma during pregnancy is rare, and limited information is available about how to optimize fetal outcomes after injury. Early recognition and appropriate management of direct fetal trauma may improve outcomes for the fetus. There are currently no available guidelines to direct management of the injured fetus. We provide a detailed literature review of the management and outcomes of direct fetal injury following blunt and penetrating injury during pregnancy, and describe a suggested initial approach to the injured pregnant patient with a focus on evaluation for fetal injury. We identified 45 reported cases of blunt trauma resulting in direct fetal injury, with 21 surviving past the neonatal period, and 33 of penetrating trauma resulting in direct fetal injury, with 24 surviving past the neonatal period. Prenatal imaging identified fetal injury in 19 cases of blunt trauma and was used to identify bullet location relative to the fetus in 6 cases. These reports were used to develop management algorithms for the injured fetus.

Gender Differences in Insurance, Surgical Admissions and Outcomes at a Kenyan Referral Hospital

INTRODUCTION

Gender is an important factor in determining access to healthcare resources. Women face additional barriers, especially in low- and middle-income countries. Surgical costs can be devastating, which can exacerbate engendered disparities. Kenya's National Hospital Insurance Fund (NHIF) aims to achieve universal coverage and protect beneficiaries from catastrophic health expenditures. We examine gender differences in NHIF coverage, health-seeking behavior, and surgical outcomes at a tertiary care hospital in Eldoret, Kenya.

MATERIALS AND METHODS

All patients ≥13 years admitted to the general surgery service at Moi Teaching and Referral Hospital from January 2018-July 2018 were enrolled. Health records were retrospectively reviewed for demographic data, clinical parameters, NHIF enrollment, and cost information. Descriptive analyses utilized Wilcoxon Rank Sum, Pearson's Chi-square, and Fisher's Exact tests.

RESULTS

366 patients were included for analysis. 48.6% were enrolled in NHIF with significant female predominance (64.8% versus 37.9%, P < 0.0001). Despite differing coverage rates, male and female patients underwent surgery and suffered in-hospital mortality at similar rates. However, women only comprised 39.6% of admissions and were significantly more likely to delay care (median 60 versus 7 days, P < 0.0001), be diagnosed with cancer (26.6% versus 13.2%, P = 0.0024), and require a palliative procedure for cancer (44.1% versus 13.0%, P = 0.013).

CONCLUSION

Many financial and cultural barriers exist in Kenya that prevent women from accessing healthcare as readily as men, persisting despite higher rates of NHIF coverage amongst female patients. Investigation into extra-hospital costs and social disempowerment for women may elucidate key needs for achieving health equity.

Surgical and Trauma Capacity Assessment in Rural Haryana, India

Background

Trauma is a major global health problem and majority of the deaths occur in low- and middle-income countries (LMICs), at even higher rates in the rural areas. The three-delay model assesses three different delays in accessing healthcare and can be applied to improve surgical and trauma healthcare delivery. Prior to implementing change, the capacities of the rural India healthcare system need to be identified.

Objective

The object of this study was to estimate surgical and trauma care capacities of government health facilities in rural Nanakpur, Haryana, India using the Personnel, Infrastructure, Procedures, Equipment and Supplies (PIPES) and International Assessment of Capacity for Trauma (INTACT) tools.

Methods

The PIPES and INTACT tools were administered at eight government health facilities serving the population of Nanakpur in June 2015. Data analysis was performed per tool subsection, and an overall score was calculated. Higher PIPES or INTACT indices correspond to greater surgical or trauma care capacity, respectively.

Findings

Surgical and trauma care capacities increased with higher levels of care. The median PIPES score was significantly higher for tertiary facilities than primary and secondary facilities [13.8 (IQR 9.5, 18.2) vs. 4.7 (IQR 3.9, 6.2), p = 0.03]. The lower-level facilities were mainly lacking in personnel and procedures.

Conclusions

Surgical and trauma care capacities at healthcare facilities in Haryana, India demonstrate a shortage of surgical resources at lower-level centers. Specifically, the Primary Health Centers were not operating at full capacity. These results can inform resource allocation, including increasing education, across different facility levels in rural India.

Role reversal for substrates and inhibitors. Slow inactivation of D-amino acid transaminase by its normal substrates and protection by inhibitors

D-Amino acid transaminase, which catalyzes the synthesis of D-alanine and D-glutamate for the bacterial cell wall, is a candidate for the design of specific inhibitors that could be novel antimicrobial agents. Under the experimental conditions usually employed for enzyme assays, kinetic parameters for its substrates were determined for short incubation periods, when intermediates and products do not accumulate and the enzyme activity is linear with time. Such kinetic analyses indicate that the enzyme accepts most D-amino acids but D-aspartate and D-glutamate are the best substrates. Under a different type of experimental conditions when the enzyme is exposed to D-alanine, intermediates, and products for periods of hours, it slowly becomes inactivated (Martinez del Pozo, A., Yoshimura, T., Bhatia, M. B., Futaki, S., and Manning, J. M. (1992) Biochemistry 31, 6018-6023). We now report that D-aspartate, D-glutamate, and L-alanine also lead to slow inactivation. Methylation or amidation of the alpha-COOH group of D-alanine prevents inactivation, indicating that decarboxylation is required for inactivation; the slow release of CO2 from substrate is demonstrated. The alpha-methyl analog of D-alanine, D-aspartate, and D-glutamate do not lead to inactivation, showing that the alpha-hydrogen of the substrate is required, i.e. that some processing is required. Lys145, which binds pyridoxal 5'-phosphate in the wild-type enzyme, is not involved in the inactivation since two active site mutant enzymes, K145Q and K145N, are also inactivated. Reactivation of the inactive enzyme at acidic pH is accompanied by the release of ammonia corresponding to 1 mol/mol of dimeric enzyme. Competitive inhibitors, amine-containing buffers, and thiols effectively impede the inactivation. This reversal in the roles of substrates and inhibitors, i.e. when a substrate can be an inactivator and an inhibitor can act as a protector, occurs during a time period not usually used to measure steady-state kinetics or initial velocities of enzyme reactions and could have physiological relevance in cells.

The role of divalent magnesium in activating the reaction catalyzed by orotate phosphoribosyltransferase

Orotate phosphoribosyltranferase (OPRTase) catalyzes the formation of orotidine 5'-monophosphate from the nitrogenous base orotate and alpha-D-5-phosphoribosyl-1-pyrophosphate (PRPP). While it is known that Mg2+ is necessary for catalysis, the mechanism of activation of the phosphoribosyl transfer by Mg2+ remains unclear. The divalent cation may activate the phosphoribosyl transfer by binding to either or both substrates PRPP and orotate or/and the enzyme. In this work we chose to explore the role of divalent magnesium in activating the phosphoribosyl transfer in bacterial OPRTase. Studies on the effect of Mg2+ on the OPRTase-catalyzed reaction indicated that the divalent metal was necessary for catalysis. A maximal rate of 70 units/mg was achieved at 2 mM MgCl2. Mn2+ could replace Mg2+ as the divalent metal. Orotate methyl ester (OAME) and uracil, neither of which form chelates with divalent metal, were found to be substrates for OPRTase. The KM for OAME and uracil were 190 microM and 2.63 mM and kcat/KM were 0.91 x 10(5) and 6 M-1 s-1, respectively. These values compare with a KM of 27 microM for orotate, 44 microM for PRPP, and a kcat/KM of 1.3 x 10(6) M-1 s-1 for orotate. Spectroscopic studies failed to reveal the existence of Mg(2+)-orotate complexes. Thus we have concluded that an orotate-metal complex is not necessary for OPRTase catalysis. Metal-enzyme binding studies indicate that only weak metal-enzyme complexes may form in bacterial OPRTase. Thus the role of divalent metal in bacterial OPRTase must be to bind PRPP.

Kinetic and stereochemical comparison of wild-type and active-site K145Q mutant enzyme of bacterial D-amino acid transaminase

D-Amino acid transaminase (EC 2.6.1.21), from Bacillus sp. YM-1, a thermostable enzyme with pyridoxal 5'-phosphate as coenzyme and a target for the design of novel antimicrobial agents, catalyzes the reversible transfer of an amino group between D-alanine and alpha-ketoglutarate to form pyruvate and D-glutamate, respectively. To explore the catalytic role of Lys-145, which binds the coenzyme, a site-specific mutant enzyme, K145Q (in which Lys-145 had been mutated to glutamine) constructed earlier (Futaki, S., Ueno, H., Martinez del Pozo, A., Pospischil, M. A., Manning, J. M., Ringe, D., Stoddard, B., Tanizawa, K., Yoshimura, T., and Soda, K. (1990) J. Biol. Chem. 265, 22306-22312) was compared to the wild-type enzyme for its kinetic parameters. Initial velocity studies and partial reaction isotope exchange experiments showed that the low activity of the mutant enzyme (about 1.5% the activity of the wild-type enzyme with saturating substrates) is an intrinsic property, confirming that contaminating enzymes do not account for the low activity of the K145Q mutant enzyme. The rates of the forward reaction for both wild-type and mutant enzymes were 30-40 times higher than the rates of the reverse reaction. KM values for the four substrates were 10 to 100 higher for the mutant compared to the wild-type enzyme. Whereas D-alanine is preferred over L-alanine by the wild-type enzyme (10(3) higher kcat/KM for D- over L-alanine), the K145Q enzyme does not efficiently discriminate between L- and D-alanine. Both wild-type and mutant enzymes also catalyze the slow racemization of L- and D-alanine. Proton NMR studies showed that wild-type enzyme catalyzed a time-dependent exchange of the C alpha proton of D-alanine with solvent D2O and a slow exchange of the alpha proton of L-alanine; the latter slow exchange rate is the same for the C alpha proton of both L- and D-alanine with the K145Q mutant enzyme. Thus, in addition to binding pyridoxal 5'-phosphate, the active-site Lys-145 of D-amino acid transaminase is involved in several other important functions, i.e. it optimizes catalytic efficiency and it maintains stereochemical fidelity. The steady-state kinetic results on the K145Q mutant enzyme together with the findings on the relative racemization rates and the NMR protein exchange data suggest that an alternate base catalyzes abstraction of the alpha proton of substrate in this mutant D-amino acid transaminase.

Partial reactions of bacterial D-amino acid transaminase with asparagine substituted for the lysine that binds coenzyme pyridoxal 5'-phosphate

In bacterial D-amino acid transaminase (EC 2.6.1.21) replacement of Lys-145, which is covalently linked to the coenzyme pyridoxal 5'-phosphate in the wild-type enzyme, by an Asn residue gave a mutant enzyme (K145N) that slowly performed each half-reaction, as determined by spectral measurements. With the wild-type enzyme, the kinetics of these events were so rapid that pre-steady-state conditions were needed for their determination. The internal aldimine between coenzyme and Lys-145 was rapidly reduced with NaCNBH3 in the wild-type enzyme, whereas in the mutant enzyme the coenzyme, which is not covalently linked to the protein, was more resistant to reduction; the reduced forms of both wild-type and mutant enzymes were inactive. With large amounts of the K145N mutant enzyme and either amino acid or keto acid substrate alone, the formation of some reaction intermediates, i.e., the external aldimine with D-alanine and the ketimine with alpha-ketoglutarate, can be measured by conventional spectroscopy. Suicide substrates also induced slow spectral shifts of the E-PLP form of the enzyme. For the K145N enzyme, exogenous amines affected only the rate of the transaldimination but not the removal of the alpha-proton of the substrate. These results suggest that in the mutant enzyme some amino acid side chain other than Lys-145 performs this function. In order to identify this site, the K145N mutant enzyme was completely inactivated by the radiolabeled suicide substrate D-serine. Peptide mapping of tryptic digests showed that Lys-267 was the modified site.(ABSTRACT TRUNCATED AT 250 WORDS)

Inactivation of dimeric D-amino acid transaminase by a normal substrate through formation of an unproductive coenzyme adduct in one subunit

D-amino acid transaminase, which contains pyridoxal 5'-phosphate (vitamin B6) as coenzyme, catalyzes the formation of D-alanine and D-glutamate from their corresponding alpha-keto acids; these D-amino acids are required for bacterial cell wall biosynthesis. Under conditions usually used for kinetic assay of enzyme activity, i.e., short incubation times with dilute enzyme concentrations, D-alanine behaves as one of the best substrates. However, the enzyme slowly loses activity over a period of hours when exposed to substrates, intermediates, and products at equilibrium. The rate of inactivation is dependent on enzyme concentration but independent of substrate concentration greater than Km values. Continuous removal of the product pyruvate by enzymic reduction precludes the establishment of equilibrium and prevents inactivation. The formation of small but detectable amounts of a quinonoid intermediate absorbing at 493 nm is proportional to inactivation. Studies with [14C]-D-alanine labeled on different carbon atoms indicate that the alpha-carboxyl group of the substrate is absent in the inactive enzyme; such decarboxylation is not a usual function of this enzyme. The inactive transaminase contains 1.1 mol of [14C]-D-alanine-derived adduct per mole of dimeric enzyme; this finding is consistent with the 50% reduction in the fluorescence intensity at 390 nm (due to the PMP form of the coenzyme) for the inactive enzyme. Thus, inactivation of one subunit of the dimeric enzyme renders the entire molecule inactive. Inactivation may occur when a coenzyme intermediate, perhaps the ketimine, is slowly decarboxylated and then undergoes a conformational change from its catalytically competent location.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic mechanism of orotate phosphoribosyltransferase from Salmonella typhimurium

The chemical mechanism of the phosphoribosyltransferases (PRTases), although largely unknown, may proceed either via a concerted direct-transfer mechanism or with a two-step mechanism involving a carboxonium-like intermediate. To study this question, we have cloned the Salmonella typhimurium pyrE gene, coding for the enzyme orotate phosphoribosyltransferase (EC 2.2.4.10, OPRTase), and developed a bacterial strain that overproduces the enzyme, which we have purified to homogeneity. Initial velocity and product inhibition studies indicated that S. typhimurium OPRTase follows a random sequential kinetic mechanism. This result was further confirmed by equilibrium isotope exchange studies on two substrate-product pairs, PRPP-PPi and OMP-orotate. In addition, the rates of the individual equilibrium isotope exchanges allowed us to conclude that PPi release and PRPP release were the rate-determining steps in the forward and reverse reactions, respectively. Although partial reactions between the two substrate-product pairs, PRPP-PPi and OMP-orotate, were observed, further studies revealed that these exchanges were a result of contaminations. Our results are significant in that S. typhimurium OPRTase, like most PRTases but in contrast to its yeast homologue, follows sequential kinetics. The artifactual partial isotope exchanges found in this work may have implications for similar prior work on the yeast enzyme. In view of the careful isotope effect studies of Parsons and co-workers [Goitein, R.K., Chelsky, D., & Parsons, S.M. (1978) J. Biol. Chem. 253, 2963-2971] and the results obtained by us, we propose that PRTases may involve a direct-transfer mechanism but with low bond order to the leaving pyrophosphate moiety and attacking base.