Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study

BACKGROUND

Constitutional mismatch repair deficiency (CMMRD) syndrome is a rare and aggressive cancer predisposition syndrome. Because a scarcity of data on this condition contributes to management challenges and poor outcomes, we aimed to describe the clinical spectrum, cancer biology, and impact of genetics on patient survival in CMMRD.

METHODS

In this cohort study, we collected cross-sectional and longitudinal data on all patients with CMMRD, with no age limits, registered with the International Replication Repair Deficiency Consortium (IRRDC) across more than 50 countries. Clinical data were extracted from the IRRDC database, medical records, and physician-completed case record forms. The primary objective was to describe the clinical features, cancer spectrum, and biology of the condition. Secondary objectives included estimations of cancer incidence and of the impact of the specific mismatch-repair gene and genotype on cancer onset and survival, including after cancer surveillance and immunotherapy interventions.

FINDINGS

We analysed data from 201 patients (103 males, 98 females) enrolled between June 5, 2007 and Sept 9, 2022. Median age at diagnosis of CMMRD or a related cancer was 8·9 years (IQR 5·9-12·6), and median follow-up from diagnosis was 7·2 years (3·6-14·8). Endogamy among minorities and closed communities contributed to high homozygosity within countries with low consanguinity. Frequent dermatological manifestations (117 [93%] of 126 patients with complete data) led to a clinical overlap with neurofibromatosis type 1 (35 [28%] of 126). 339 cancers were reported in 194 (97%) of 201 patients. The cumulative cancer incidence by age 18 years was 90% (95% CI 80-99). Median time between cancer diagnoses for patients with more than one cancer was 1·9 years (IQR 0·8-3·9). Neoplasms developed in 15 organs and included early-onset adult cancers. CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%]). Patients with CNS tumours had the poorest overall survival rates (39% [95% CI 30-52] at 10 years from diagnosis; log-rank p<0·0001 across four cancer types), followed by those with haematological cancers (67% [55-82]), gastrointestinal cancers (89% [81-97]), and other solid tumours (96% [88-100]). All cancers showed high mutation and microsatellite indel burdens, and pathognomonic mutational signatures. MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival (overall survival at age 15 years 63% [95% CI 55-73] for PMS2, 49% [35-68] for MSH6, 19% [6-66] for MLH1, and 0% for MSH2; p<0·0001). Frameshift or truncating variants within the same gene caused earlier cancers and inferior outcomes compared with missense variants (p<0·0001). The greater deleterious effects of MLH1 and MSH2 variants as compared with PMS2 and MSH6 variants persisted despite overall improvements in survival after surveillance or immune checkpoint inhibitor interventions.

INTERPRETATION

The very high cancer burden and unique genomic landscape of CMMRD highlight the benefit of comprehensive assays in timely diagnosis and precision approaches toward surveillance and immunotherapy. These data will guide the clinical management of children and patients who survive into adulthood with CMMRD.

FUNDING

The Canadian Institutes for Health Research, Stand Up to Cancer, Children's Oncology Group National Cancer Institute Community Oncology Research Program, Canadian Cancer Society, Brain Canada, The V Foundation for Cancer Research, BioCanRx, Harry and Agnieszka Hall, Meagan's Walk, BRAINchild Canada, The LivWise Foundation, St Baldrick Foundation, Hold'em for Life, and Garron Family Cancer Center.

Malignant A-to-I RNA editing by ADAR1 drives T cell acute lymphoblastic leukemia relapse via attenuating dsRNA sensing

Leukemia-initiating cells (LICs) are regarded as the origin of leukemia relapse and therapeutic resistance. Identifying direct stemness determinants that fuel LIC self-renewal is critical for developing targeted approaches. Here, we show that the RNA-editing enzyme ADAR1 is a crucial stemness factor that promotes LIC self-renewal by attenuating aberrant double-stranded RNA (dsRNA) sensing. Elevated adenosine-to-inosine editing is a common attribute of relapsed T cell acute lymphoblastic leukemia (T-ALL) regardless of molecular subtype. Consequently, knockdown of ADAR1 severely inhibits LIC self-renewal capacity and prolongs survival in T-ALL patient-derived xenograft models. Mechanistically, ADAR1 directs hyper-editing of immunogenic dsRNA to avoid detection by the innate immune sensor melanoma differentiation-associated protein 5 (MDA5). Moreover, we uncover that the cell-intrinsic level of MDA5 dictates the dependency on the ADAR1-MDA5 axis in T-ALL. Collectively, our results show that ADAR1 functions as a self-renewal factor that limits the sensing of endogenous dsRNA. Thus, targeting ADAR1 presents an effective therapeutic strategy for eliminating T-ALL LICs.

Unmet Needs, Barriers, and Facilitators for Conducting Adolescent and Young Adult Cancer Survivorship Research in Southern California: A Delphi Survey

Introduction: An adolescent and young adult cancer (AYAC) patient is an individual who has received a cancer diagnosis between 15 and 39 years of age. They require significant survivorship care due to a combination of practical, physical, and mental health problems, but research in these areas is sparse. This study aimed to identify the unmet needs, barriers, and facilitators for conducting AYAC survivorship research in Southern California (SoCal) from the providers' and researchers' perspectives. Methods: A two-round, electronically administered Delphi survey study was conducted, involving a panel of 12 health care professionals and/or researchers with substantial work experience in AYAC. A 10-point Likert scale was used to evaluate 24 areas of unmet needs in AYAC survivors, 39 barriers, and 25 facilitators. Results: The top unmet needs in AYAC survivorship requiring research were in mental health issues, improving school/occupational performance, neurocognitive disorders, subsequent malignant neoplasms, and reproductive health. The top barriers identified were as follows: (1) institutions are too short-staffed to administer survivorship studies; (2) oncologists do not have the time/resources; and (3) lack of available funding. The top facilitators identified were as follows: (1) development of a mechanism/program to fund AYAC survivorship research studies; (2) in-person or virtual investigator engagement between children's hospitals and adult cancer centers to discuss research studies; and (3) developing personalized survivorship goals with AYAC patients and survivors to facilitate enrollment into survivorship studies. Conclusion: Experts identified the lack of time, manpower, funding, and resources as major barriers in AYAC survivorship research. Enhancing communication and collaboration with different stakeholders may facilitate AYAC survivorship research efforts within the SoCal region.

Malignant A-to-I RNA editing by ADAR1 drives T-cell acute lymphoblastic leukemia relapse via attenuating dsRNA sensing

Leukemia initiating cells (LICs) are regarded as the origin of leukemia relapse and therapeutic resistance. Identifying direct stemness determinants that fuel LIC self-renewal is critical for developing targeted approaches to eliminate LICs and prevent relapse. Here, we show that the RNA editing enzyme ADAR1 is a crucial stemness factor that promotes LIC self-renewal by attenuating aberrant double-stranded RNA (dsRNA) sensing. Elevated adenosine-to-inosine (A-to-I) editing is a common attribute of relapsed T-ALL regardless of molecular subtypes. Consequently, knockdown of ADAR1 severely inhibits LIC self-renewal capacity and prolongs survival in T-ALL PDX models. Mechanistically, ADAR1 directs hyper-editing of immunogenic dsRNA and retains unedited nuclear dsRNA to avoid detection by the innate immune sensor MDA5. Moreover, we uncovered that the cell intrinsic level of MDA5 dictates the dependency on ADAR1-MDA5 axis in T-ALL. Collectively, our results show that ADAR1 functions as a self-renewal factor that limits the sensing of endogenous dsRNA. Thus, targeting ADAR1 presents a safe and effective therapeutic strategy for eliminating T-ALL LICs.

Tumor Genomic Profiling and Ex Vivo Drug Sensitivity Testing for Pediatric Leukemia and Lymphoma Patients

OBJECTIVE

To describe the frequency of use of tumor genomic profiling and functional ex vivo drug sensitivity testing in pediatric patients with hematologic malignancies at our institution, and to determine how the results affected treatment selection.

METHODS

A retrospective chart review was conducted to analyze the frequency of tumor genomic profiling and functional drug sensitivity screening in our institution in pediatric patients with hematologic malignancies and to ask if the results were used to direct treatment. A case series of patients for whom these testing recommendations resulted in therapeutic interventions is reported.

RESULTS

Thirty-three patients underwent tumor genomic profiling assays, functional ex vivo testing, or both. Nineteen patients (58%) had genomic profiling assays performed alone, 3 (9%) had functional ex vivo testing performed alone, and 11 (33%) had both tests performed. Twenty-one (64%) patients had potentially actionable mutations detected by the genomic profiling assay. Seven (21%) patients received at least 1 chemotherapeutic agent in accordance with the tumor genomic profiling or functional ex vivo drug sensitivity testing results. Three (43%) of the 7 patients who were treated with testing directed therapy had a favorable treatment response (PR or CR) to treatments selected based upon results of genomic or functional ex vivo testing.

CONCLUSIONS

This retrospective case series demonstrates that precision medicine techniques such as genomic profiling and drug sensitivity testing can positively inform treatment selection in pediatric patients with relapsed or refractory leukemia and lymphoma.

Improving vitamin D testing and supplementation in children with newly diagnosed cancer: A quality improvement initiative at Rady Children's Hospital San Diego

BACKGROUND

Vitamin D deficiency and insufficiency have been associated with poorer health outcomes. Children with cancer are at high risk for vitamin D deficiency and insufficiency. At our institution, we identified high variability in vitamin D testing and supplementation in this population. Of those tested, 65% were vitamin D deficient/insufficient. We conducted a quality improvement (QI) initiative with aim to improve vitamin D testing and supplementation among children aged 2-18 years with newly diagnosed cancer to ≥80% over 6 months.

METHODS

An inter-professional team reviewed baseline data, then developed and implemented interventions using Plan-Do-Study-Act (PDSA) cycles. Barriers were identified using QI tools, including lack of automated triggers for testing and inconsistent supplementation criteria and follow-up testing post supplementation. Interventions included an institutional vitamin D guideline, clinical decision-making tree for vitamin D deficiency, insufficiency and sufficiency, electronic medical record triggers, and automated testing options.

RESULTS

Baseline: N = 26 patients, four (15%) had baseline vitamin D testing; two (8%) received appropriate supplementation. Postintervention: N = 33 patients; 32 (97%) had baseline vitamin D testing; 33 (100%) received appropriate supplementation and completed follow-up testing timely (6-8 weeks post supplementation). Change was sustained over 24 months.

CONCLUSIONS

We achieved and sustained our aim for vitamin D testing and supplementation in children with newly diagnosed cancer through inter-professional collaboration of hematology/oncology, endocrinology, hospital medicine, pharmacy, nursing, and information technology. Future PDSA cycles will address patient compliance with vitamin D supplementation and impact on patients' vitamin D levels.

To test or not to test: genetic cancer predisposition testing in paediatric patients with cancer

Genetic cancer predisposition testing in the paediatric population poses unique ethical dilemmas. Using the hypothetical example of a teenager with cancer with a high probability of having an underlying cancer predisposition syndrome, we discuss the ethical considerations that affect the decision-making process. Because legally these decisions are made by parents, genetic testing in paediatrics can remove a child's autonomy to preserve his or her own 'open future'. However, knowledge of results confirming a predisposition syndrome can potentially be beneficial in modifying treatment and surveillance plans and enabling at-risk family members to obtain cascade testing for themselves. Considering virtue ethics to envision the best characters of the patient, parents and healthcare providers can guide them to the better choice to test or not to test, with the ultimate goal of achieving the best outcome for survival and eudaimonia, human flourishing reliably sought out.

Population pharmacokinetic analysis of high-dose methotrexate in pediatric and adult oncology patients

PURPOSE

High-dose methotrexate (HD-MTX) is widely used in pediatric and adult oncology treatment regimens. This study aimed to develop a population pharmacokinetic model to characterize pediatric and adult MTX exposure across various disease types and dosing regimens, and to evaluate exposure-toxicity relationships.

METHODS

MTX pharmacokinetic data from pediatric and adult patients were collected. A population pharmacokinetic model was developed to determine the effects of age, liver function, renal function, and demographics on MTX disposition. The final model was used in Monte Carlo simulations to generate expected exposures for different dosing regimens. The association of toxicity, determined through chart review, and MTX area under the curve (AUC) was modeled using logistic regression.

RESULTS

The analysis included 5116 MTX concentrations from 320 patients (135 adult, age 19-79 years; 185 pediatric, age 0.6-19 years). Estimated glomerular filtration rate (eGFR) and treatment cycle number were independent predictors of clearance (CL). CL varied 2.1-fold over the range of study eGFR values and increased 14% for treatment cycle numbers greater than 7. Higher MTX AUC was associated with higher risk of nephrotoxicity in adults, and neurotoxicity and hepatotoxicity in pediatrics.

CONCLUSIONS

This study represents one of the most comprehensive evaluations of HD-MTX PK across a wide range of ages and disease types. After accounting for differences in renal function, age did not impact CL, although toxicity patterns differed by age. The model allows for early identification of patients with slowed MTX clearance and at higher risk of toxicity.

'I just need an opiate refill to get me through the weekend'

In this article, we discuss the ethical dimensions for the prescribing behaviours of opioids for a chronic pain patient, a scenario commonly witnessed by many physicians. The opioid epidemic in the USA and Canada is well known, existing since the late 1990s, and individuals are suffering and dying as a result of the easy availability of prescription opioids. More recently, this problem has been seen outside of North America affecting individuals at similar rates in Australia and Europe. We argue that physicians are also confronted with an ethical crisis where a capitalist-consumerist society is contributing to this opioid crisis in which societal, legal and business interests push physicians to overprescribe opioids. Individual physicians often find themselves unequipped and unsupported in attempts to curb the prescribing of opioid medications and balance competing goals of alleviating pain against the judicious use of pain medications. Physicians, individually and as a community, must reclaim the ethical mantle of our profession, through a more nuanced understanding of autonomy and beneficence. Furthermore, physicians and the medical community at large have a fiduciary duty to patients and society to play a more active role in curbing the widespread distribution of opioids in our communities.

MLL rearranged acute lymphoblastic leukaemia presenting as a maxillary sinus mass with a discordant immunophenotypic profile from the bone marrow

We describe an unusual case of pre-B lymphoblastic leukaemia presenting with a unilateral maxillary sinus mass in which biopsies of the primary mass and the bone marrow demonstrated conflicting immunophenotyping results. The extramedullary mass was consistent with a precursor B-cell malignancy, while the bone marrow was initially reported as a possible mature B-cell malignancy. The treatments for the two are fundamentally different, which necessitated a delay in the initiation of his chemotherapy until a clear diagnosis was made. Mixed lineage leukaemia gene rearrangement was confirmed by fluorescence in situ hybridisation in both the primary mass and bone marrow, which unified the diagnosis as pre-B acute lymphoblastic leukaemia given the common cytogenetic feature.

Severe Neuropathic Pain With Concomitant Administration of Vincristine and Posaconazole

Vincristine is a chemotherapeutic agent with a potential toxicity of sensorimotor peripheral neuropathy. Patients receiving chemotherapy are in an immunocompromised state and may require antifungal agents. Triazole antifungals are known inhibitors of cytochrome P450 (CYP) enzymes. Vincristine is a known CYP3A4 and CYP3A5 substrate, and concomitant administration with fluconazole or voriconazole has been reported to increase vincristine toxicity and peripheral neuropathy, but there is limited literature on posaconazole in this regard. This 5-year-old girl with pre-B-cell acute lymphoblastic leukemia received vincristine while receiving posaconazole for a mucormycosis infection and developed unexpectedly severe peripheral neuropathy. After recovery, the child continued on mucormycosis prophylaxis with posaconazole with instructions to hold for 2 days before and on the day of vincristine administration. This case illustrates the potentiating effect that posaconazole had on vincristine-associated neurotoxicity, and our approach to mitigating that negative interaction.

Challenges of assessing response to therapy in non-Hodgkin's lymphoma of the bone

A 17-year-old male adolescent was diagnosed with diffuse large B cell lymphoma arising from the right humeral head. The lymphoma was found to be isolated to the bone, with a very small extraosseous component. After completion of a standard chemotherapy regimen, the Positron Emission Tomography-Computed Tomography (PET-CT) became PET negative but the CT and MRI appeared mostly unchanged in appearance, suggesting refractory disease. Repeat biopsy confirmed that no lymphoma remained, and he was in complete remission. Three months after completion of therapy, MRI continued to be abnormal. This case illustrates the unique challenges posed by the imaging characteristics of bone lymphoma, both during and after therapy. Biopsy is definitive but causes additional morbidity and may not be necessary. Imaging is done routinely to assess response to therapy, as with this patient, but in lymphomas of the bone imaging can be misleading and can lead to unnecessary procedures or follow-up imaging.

Treatment of immune thrombocytopenic purpura associated with cytomegalovirus infection in a child with pre-B cell acute lymphoblastic leukaemia after central nervous system relapse

A 13-year-old male patient with a history of pre-B cell acute lymphoblastic leukaemia (ALL) with isolated central nervous system relapse on maintenance chemotherapy presented with severe thrombocytopenia refractory to platelet transfusions. The patient showed only modest responses to two courses of intravenous immunoglobulin and steroids. He was found to be positive for cytomegalovirus (CMV) with modest viral load. His thrombocytopenia normalised with rituximab therapy and CMV treatment supporting the diagnosis of CMV-associated immune thrombocytopenic purpura (ITP). Following treatment, the patient continued to have a stable platelet count well above the threshold for transfusions. He continued to be intermittently treated for CMV when viral loads became detectable. This report discusses the unique management considerations of ITP in a patient undergoing therapy for ALL with a review of previously reported cases and discusses the possibility of CMV viraemia as a modulating factor.

"Please amputate my child's arms"

Jeremy sustained bilateral complete brachial plexus injuries in an auto collision on an icy road a month before his third birthday. The accident rendered both upper extremities completely flail and insensate: he has no motor or sensory function of his shoulders, elbows, wrists, or digits. Jeremy does, however, have normal function of the lower extremities. Physical therapists have worked with the child for over a year with no noted improvement in arm function. Jeremy falls frequently, causing injury to his face and head, and occasionally, his arms get twisted or caught in his crib and his fingers turn blue. Jeremy's mother, who carries the main responsibility for his daily care, believes that his insensate arms are too heavy and "get in his way," causing the falls. She and Jeremy's father present to the orthopedic clinic at the children's hospital with the request of having both arms amputated. The primary orthopedic surgeon and the orthopedic team disagree with the parents that bilateral upper-extremity amputation offers any medical benefit, but Jeremy's mother tells the surgeon that, if he will not perform the surgery, her family will find a doctor who will. The surgeon, who feels ethically distressed by the parental insistence on this amputation in such a young child, requests an ethics consultation.

Persistent sacral chloroma in refractory acute myelogenous leukaemia

Acute myelogenous leukaemia (AML) is a clonal process involving the myeloid subgroup of white blood cells. Chloromas, or myeloid sarcomas, are masses of myeloid leukaemic cells and are a unique aspect of AML. This case involves a 14-year-old boy with AML who presented with multiple chloromas at diagnosis. The patient's extra-calvarial masses and bone marrow involvement responded to chemotherapy; however, his sacral epidural chloromas persisted despite four courses of chemotherapy. The central nervous system, bone marrow and testes have been known to be sanctuary sites for AML. This case illustrates that the sacral spinal canal may potentially be a sanctuary site for the disease process also.

Severe medication-induced peripheral neuropathy treated with topical doxepin cream in a paediatric patient with leukaemia

A 17-year-old female with recently relapsed acute lymphoblastic leukaemia and a treatment course complicated by rhinocerebral mucormycosis infection developed severe peripheral neuropathy during the treatment for mucormycosis infection. This was felt to be a medication side effect. Her peripheral neuropathy was refractory to many well-established treatments, but ultimately responded dramatically and consistently to a novel therapy, topical doxepin cream (5%). This case report is the first published report of the application of topical doxepin cream for treatment of peripheral neuropathy in a paediatric patient.

Fever without a source: evolution of a diagnosis from Kawasaki disease to acute myelogenous leukaemia

A previously healthy 11-month-old male patient presented with fever, abdominal pain and irritability. As part of an extensive evaluation for the cause of his fevers, an echocardiogram was performed and showed mildly dilated coronary arteries, leading to a diagnosis of incomplete Kawasaki disease (KD). He was treated with intravenous immunoglobulin (IVIG), defervesced and was discharged home. Two weeks later, he presented with anaemia initially attributed to haemolytic anaemia secondary to IVIG and received a red blood cell transfusion. However, his anaemia recurred 2  weeks later with leucocytosis, prompting a bone marrow aspirate 4  weeks after his diagnosis of KD. This demonstrated acute myelogenous leukaemia most consistent with acute megakaryocytic leukaemia. This case highlights the potentially subtle presentation of acute leukaemia and the need to keep an open mind and reconsider the initial diagnosis as new information comes to light in the care of an ill child.

Inadvertently boarding a pirate ship: disease progression in a paediatric patient with relapsed metastatic Ewing sarcoma receiving treatment at a centre for alternative therapy in Mexico

Complementary and alternative medicine (CAM) therapies are commonly incorporated into the care of patients with paediatric cancer. Many modalities are safe and effective during cancer treatment and have proved beneficial for symptom relief and quality of life. However, situations where alternative therapy is provided without allopathic medical care supportive care resources can pose a safety risk to patients. This report describes the case of a 16-year-old Chinese girl with metastatic Ewing sarcoma who sought treatment with alternative treatment in Mexico. When her disease progressed with an ensuing significant loss of function, the centre personnel were unable to respond to her acute deterioration or provide necessary medical care. This resulted in her being stranded in a foreign country paralysed, isolated, and with large unanticipated financial expenditures.

Proton transfer in catalysis by fumarase

Using 3T[14C]malate it was possible to show intermolecular T-transfer to unlabeled fumarate. The rate of dissociation of ET derived from the malate was not rapid, only about as fast as required for KMcat. Because of the slow dissociation of ET derived from T-malate, the awkward complex ET-malate is readily formed. As shown by the effect of added malate on the partition of ET, otherwise captured by fumarate, ET.malate must be functional. Its rate of dissociation to E.M determines the V/Km value of malate. Hydrogen dissociation of the complex ET.F was linearly related to the concentration and basicity of the buffer provided, varying from < 10% to > 60% of the overall rate with alkyl phosphonates. Partition of EH.F to free malate or fumarate occurs in a ratio approximately 2:1 at both low and high buffer. This agrees well with the comparison of the equilibrium exchange rates: malate with [18O]water to malate with [14C]-fumarate [Hansen, J.N., Dinovo, E.C., & Boyer, P.D. (1969) J. Biol. Chem. 244, 6270-6279]. Therefore, the abstracted hydroxyl group is fully exchanged from the enzyme when the bound hydrogen and fumarate return to malate and must be much more accessible to the medium than the abstracted proton. The fact that buffer increases the rate of proton transfer to the medium in the central complex makes it appear that a proton relay connects the active site donor with a remote site that interfaces with the ultimate proton source, water.

Role of CO2 in proton activation by histidine decarboxylase (pyruvoyl)

The amino acid decarboxylases that use an intrinsic pyruvoyl cofactor have been viewed in terms of the pyridoxal-P paradigm whereby a Schiff base is formed between the enzyme-bound cofactor and the substrate, setting up a cation sink for electrons of the C alpha-CO2- bond, ejecting CO2, and the reversal of these steps with a proton with overall retention stereochemistry. With histidine decarboxylase (pyruvoyl) it is found that the presence of CO2 is required for T-exchange between histamine and water. Since the forward reaction including formation of the C-H bond does not require added CO2, it might be assumed that the CO2 that is formed in the fragmentation step is retained by the enzyme perhaps to assist in proton transfer. No such requirement for CO2 has been reported for the pyridoxal-P-dependent decarboxylases which are generally thought to liberate CO2 prior to proton transfer. In seeking a connection between bound CO2 and proton transfer in the histidine decarboxylase reaction, one is reminded of the carboxybiotin enzymes also known for an invariant stereochemistry of retention and for the requirement that the biotin be in the carboxylated form for H-exchange to occur. Perhaps the bound CO2 of histidine decarboxylase forms a carbamate by addition to Lys155 or to an amide group of the active site. The new carboxy group could then be the vehicle for protonating the carbon from which it originated, giving overall retention of the stereochemistry at the alpha-C.(ABSTRACT TRUNCATED AT 250 WORDS)

A rate-determining proton relay in the pyruvate kinase reaction

This study ascribes the large steady-state D2O isotope effect on kcat of pyruvate kinase (PEP + ADP----pyruvate + ATP) to the reprotonation of the product form of the enzyme for use in forming pyruvate. Previous tritium trapping experiments [Rose, I. A., & Kuo, D. J. (1989) Biochemistry 28, 9579-9585] with muscle pyruvate kinase showed that the proton used for ketonization of enolpyruvate is derived from an enzyme "pool" that contains three kinetically equivalent hydrogens that could be trapped in a nontritiated "chase" medium by high levels of ADP and PEP. The exchange of this pool with the medium was rapid in the free enzyme (approximately 1400 s-1), prior to addition of PEP, and apparently much less in the completed complex. The dissociation rate constant was determined by using the dissociation-competition equation koffT = K1/2kcat/Km, where kcat/Km is the steady-state parameter for PEP and K1/2 is the concentration of PEP required to trap 50% of the isotope that could be trapped. The present study shows that the competition constant, K1/2, is decreased by approximately 5-fold in D2O, the same effect see on kcat under conditions where kcat/Km, measured in the steady state, is not changed. The common effect of D2O on kcat in the steady state and koffT in pulse/chase suggests that the forward reaction rate is determined by hydrogen transfer to the enzyme. Further evidence indicates that the kinetically important proton in question is the proton used for ketonization of enolpyruvate, the substrate proton.(ABSTRACT TRUNCATED AT 250 WORDS)

The substrate proton of the pyruvate kinase reaction

The pyruvate kinase reaction occurs in separate phosphate- and proton-transfer stages: (formula; see text) K+, Mg2+, and Mg.ADP are known to be required for the phosphoryl transfer step, and K+ and Mg2+ with allosteric stimulation by MgATP are important for proton transfer. This paper uses the isotope trapping method with 3H-labeled water to identify the proton donor and determine when in the sequence of the catalytic cycle it is generated. When the enzyme was allowed to exchange briefly with 3H2O (pulse phase) and then diluted into a mixture containing PEP, ADP, and the cofactor K+, Mg2+, or Co2+ in D2O (chase phase), an amount of [3H]pyruvate was formed in great excess of the amount expected from steady-state catalysis in the diluted 3H-labeled water. With K+, Mg2+, and ADP at pH 6-9.5 in the pulse phase, a limit of 1.25 enzyme equiv of 3H were trapped. The concentration of PEP required for half-maximum trapping was 14-fold greater than its steady-state Km. Therefore, the rate constant for dissociation of the donor proton is estimated to be 14 times the steady-state rate of [3H]pyruvate formation, approximately 109 s-1, or 1500 s-1. At pD 6.4, Mg2+ and ADP were required in the chase, indicating that the ADP in the pulse was not bound tightly enough to be used in the chase. At pD 9.4, ADP was not required in the chase, only Mg2+ or Co2+, making it possible to limit the chase to one turnover from hybrid labeled complexes such as E.K.Mg.CoADP or E.K.Co.MgADP and PEP.(ABSTRACT TRUNCATED AT 250 WORDS)

Aconitase: its source of catalytic protons

An ordinary isotope partition experiment was performed to determine the rate of dissociation of the proton from the donor site for the hydration of cis-aconitate. Aconitase in [3H]water was efficiently diluted into well-mixed solutions of cis-aconitate. Citrate and isocitrate that were formed within 2 s were more heavily labeled than could be explained by consideration of an isotope effect in the processing of one proton per enzyme equivalent. Control experiments indicate that mixing was much more rapid than catalytic turnover, ruling out incompletely diluted [3H]water as a significant isotope source. Therefore, it appears that significantly more than one enzyme-bound tritium atom (protons) must have been used in the course of the multiple turnover of the enzyme after the dilution was complete. Isotope incorporation reached values in excess of four proton equivalents as a limit with simple Michaelis dependence on cis-aconitate. From the half-saturation concentration value for trapping, 0.15 mM, the t 1/2 for exchange of each of these protons with solvent appears to be approximately 0.1 s at 0 degrees C. The large number of protons trapped seems to suggest the existence of a structurally stabilized pool of protons, or water, that communicates between the active site base and the medium in the hydration of cis-aconitate. The proton abstracted in the dehydration of [3H]citrate is transferred directly to undissociated cis-aconitate to form isocitrate without dilution, or cis-aconitate having dissociated, the tritium passes to the medium, presumably through the pool of bound protons indicated above. All of the citrate-derived protons can be found in isocitrate if cis-aconitate is added in sufficient concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentration and partitioning of intermediates in the fructose bisphosphate aldolase reaction. Comparison of the muscle and liver enzymes

Chemical analysis of enzyme reaction intermediates has been used to compare the liver and muscle isozymes of rabbit aldolase at equilibrium and in their steady states to determine if they have properties that favor the direction of flow of glycolytic intermediates in their tissues of origin. For both enzymes at saturating concentrations of fructose 1,6-P2, the sum of intermediates in the steady state agreed with the total active enzyme calculated to be present. The two half-reactions, characterized by fructose 1,6-bisphosphate(Fru-P2):aldehyde exchange and DHAP:proton exchange were found to be of different importance in determining the rate of reaction with Fru-P2 with the liver enzyme being much more limited in the processing of DHAP. The chemical interconversions within each half-reaction are generally rapid compared with the release of products. The greater sensitivity of liver aldolase to inhibition by aldehydes in Fru-P2 cleavage seems to be a normal consequence of the higher level of the eneamine of DHAP in the forward steady state with the liver enzyme and probably should not be ascribed to a greater intrinsic affinity. An earlier report (Grazi, E., and Trombetta, G. (1979) Eur. J. Biochem. 100, 197-202) purporting to show a special interaction of glyceraldehyde-3-P with liver enzyme prior to proton abstraction from DHAP could not be reproduced. Examples are presented from the data that validate the use of the analytical methods used for analysis of intermediates in the case of the Schiff's base aldolases.

Resolution of brewers' yeast pyruvate decarboxylase into two isozymes

A novel purification method was developed for brewers' yeast pyruvate decarboxylase (EC 4.1.1.1) that for the first time resolved the enzyme into two isozymes on DEAE-Sephadex chromatography. The isozymes were found to be distinct according to sodium dodecyl sulfate polyacrylamide gel electrophoresis: the first one to be eluted gave rise to one band, the second to two bands. The isozymes were virtually the same so far as specific activity, KM, inhibition kinetics and irreversible binding properties by the mechanism-based inhibitor (E)-4-(4-chlorophenyl)-2-oxo-3-butenoic acid are concerned. This finding resolves a longstanding controversy concerning the quaternary structure of this enzyme.

Chemical trapping of complexes of dihydroxyacetone phosphate with muscle fructose-1,6-bisphosphate aldolase

Dihydroxyacetone phosphate (DHAP) in equilibrium with FDP aldolase of muscle is present in the form of two major covalent complexes. One, representing approximately 60% of total bound substrate, decomposes to Pi and methylglyoxal upon acid denaturation of the enzyme as first reported by Grazi and Trombetta [Grazi, E., & Trombetta, G. (1979) Biochem. J. 175, 361-365]. This is now shown to be the enzyme-eneamine phosphate reaction intermediate since Pi formation is prevented if the acid denaturation is done in the presence of potassium ferricyanide, an oxidant of the eneamine. The enzyme-eneamine aldehyde X Pi 6, presumed to be an intermediate of the slow methylglyoxal synthetase reaction of aldolase, must not be a significant source of the Pi produced upon denaturation and is probably not a significant component of the equilibrium. The oxidation product, the enzyme-imine of phosphopyruvaldehyde, is sufficiently stable in 1 N HCl, t1/2 = 76 min at 0 degree C, to be isolated with the trichloroacetic acid precipitated protein. A second covalent complex, approximately 20-24% of bound dihydroxyacetone [32P]phosphate, remains with the protein during acid denaturation and centrifugation. This acid-stable complex is formed rapidly and is chased rapidly by unlabeled substrate. Its stability in 1 N HCl is similar to that of the ferricyanide-oxidized derivative mentioned above. From this and its reactivity with cyanoborohydride in acid, this complex is thought to be the imine adduct of DHAP with aldolase 4 and/or the carbinolamine complex 3 present in the initial equilibrium. D-Glyceraldehyde 3-phosphate in the carbonyl form also forms an acid-precipitable complex with aldolase.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct spectroscopic observation of a brewer's yeast pyruvate decarboxylase-bound enamine intermediate produced from a suicide substrate. Evidence for nonconcerted decarboxylation

The conjugated alpha-keto acid (E)-4-(4-chlorophenyl)-2-oxo-3-butenoic acid, a suicide substrate (Kuo, D. J., and Jordan, F. (1983) Biochemistry 22, 3735-3740), when reacted with brewer's yeast pyruvate decarboxylase (EC 4.1.1.1), was found to produce a new absorption band centered at 440 nm. The band was attributed to the formation of a thiamindiphosphate-bound intermediate produced upon decarboxylation and was not observed in the absence of either thiamindiphosphate or apoprotein. Simultaneously, with the appearance of the spectral band, the enzyme was inactivated irreversibly. The combined evidence suggested that the spectral band pertains to an enzyme-bound conjugated enamine and the results constitute the first direct observation of such an intermediate in any thiamindiphosphate requiring enzymatic reaction. It could be concluded that for the alpha-keto acid employed, CO2 loss and C-protonation of the enamine take place in a stepwise, rather than concerted, manner and that the steps culminating in CO2 loss are faster overall than the subsequent steps that lead to the release of the product. The observation of stoichiometric concentration of enzyme-bound enamine intermediate supports the previous suggestion that alpha-hydroxyethyl thiamindiphosphate is not the true intermediate, but rather the enamine is (Ullrich, J., and Mannshreck, A. (1967) Eur. J. Biochem 1, 110-116).

Active site directed irreversible inactivation of brewers' yeast pyruvate decarboxylase by the conjugated substrate analogue (E)-4-(4-chlorophenyl)-2-oxo-3-butenoic acid: development of a suicide substrate

(E)-4-(4-Chlorophenyl)-2-oxo-3-butenoic acid (CPB) was found to irreversibly inactivate brewers' yeast pyruvate decarboxylase (PDC, EC 4.1.1.1) in a biphasic, sigmoidal manner, as is found for the kinetic behavior of substrate. An expression was derived for two-site irreversible inhibition of allosteric enzymes, and the kinetic behavior of CPB fit the expression for two-site binding. The calculated Ki's of 0.7 mM and 0.3 mM for CPB were assigned to the catalytic site and the regulatory site, respectively. The presence of pyruvic acid at high concentrations protected PDC from inactivation, whereas low concentrations of pyruvic acid accelerated inactivation by CPB. Pyruvamide, a known allosteric activator of PDC, was found to enhance inactivation by CPB. The results can be explained if pyruvamide binds only to a regulatory site, but CPB and pyruvic acid compete for both the regulatory and the catalytic centers. [1-14C]CPB was found to lose 14CO2 concurrently with the inactivation of the enzyme. Therefore, CPB was being turned over by PDC, in addition to inactivating it. CPB can be labeled a suicide-type inactivator for PDC.