Dose-Related Effect of Chemotherapy on Bone Mineral Density Among Pediatric Acute Lymphoblastic Leukemia Survivors

OBJECTIVES

Reduced bone mineral density (BMD) can negatively affect lifelong skeletal health by -increasing the risk for developing osteopenia and osteoporosis. This study evaluated the relationship between BMD and cumulative doses of intravenous (IV) methotrexate (MTX) and glucocorticoids in pediatric acute lymphoblastic leukemia (ALL) survivors. The association between BMD and vitamin D concentrations measured at the time of entry into the long-term follow-up program was also assessed.

METHODS

This retrospective study included pediatric ALL survivors who had received a dual-energy X-ray absorptiometry (DXA) scan after the end of therapy (EOT) or within the 6 months prior to the EOT. Low/-intermediate and high cumulative IV MTX doses were defined as doses less than 20,000 mg/m2 and -greater than or equal to 20,000 mg/m2, respectively. Descriptive statistics, Student t test, and linear -regression were used to analyze the data.

RESULTS

A total of 62 patients, with 34 patients in the low/intermediate and 28 patients in the high -cumulative IV MTX dose groups, were analyzed. The median time from EOT to DXA scan was 2.3 years. The mean DXA lumbar spine z score was significantly lower in the high cumulative IV MTX dose group -compared with the low/intermediate dose group (-0.86 vs -0.14; p = 0.008). Cumulative glucocorticoid doses and vitamin D concentrations were not associated with BMD.

CONCLUSIONS

Pediatric patients who had received cumulative IV MTX doses of greater than or equal to 20,000 mg/m2 during their ALL treatment had lower BMD than those who had received lower cumulative doses.

Genome-Wide Association Study of Pregnancy in Childhood Cancer Survivors: A Report from the Childhood Cancer Survivor Study

BACKGROUND

Gonadotoxic treatment-related infertility has a significant impact on quality of life in childhood cancer survivors. Genome-wide association analyses to delineate the risk of infertility in childhood cancer survivors have not been previously reported.

METHODS

Leveraging genotype data from a large survivor cohort, the Childhood Cancer Survivor Study (CCSS), we investigated the role of SNPs on future pregnancy or siring a pregnancy in survivors without pelvic, testicular, or brain radiation who had ever been married. We calculated sex-stratified hazard ratios, using Cox proportional hazards modeling, adjusting for birth cohort (before 1965 vs. 1965 or later) and doses of relevant chemotherapies; replication was attempted in the independent St. Jude Lifetime Cohort study (SJLIFE).

RESULTS

In the CCSS cohort, nine SNPs were found to be suggestive (P < 10-7) or statistically significantly (P < 5 × 10-8) associated with pregnancy, however, none of the SNPs were replicated in SJLIFE. Cohorts differed based on the overall pregnancy rate, frequency of sterilizing procedures, and birth cohort.

CONCLUSIONS

We were not able to replicate our findings of SNPs associated with pregnancy in childhood cancer survivors.

IMPACT

Future attempts at replication should be considered in cohorts treated in a comparable era. In addition, understanding the role of genetics in fertility in childhood cancer survivors may be better approached using more advanced sequencing techniques.

Southern California Pediatric and Adolescent Cancer Survivorship (SC-PACS): Establishing a Multi-Institutional Childhood, Adolescent, and Young Adult Cancer Survivorship Consortium in Southern California

Introduction

Given their risk for late effects and early mortality, childhood/adolescent cancer survivors (CACSs) should receive longitudinal monitoring and care. The Southern California Pediatric and Adolescent Cancer Survivorship (SC-PACS) consortium was established in February 2017 to combine resources and expertise across seven participating survivorship programs. Its over-arching objective is to address the unique needs of its demographically diverse CACS population through collaborative survivorship research and care initiatives. The first SC-PACS study was an assessment of survivorship needs and evaluation of current services as reported by CACSs and their parents/primary care givers (PPCGs) receiving survivorship care at consortium sites.

Methods

As an initial investigation, a cross-sectional survey for CACSs and their parents/primary care givers was conducted. The goal was to enroll 10 CACSs and 10 PPCGs from each of the seven institutions (total of 140 participants). The eligibility criteria for CACSs were age ≥13 years at the time of enrollment, >2 years from the end of treatment, sufficient cognitive function to complete the survey, and English or Spanish language proficiency. For CACSs <13 years old, their PPCGs completed the survey. This was a convenience sample using frequencies and proportions to describe participant characteristics and survey responses, which were entered into a Research Electronic Data Capture (REDCap) database.

Results

Across the consortium, of the recruitment target of 140 participants (CACSs, n=70; PPCGs, n=70), 127 (90.7%) participants were enrolled. Of the 127 participants enrolled, 65 (51.2%) were CACSs and 62 (48.8%) were PPCGs. The majority of participants were female (51.2%), were Hispanic (62.2%), spoke English as the primary language at home (57.5%), and were diagnosed between one to four years of age (45.7%). Information considered most important by both CACSs and PPCGs was related to cancer diagnosis (90.8%) and future risks as a result of cancer treatment received (98.0%). Overall, 78% of CACSs and PPCGs found the survivorship information (treatment summary) useful, and 83% felt that they received the right amount of information about their cancer.

Conclusion

Our aim was to obtain baseline data that would characterize our CACS population, inform consortium priorities, and establish a collaborative research platform. The ultimate goal of the consortium is to develop a comprehensive survivorship care approach that addresses the most important needs of cancer survivors in our catchment area and promotes best practice interventions. Future plans are to expand the needs assessment survey to obtain a wider representation of the survivor population at SC-PACS institutions, helping create strategies to improve cancer-specific education, delivery of treatment summary, and access to community resources for this demographically and socioeconomically diverse population.

The Application of Next-generation Sequencing Tumor Molecular Profiling in the Diagnosis and Management of a Case of Acute Myelogenous Leukemia With MLL-PTD in a Pediatric Heart Transplant Recipient

A 6-year-old girl with a history of heart transplantation was diagnosed with myelodysplastic syndrome, which progressed to acute myelogenous leukemia. Comprehensive genomic profiling of her tumor discovered an MLL-PTD (partial tandem duplication) and she received chemotherapy and a hematopoietic stem cell transplant (HSCT). She subsequently relapsed and tumor molecular profiling was repeated, revealing 2 new potentially targetable mutations (FLT3 and IDH2). A novel treatment regimen targeting these mutations with sorafenib and azacitidine without using cytotoxic chemotherapy produced remission and she subsequently pursued a second HSCT. She remains disease-free 17 months after HSCT. This case report demonstrates how repeated tumor molecular profiling provided novel actionable information for the diagnosis and management at 2 timepoints.

Molecular and phenotypic diversity of CBL-mutated juvenile myelomonocytic leukemia

Mutations in the CBL gene were first identified in adults with various myeloid malignancies. Some patients with juvenile myelomonocytic leukemia (JMML) were also noted to harbor mutations in CBL, but were found to have generally less aggressive disease courses compared to patients with other forms of Ras pathway-mutant JMML. Importantly, and in contrast to most reports in adults, the majority of CBL mutations in JMML patients are germline with acquired uniparental disomy occurring in affected marrow cells. Here, we systematically studied a large cohort of 33 JMML patients with CBL mutations and found that this disease is highly diverse in presentation and overall outcome. Moreover, we discovered somatically acquired CBL mutations in 15% of pediatric patients who presented with more aggressive disease. Neither clinical features nor methylation profiling were able to distinguish patients with somatic CBL mutations from those with germline CBL mutations, highlighting the need for germline testing. Overall, we demonstrate that disease courses are quite heterogeneous even among patients with germline CBL mutations. Prospective clinical trials are warranted to find ideal treatment strategies for this diverse cohort of patients.

Real-world data from a molecular tumor board demonstrates improved outcomes with a precision N-of-One strategy

Next-generation sequencing (NGS) can identify novel cancer targets. However, interpreting the molecular findings and accessing drugs/clinical trials is challenging. Furthermore, many tumors show resistance to monotherapies. To implement a precision strategy, we initiated a multidisciplinary (basic/translational/clinical investigators, bioinformaticians, geneticists, and physicians from multiple specialties) molecular tumor board (MTB), which included a project manager to facilitate obtaining clinical-grade biomarkers (blood/tissue NGS, specific immunohistochemistry/RNA expression including for immune-biomarkers, per physician discretion) and medication-acquisition specialists/clinical trial coordinators/navigators to assist with medication access. The MTB comprehensively reviewed patient characteristics to develop N-of-One treatments implemented by the treating physician's direction under the auspices of a master protocol. Overall, 265/429 therapy-evaluable patients (62%) were matched to ≥1 recommended drug. Eighty-six patients (20%) matched to all drugs recommended by MTB, including combinatorial approaches, while 38% received physician's choice regimen, generally with unmatched approach/low degree of matching. Our results show that patients who receive MTB-recommended regimens (versus physician choice) have significantly longer progression-free (PFS) and overall survival (OS), and are better matched to therapy. High (≥50%) versus low (<50%) Matching Score therapy (roughly reflecting therapy matched to ≥50% versus <50% of alterations) independently correlates with longer PFS (hazard ratio [HR], 0.63; 95% confidence interval [CI], 0.50-0.80; P < 0.001) and OS (HR, 0.67; 95% CI, 0.50-0.90; P = 0.007) and higher stable disease ≥6 months/partial/complete remission rate (52.1% versus 30.4% P < 0.001) (all multivariate). In conclusion, patients who receive MTB-based therapy are better matched to their genomic alterations, and the degree of matching is an independent predictor of improved oncologic outcomes including survival.

Safety, pharmacokinetics, and pharmacodynamics of panobinostat in children, adolescents, and young adults with relapsed acute myeloid leukemia

BACKGROUND

Novel therapies are urgently needed for pediatric patients with relapsed acute myeloid leukemia (AML).

METHODS

To determine whether the histone deacetylase inhibitor panobinostat could be safely given in combination with intensive chemotherapy, a phase 1 trial was performed in which 17 pediatric patients with relapsed or refractory AML received panobinostat (10, 15, or 20 mg/m² ) before and in combination with fludarabine and cytarabine.

RESULTS

All dose levels were tolerated, with no dose-limiting toxicities observed at any dose level. Pharmacokinetic studies demonstrated that exposure to panobinostat was proportional to the dose given, with no associations between pharmacokinetic parameters and age, weight, or body surface area. Among the 9 patients who had sufficient (>2%) circulating blasts on which histone acetylation studies could be performed, 7 demonstrated at least 1.5-fold increases in acetylation. Although no patients had a decrease in circulating blasts after single-agent panobinostat, 8 of the 17 patients (47%), including 5 of the 6 patients treated at dose level 3, achieved complete remission. Among the 8 complete responders, 6 (75%) attained negative minimal residual disease status.

CONCLUSIONS

Panobinostat can be safely administered with chemotherapy and results in increased blast histone acetylation. This suggests that it should be further studied in AML.

High-Dose Methotrexate in Pediatric Acute Lymphoblastic Leukemia: Predictors of Delayed Clearance and the Effect of Increased Hydration Rate on Methotrexate Clearance

Objectives High-dose methotrexate (HDMTX) is an important chemotherapeutic agent in the treatment of many cancers. Identification of the predictors of poor clearance during HDMTX infusions could advance the introduction of improved supportive care to prevent toxicities and reduce hospital length of stay. The purpose of this study was to identify relationships between patient physical characteristics and HDMTX clearance in the treatment of pediatric acute lymphoblastic leukemia (ALL). At our hospital, patients who have delayed methotrexate (MTX) clearance during a cycle of HDMTX receive an increased rate of hydration with subsequent cycles. This increase in hydration rate was examined for its potential to mitigate predictors of poor clearance and to prevent nephrotoxicity. Methods This study retrospectively examined the treatment records of 87 pediatric patients diagnosed with ALL who were treated on or according to Children's Oncology Group (COG) protocols AALL0232, AALL0434, AALL1131, and AALL1231. Each patient received four cycles of HDMTX (5 g/m² over 24 hours) at two-week intervals. Patients received either 125 ml/m²/hour (standard) or 200 ml/m²/hour (delayed clearance protocol) hydration before, with, and after each infusion. MTX levels taken at 24-, 42-, and 48-hour time points were used as an indirect measure of drug clearance. Two-tailed inference for ordinary least squares regression and both heteroskedastic and paired two-tailed t-tests were performed to identify physical characteristics associated with delayed MTX clearance and the effects of hydration rate on MTX clearance, respectively. Results Patient age and body surface area (BSA) were found to have statistically significant (p<0.05) positive associations with the serum MTX levels at 24, 42, and 48 hours in cycle 1. Age and BSA were significant only at the 24-hour time point in cycles 2 and 4. Weight alone was not associated with delayed MTX clearance. For patients who had delayed MTX clearance once and thus received the delayed clearance protocol in subsequent cycles, increasing the hydration rate from 125 to 200 ml/m²/hour was associated with a statistically significant decrease in average MTX levels as well as serum creatinine levels at the 24-, 42-, and 48-hour time points. Once patients with delayed clearance received the 200 ml/m²/hour rate of hydration, the history of prior poor clearance lost its predictive value for serum MTX levels and delayed clearance. Conclusions These results suggest that patient age and BSA are significant predictors of MTX clearance if all patients receive the same rate of hydration. Age and BSA affect the distribution phase of MTX kinetics, with downstream effects in the elimination phase. Increased hydration mitigates the effects of these physical characteristics on the elimination phase kinetics by improving renal elimination of MTX, causing a loss of significance of age and BSA as predictors of MTX levels in subsequent cycles at the 42- and 48-hour time points, but with less effect at 24 hours. Thus, hyperhydration regimens prior to cycle 1 of HDMTX could be considered for patients presenting with risk factors of advanced age or high BSA to avoid delayed clearance.

Yoga as a Complementary and Alternative Therapy in Children with Hematologic and Oncologic Disease

A diagnosis of a hematologic or oncologic disease in a child can be stressful for the patient and the family. Yoga as an intervention has been reported to decrease stress in adults diagnosed with chronic disorders but few studies have been reported with children and their families. A convenience sample of patients diagnosed with cancer or a blood disorder (ages 7-17 years) and their caregivers was selected to participate in a single bedside yoga class. Participants were surveyed pre and post yoga with the Spielberger State Trait Anxiety Scale. Children were also surveyed pre and post yoga with the Wong-Baker Faces Pain Scale. Children had a significant decrease in pain post yoga but no change in anxiety. Adolescents and parents had a significant decrease in anxiety post yoga intervention.

Clinical Impact of Next-generation Sequencing in Pediatric Neuro-Oncology Patients: A Single-institutional Experience

The implementation of next-generation sequencing (NGS) in pediatric neuro-oncology may impact diagnosis, prognosis, therapeutic strategies, clinical trial enrollment, and germline risk. We retrospectively analyzed 58 neuro-oncology patients (31 boys, 27 girls, average age 7.4 years) who underwent NGS tumor profiling using a single commercially available platform on paraffin-embedded tissue obtained at diagnosis (20 low-grade gliomas, 12 high-grade gliomas, 11 embryonal tumors, four ependymal tumors, three meningeal tumors, and eight other CNS tumors) from May 2014 to December 2016. NGS results were analyzed for actionable mutations, variants of unknown significance and clinical impact. Seventy-four percent of patients (43 of 57) had actionable mutations; 26% had only variants of uncertain significance (VUS). NGS findings impacted treatment decisions in 55% of patients; 24% were given a targeted treatment based on NGS findings. Seven of eight patients with low-grade tumors treated with targeted therapy (everolimus, trametinib, or vemurafenib) experienced partial response or stable disease. All high-grade tumors had progressive disease on targeted therapy. Forty percent of patients had a revision or refinement of their diagnosis, and nine percent of patients were diagnosed with a previously unconfirmed cancer predisposition syndrome. Turnaround time between sample shipment and report generation averaged 13.4 ± 6.4 days. One sample failed due to insufficient DNA quantity. Our experience highlights the feasibility and clinical utility of NGS in the management of pediatric neuro-oncology patients. Future prospective clinical trials using NGS are needed to establish efficacy.

Treatment of Recurrent Refractory Pediatric Pre-B Acute Lymphoblastic Leukemia Using Inotuzumab Ozogamicin Monotherapy Resulting in CD22 Antigen Expression Loss as a Mechanism of Therapy Resistance

BACKGROUND

Inotuzumab ozogamicin is a novel antibody-drug conjugate that targets CD22, a common antigen on pre-B acute lymphoblastic leukemia cells.

OBSERVATIONS

A 7-year-old boy with pre-B acute lymphoblastic leukemia in his second relapse was given 2 cycles of inotuzumab ozogamicin. He responded morphologically with a negative bone marrow evaluation. However, he relapsed in cycle 3 of therapy with a loss of CD22 expression on his lymphoblast population.

CONCLUSION

To our knowledge, this is the first published case of CD22 expression loss as a mechanism of therapy resistance for inotuzumab ozogamicin.

Identification of NTRK fusions in pediatric mesenchymal tumors

BACKGROUND

NTRK fusions are known oncogenic drivers and have recently been effectively targeted by investigational agents in adults. We sought to assess the frequency of NTRK fusions in a large series of pediatric and adolescent patients with advanced cancers.

PROCEDURE

Genomic profiles from 2,031 advanced cancers from patients less than 21 years old who were assayed with comprehensive genomic profiling were reviewed to identify NTRK fusions.

RESULTS

Total of nine cases (0.44%) harbored NTRK fusions, including novel partners. Four of these cases were in children less than 2 years old for which infantile fibrosarcoma was considered as a diagnosis, and two harbored the canonical ETV6-NTRK3. The remaining cases carried other diagnoses, at least one that carried the diagnosis of inflammatory myofibroblastic tumor.

CONCLUSIONS

NTRK fusions occur in a subset of young patients with mesenchymal or sarcoma-like tumors at a low frequency, and are eminently druggable targets via either investigational agents or approved drugs.

Effect of Vitamin D Supplementation on Delayed Hyperphosphatemia in Pediatric Acute Lymphoblastic Leukemia Patients During Induction Chemotherapy

OBJECTIVES

Vitamin D plays a role in maintaining bone health and calcium metabolism, but recent studies cast doubt on vitamin D supplementation's benefits in survivors of acute lymphoblastic leukemia (ALL). Vitamin D supplementation could increase serum phosphate through increased intestinal absorption of phosphate and suppression of parathyroid hormone, which would lead to decreased renal phosphate excretion. Because of the potential for renal injury during induction chemotherapy for ALL, Vitamin D supplementation's potential for increasing hyperphosphatemia could outweigh its suggested but unproven benefits.

METHODS

To measure the interaction between vitamin D supplementation and phosphate during chemotherapy induction, a retrospective study was done. Demographic data; clinical information about the diagnosis; laboratory data regarding calcium, phosphate, and vitamin D concentrations; and medication histories were reviewed.

RESULTS

A retrospective study of 41 children with ALL showed no statistically significant difference in the final phosphate concentrations that were obtained (4.41 mg/dL vs. 4.53 mg/dL, p = 0.635) with regard to their vitamin D supplementation status. Longitudinal effects with vitamin D and phosphate showed a trend toward increasing phosphate concentrations in patients who received supplemental vitamin D (0.035 vs. 0.010 mg/dL per day; p = 0.102).

CONCLUSIONS

Vitamin D potentially poses a risk of hyperphosphatemia in children undergoing induction chemotherapy for ALL.

Melanotic Neuroectodermal Tumor of Infancy in the Epididymis: A Brief Report and Review of the Role of Chemotherapy in Management

Melanotic neuroectodermal tumor of infancy is a rare benign tumor of neural crest origin. The tumor generally presents in the jawbones; however, it occasionally occurs in extracranial sites. Although 95% of these tumors present within the first year of life and 15% in extracranial locations, we report an unusual case of a 15-month-old male with melanotic neuroectodermal tumor of infancy of the epididymis. The patient underwent orchiectomy without adjuvant chemotherapy or radiation. Twenty months later, there was no sign of recurrence. In addition, we discuss the role of chemotherapy and radiation and the potential importance of molecular genetics in establishing guidelines for management.

Darbepoetin alfa and ferric gluconate ameliorate the anemia associated with recessive dystrophic epidermolysis bullosa

Our objective was to describe the efficacy of darbepoetin alfa and ferric gluconate complex in the treatment of the anemia associated with recessive dystrophic epidermolysis bullosa. To accomplish this aim, we retrospectively reviewed a series of patients with this disease treated in a single institution with darbepoetin alfa and parenteral iron for anemia. Four patients with recessive dystrophic epidermolysis bullosa were treated for a mean length of treatment of 14.5 months (4-18 months). Three patients received parenteral iron in the form of ferric gluconate complex and one received iron dextran. The mean pretreatment hemoglobin was 6.8 g/dL (4.9-9.6 g/dL). All four had improvements in their hemoglobin levels and energy levels with a mean increase in hemoglobin level of 2.8 g/dL (p = 0.003). We found darbepoetin alfa and ferric gluconate complex to be effective in the treatment of anemia associated with recessive dystrophic epidermolysis bullosa and to have distinct advantages over previously described formulations, and we concluded that they should be considered in the supportive care of this disease. We recommend the development of a standardized protocol for the evaluation and management of recessive dystrophic epidermolysis bullosa-associated anemia.

Temozolomide is active in childhood, progressive, unresectable, low-grade gliomas

PURPOSE

To assess the activity and tolerability of temozolomide in children with progressive low-grade gliomas (LGGs).

PATIENTS AND METHODS

The authors reviewed the records of 13 children (6 months to 19 years old) with progressive LGGs and magnetic resonance imaging evidence of unresectable tumors who were treated with temozolomide at the authors' institution since 1999.

RESULTS

Four patients received a 5-day regimen of temozolomide (150 mg/m2 per day) repeated every 28 days, and nine patients received a 42-day regimen (75 mg/m2 per day) repeated every 56 days. Three patients demonstrated partial responses to temozolomide, with a median time to maximal response of 5 months (range 4-12 months), and one had a minor response at 9 months. Four patients developed progression while on temozolomide, with a median time to progression of 7 months (range 1-12 months). Five patients had disease stabilization. Among the five patients with prior chemotherapy and/or radiation therapy, temozolomide was associated with disease stabilization in three and tumor response in one. In the three patients with neurofibromatosis type 1, two patients experienced tumor responses and one disease stabilization. Thrombocytopenia, nausea, emesis, and fatigue were the most common toxicities. Four patients discontinued therapy because of the side effects.

CONCLUSIONS

Temozolomide is active in children with LGGs. It is effective in previously treated patients and in patients with neurofibromatosis type 1. The 42-day regimen appears less toxic than the 5-day regimen. Any impact on survival for these patients remains to be demonstrated.

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.