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J Nachman

Publications and source records attributed to J Nachman.

At least 37 records · Page 2Linked to original sources
PubMed1997

Clinical features and treatment outcome of childhood T-lineage acute lymphoblastic leukemia according to the apparent maturational stage of T-lineage leukemic blasts: a Children's Cancer Group study.

PURPOSE: Leukemic cells from T-lineage acute lymphoblastic leukemia (ALL) patients are thought to originate from T-lymphocyte precursors corresponding to discrete stages of T-cell ontogeny. Here we sought to determine the influence of leukemic cell apparent maturational stage on treatment outcomes in pediatric T-lineage ALL. PATIENTS AND METHODS: From 1983 through 1993, 407 pediatric T-lineage ALL patients were enrolled onto two sequential series of risk-adjusted treatment protocols of the Children's Cancer Group. In the current analysis, T-lineage ALL patients were immunophenotypically classified as follows: CD7+ CD2- CD5- pro-thymocyte leukemia (pro-TL), CD7+ (CD2 or CD5)+ CD3- immature TL, and CD7+ CD2+ CD5+ CD3+ mature TL. RESULTS: Similar induction outcomes of 91.4%, 97.1%, and 98.3% were obtained by the pro-, immature, and mature TL groups, respectively. Four-year event-free survival (EFS) was lower for pro-TL patients (57.1%; SD = 8.4%,) compared with immature and mature TL patients (68.5%; SD = 3.5%; and 77.1%; SD = 4.0%, respectively) with an overall significance of .05 (log-rank test) or .04 (log-rank trend test). Relative hazards rates (RHR) were 2.11 and 1.22 for pro-TL and immature TL versus mature TL, respectively. Highly significant differences were found for overall survival (P = .005, log-rank test; P = .009, log-rank trend test). Multivariate analysis confirmed that the prognostic influence of ontogeny grouping was independent of that of other prognostic factors. CONCLUSION: Leukemic cells of the pro-TL maturation stage identify a small subgroup of T-lineage ALL patients who have a significantly worse EFS outcome than patients whose cells are of a more mature stage of development.

Analysis of Variance↗
PubMed1997

Augmented Berlin-Frankfurt-Munster therapy abrogates the adverse prognostic significance of slow early response to induction chemotherapy for children and adolescents with acute lymphoblastic leukemia and unfavorable presenting features: a report from the Children's Cancer Group.

PURPOSE: Compared with previous Children's Cancer Group (CCG) acute lymphoblastic leukemia (ALL) trials, therapy based on the Berlin-Frankfurt-Munster (BFM) 76 trial has effected an improvement in event-free survival (EFS). In an attempt to improve EFS further, CCG investigators formulated an augmented BFM (A-BFM) regimen that provides prolonged, intensified postinduction chemotherapy relative to the CCG-modified BFM regimen. PATIENTS AND METHODS: We tested A-BFM in 101 patients with ALL and unfavorable presenting features that showed slow early response (SER) to induction therapy who attained remission on day 28. Their outcome was compared with that of 251 concurrent patients with unfavorable presenting features, a rapid early response to therapy (RER), and remission by day 28, treated with CCG-BFM with or without cranial radiation (CRT). RESULTS: The 4-year EFS rate from the end of induction for SER patients treated with A-BFM was 70.8% +/- 4.6%. Seventeen patients remain in continuous remission beyond 5 years. Vincristine (VCR) neurotoxicity developed in 50% of patients, but was rarely debilitating. Allergies to Escherichia coli L-asparaginase (L-ASP) occurred in 35% of patients. Avascular necrosis of bone (AVN) developed in 9% of patients. In comparison, a concurrent RER group treated with standard BFM +/- CRT had a 4-year EFS rate of 73.1% +/- 4.6%. CONCLUSION: The toxicity of A-BFM is significant, but acceptable. Compared with historical control SER patients treated with CCG-modified BFM, A-BFM therapy appears to produce a significant improvement in EFS. This is the first study to show that intensive chemotherapy, as given in the A-BFM regimen, can abrogate the adverse prognostic significance of SER.

Adolescent↗
PubMed1997

Prognostic significance of the CD10+CD19+CD34+ B-progenitor immunophenotype in children with acute lymphoblastic leukemia: a report from the Children's Cancer Group.

Leukemic cells from most patients with B-lineage acute lymphoblastic leukemia (ALL) appear to originate from normal B-lymphocyte precursors. The earliest B-cell progenitors coexpress the antigens CD10, CD19, or CD34 on their cell surfaces. In a large cohort of 2028 children with ALL, we compared treatment outcomes of a subset of B-lineage ALL patients with CD10+CD19+CD34+ immature B-progenitor leukemia (BPL) to the treatment outcomes of the remaining CD19+ B-lineage ALL patients. Pediatric B-lineage ALL cases enrolled on risk-adjusted ALL treatment protocols of the Children's Cancer Group were immunophenotypically classified as BPL or non-BPL. Patients were stratified further into age groups of > or = 1 year and <12 months. Event-free survival (EFS) outcomes were calculated by standard life table methods. BPL patients in both age groups generally had more favorable presenting characteristics than non-BPL controls. Within the age group of > or = 1 year, BPL patients had a slightly better EFS outcome than non-BPL patients, with 3-year estimates of 83.9% (SD = 1.1%) vs. 78.8% (SD = 1.8%), respectively (P = 0.10). Infants with BPL, representing one-fifth of the total infant patient population, had a significantly better EFS outcome than infants with non-BPL (three-year EFS: 82.4%, SD = 9.2% vs. 34.4%, SD = 5.9%, P = 0.006). In univariate analyses, the relative hazard rate (RHR) was 3.73 for non-BPL vs BPL and this marked difference in EFS outcome was maintained at 5 years of follow-up. The favorable prognostic influence of the BPL immunophenotype for infants remained significant in multivariate analyses with an RHR of 2.72 for non-BPL vs BPL (P = 0.05). CD10+CD19+CD34+ immature B-progenitor immunophenotype is associated with favorable characteristics for children with ALL and identifies a subset of infants who achieve favorable EFS outcomes.

Adult↗
PubMed1996

Osteosarcoma metastatic to the pancreas in young patients.

Two rare cases of pancreatic metastases from osteosarcoma are reported. Metastatic spread occurred after treatment with chemotherapy. This unusual phenomenon represents an alternation in the natural history of osteosarcoma with increased long-term survival of patients who are successfully treated with chemotherapy.

Adolescent↗
PubMed1995

Patterns of failure following total body irradiation and bone marrow transplantation with or without a radiotherapy boost for advanced neuroblastoma.

PURPOSE: To evaluate the patterns of failure and outcome of patients undergoing high-dose chemotherapy, total body irradiation (TBI), and bone marrow transplantation (BMT) for advanced/relapsed pediatric neuroblastoma, with emphasis on the impact of a radiotherapy boost to primary and metastatic sites. METHODS AND MATERIALS: Between May 1986 and June 1993, 26 patients with advanced neuroblastoma underwent high-dose chemotherapy and TBI followed by BMT at our institution. The majority of patients were over the age of 2 years (73%) and were Stage IV at diagnosis (81%). Multiple metastatic sites were involved including bone (17), bone marrow (15), distant nodes (11), liver (5), lung (4) and brain (1). Twenty patients (77%) received cyclophosphamide (50 mg/kg x 4 days) and TBI as consolidation therapy. TBI was delivered to a total dose of 12 Gy given in 2 Gy twice daily (b.i.d.) fractions over the 3 days preceding bone marrow infusion. A local radiotherapy boost of 8-24 Gy was given to 13 out of 26 patients (50%) to the primary and/or metastatic sites immediately prior to or following induction chemotherapy according to physician judgement. Sites not amenable to a radiotherapy boost included the bone marrow, diffuse/bilateral lung involvement, and multiple bone metastases (> four sites). RESULTS: The actuarial overall survival of the 26 patients was 40.4% at 3 and 5 years, with a progression-free survival at 5 years of 38.5%. Six patients died of transplant-related toxicity (23%). The use of cyclophosphamide as high-dose consolidation chemotherapy was significantly better than other multidrug regimens used in terms of overall survival (p < 0.0001) and progression-free survival (p = 0.0004). The presence of liver involvement prior to BMT was a significant adverse prognostic factor by multivariate analysis. Of the 20 patients surviving the transplant, 10 (50%) underwent a local radiotherapy boost. The patterns of failure were as follows: 3 out of 10 "boost" patients failed overall, none in previous (old) sites of disease only, 1 in new sites only, and 2 in old and new sites; 6 out of 10 "no boost" patients failed overall, 4 in old sites only, none in new sites only, and 2 in old and new sites. There was a trend toward improved 5-year progression-free survival in patients surviving the transplant that received a boost (68% vs. 33%, p = 0.24). A failure analysis was also performed for each of the 59 initially involved sites, of which the majority (64%) were amenable to a radiotherapy boost. Overall, there is a trend toward less failure in sites amenable to a radiotherapy boost that were irradiated (1 out of 10) vs. those not irradiated (6 out of 28). Failure in the liver occurred in three out of four of the patients with liver involvement that did not receive boost radiotherapy, whereas all seven patients with distant nodal involvement were controlled without a boost. Long-term sequelae include learning difficulties (2), cataract formation (1), and hearing loss (2). Sequelae attributable to a radiotherapy boost occurred in only one patient who received whole brain radiotherapy and developed a cataract and learning difficulties. CONCLUSION: We have found an actuarial 5-year survival rate of 40.4% for patients with advanced neuroblastoma treated with BMT, which compares favorably with results of other published series. Disease recurrence following BMT was most common in previous sites of disease. The majority (64%) of these sites were amenable to a radiotherapy boost. An analysis of failure suggests that a low-dose radiotherapy boost improves control of these sites.

Adrenal Gland Neoplasms↗
PubMed1993

Molecular basis of spectrin deficiency in hereditary pyropoikilocytosis.

Hereditary pyropoikilocytosis (HPP) is a recessively inherited hemolytic anemia characterized by severe poikilocytosis and red blood cell fragmentation. HPP red blood cells are partially deficient in spectrin and contain a mutant alpha or beta-spectrin that is defective in terms of spectrin self-association. Although the nature of the latter defect has been studied in considerable detail and many mutations of alpha-spectrin and beta spectrin have been identified, the molecular basis of spectrin deficiency is unknown. Here we report two mechanisms underlying spectrin deficiency in HPP. The first mechanism involves a thalassemia-like defect characterized by a reduced synthesis of alpha-spectrin as shown by studies involving synthesis of spectrin in two unrelated HPP probands and their parents: One parent carries the elliptocytogenic spectrin mutation, whereas the other parent is fully asymptomatic. Peripheral blood mononuclear cells as a source of erythroid burst-forming unit (BFUe) were cultured in a two-phase liquid culture system that gives rise to terminally differentiated erythroblasts. Pulse-labeling studies of an equal number of erythroblasts or morphologically identical maturity showed that the synthesis of alpha-spectrin as well as the mRNA levels as measured by the competitive polymerase chain reaction (PCR) method are markedly reduced in the presumed asymptomatic carriers and the HPP probands. In contrast, the synthesis and mRNA levels of beta-spectrin were normal. These results constitute a direct demonstration of an alpha-spectrin synthetic defect in a subset of asymptomatic carriers of HPP and HPP probands. The second mechanism underlying spectrin deficiency involves increased degradation of mutant spectrin before its assembly on the membrane. This is evidenced by pulse labeling studies of erythroblasts from a patient with HPP associated with a homozygous state for spectrin alpha I/46 mutation (leu-pro mutation at AA 207 of alpha-spectrin). These studies showed that although spectrin is synthesized in the cytosol in normal amounts, the rate of turnover of alpha-spectrin is faster resulting in about 40% to 50% reduced assembly of alpha-spectrin and beta-spectrin on the membrane. Thus, spectrin deficiency in this case is at least in part caused by increased susceptibility of the mutant spectrin to degradation before its assembly on the membrane. We conclude that at least two separate mechanisms underlie the molecular basis of spectrin deficiency in HPP.

Adolescent↗
PubMed1993

Young adults 16-21 years of age at diagnosis entered on Childrens Cancer Group acute lymphoblastic leukemia and acute myeloblastic leukemia protocols. Results of treatment.

BACKGROUND: Scant data are available on event-free survival (EFS) for young adults with leukemia who were 16-21 years of age at diagnosis. In acute lymphoblastic leukemia (ALL), it is well recognized that children have a better EFS compared with adults, whereas for acute myelocytic leukemia (AML), EFS results seem to be similar. To determine the appropriate treatment for young adults with leukemia, outcome data are essential. METHODS: Young adults entered on the Childrens Cancer Group (CCG) 100 series ALL protocols and the CCG 213 AML protocol were analyzed for EFS and survival. Prognostic factors for EFS also were determined. RESULTS: The actuarial EFS for 143 young adults with ALL treated on the CCG 100 series ALL protocols was 64 +/- 4% at 4 years and 59 +/- 4% at 6 years. The major adverse prognostic feature was leukocyte counts greater than 50,000/microliters. The actuarial EFS for 79 young adults with AML entered on CCG 213 was 32.2 +/- 5.3% at 2 years and 28.6 +/- 5.3% at 3 years. CONCLUSIONS: Young adults with ALL treated on CCG protocols have a 6-year EFS of approximately 60%. This is similar to the EFS for patients 10-15+ years of age at diagnosis treated on the same protocols and better than EFS results reported from most adult trials. Young adults with AML had a slightly inferior outcome compared with younger children.

Adolescent↗
PubMed1993

Footprinting of individual tumors and their variants by constitutive cytokine expression patterns.

We have examined the cytokine mRNA expression profile of six different human cell lines derived from Ewing sarcomas using polymerase chain reaction and found each to constitutively express a characteristic pattern. Furthermore, each cell line differed in the levels of secreted cytokines. We also analyzed the expression of several cytokines in murine UV-induced sarcomas and their heritably stable progressive variants. Each murine tumor also constitutively expressed a large number of cytokines, and in some cases, the more malignant variants differed from their parental tumors. These results demonstrate that tumors of the same type, and even in the same lineage, can have distinct cytokine expression and/or secretion profiles. Some cytokines may stimulate tumor growth while others may have antitumor effects. Cytokine therapy may be tailored depending upon the cytokine profile of the individual malignancy.

Animals↗
PubMed1993

Crystallization and preliminary X-ray diffraction data of two heparin-binding fragments of human fibronectin.

Two different heparin-binding fragments of human fibronectin have been crystallized in forms which are suitable for crystal structure analyses. The 30 kDa hep-2A fragment, consisting of type III domains 12-14, was crystallized from solutions containing ammonium sulfate or polyethylene glycol 6000. The crystals grown in ammonium sulfate solutions were orthorhombic with space group I222 or I2(1)2(1)2(1) with a = 68.1 A, b = 88.6 A, and c = 144.9 A. The crystals grown in polyethylene glycol solutions are hexagonal with space group P6(1)22 or P6(5)22 with a = b = 66.7 A and c = 245.7 A. The 40 kDa hep-2B fragment, consisting of type III domains 12-15, was also crystallized from solutions containing ammonium sulfate with the addition of glycerol. Glycerol proved an effective agent for reducing the number of crystals in the crystallization experiments, and thus, increasing the size of the crystals in these experiments. This crystal form is nearly isomorphous to the orthorhombic form of the hep-2A fragment with space group I222 or I2(1)2(1)2(1) and a = 67.5 A, b = 87.0 A, and c = 144.3 A. All crystal forms diffract to at least 3.5 A resolution and contain a single molecule in the asymmetric unit.

Crystallization↗
PubMed1992

An evaluation of abdominal staging procedures performed in pediatric patients with advanced Hodgkin's disease: a report from the Childrens Cancer Study Group.

Forty-nine children and adolescents underwent staging laparotomy in the course of a cooperative group (CCG) study of advanced Hodgkin's disease (HD). The purpose of the study was to evaluate the toxicity of a regimen of 12 cycles of doxorubicin, bleomycin, vinblastine, and imidazole carboxamide (ABVD) plus 2,100 cGy regional irradiation in patients with stage III-IV disease. Review of the biopsies and specimens from these 49 laparotomies identified distinct patterns of abdominal involvement and permitted an evaluation of the importance of different aspects of the staging procedure and an assessment of the value of non-invasive techniques in determining the extent of abdominal disease. The major observations from these studies were: (1) computed tomography (CT) and gallium 67 scans (GA) were specific (few false positives), but not sensitive, indicators of abdominal disease; (2) negative findings at laparotomy in 29 patients permitted a contraction of the abdominal radiation fields with no subsequent abdominal recurrence in these patients; (3) in five patients, stage IV status was established only by laparotomy and open hepatic biopsy; (4) all patients with lower abdominal or pelvic involvement also had upper abdominal node or splenic involvement; (5) involvement of splenic hilar nodes was a sensitive predictor of splenic involvement; (6) disease in the inferior paraaortic nodes accurately predicted the presence of more distal disease, ie, in the iliac or pelvic node groups; and (7) in a limited experience, transposition of the ovaries protected ovarian function from the effects of pelvic irradiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1991

Therapy-related myelodysplastic syndrome and acute myeloid leukemia in children: correlation between chromosomal abnormalities and prior therapy.

We have studied 20 children with therapy-related myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) who were 3 months to 16 years old at diagnosis of their primary neoplasm and 1 to 24 years old at diagnosis of their secondary neoplasm. The median interval from initial treatment for the first malignancy to diagnosis of therapy-related MDS or AML was 46 months (range, 12 to 116 months). Twelve patients had chromosomal abnormalities resulting in loss of material from the long arm of chromosomes 5 and/or 7, three patients had abnormalities of chromosome 11 band q23, one patient had both classes of abnormalities, three patients had other abnormalities, and one patient had a normal karyotype. Ten of 12 patients with chromosome 5 and/or 7 abnormalities had been exposed to an alkylating agent, and two of three patients with 11q23 abnormalities had been exposed to an epipodophyllotoxin. The patient with both classes of abnormalities had been exposed to both types of therapy. We conclude that abnormalities of chromosomes 5 and/or 7 are common in children with therapy-related MDS or AML. The proposed relationships between exposure to alkylating agents and abnormalities of chromosomes 5 and/or 7 and between exposure to epipodophyllotoxins and abnormalities of 11q23 are supported in this pediatric series.

Adolescent↗
PubMed1991

Direct correlation of cytogenetic findings with cell morphology using in situ hybridization: an analysis of suspicious cells in bone marrow specimens of two patients completing therapy for acute lymphoblastic leukemia.

Bone marrow cells from two pediatric patients completing therapy for acute lymphoblastic leukemia were studied using in situ hybridization with an alpha-satellite DNA probe specific for chromosome 17. Morphologic analysis of the end-therapy specimens from each patient had shown small numbers (7.5%, 8.5%) of cells that were suspicious for residual or recurrent disease. These cells could not be morphologically or immunophenotypically distinguished with certainty from immature lymphoid cells (hematogones), which may be present normally, sometimes in increased numbers, in the bone marrow specimens of children. In situ hybridization with a probe to chromosome 17 was used because the leukemic cells from each patient had originally been shown to have an extra copy of this chromosome. In one patient, in situ studies showed a population of cells (106 of 1,000 cells) with three hybridization signals indicating trisomy 17, and thus residual/recurrent leukemia. In the other patient trisomy 17 could not be detected. Additional hybridizations to previously stained bone marrow aspirate smears permitted a direct correlation of the cytogenetic findings with the suspicious cells on a cell-to-cell basis. The questionable cells were identified, photographed, and then re-examined after hybridization. In one patient, 13 of 18 (72%) of the suspicious cells were found to have trisomy 17, whereas in the other patient 0 of 24 (0%) demonstrated an extra copy of this chromosome. These cases illustrate a clinical application of interphase cytogenetic analysis and demonstrate how this technology can be used for direct correlation of cytogenetic findings with cell morphology. This technique should prove useful for the detection of minimal residual disease and for lineage studies in leukemia and myelodysplasia.

Antigens, CD↗
PubMed1991

Immunoglobulin gene rearrangements in acute lymphoblastic leukemia with the 9;11 translocation.

The recurring chromosomal 9;11 translocation [t(9;11) (p22;q23)] typically is associated with acute monoblastic leukemia, but a number of patients with acute lymphoblastic leukemia also have been reported to have the t(9;11). To investigate the cell lineage in the latter cases, we analyzed DNA from the leukemic cells of an 8-year-old girl with acute lymphoblastic leukemia and a t(9;11) for rearrangements of the immunoglobulin and T-cell receptor genes. Rearrangements of both immunoglobulin heavy-chain loci and of one lambda light-chain gene were detected, as well as deletions affecting both alleles of the kappa light-chain genes; T-cell receptor genes were in germline configuration. These results provide further evidence that the 9;11 translocation is not limited to myeloid lineage leukemia and may be observed in acute lymphoblastic leukemia.

Child↗
PubMed1991

Granulocytic sarcoma of the ileum treated by bone marrow transplantation.

An 8-year-old boy with a granulocytic sarcoma of the proximal ileum metastatic to mesenteric lymph nodes was placed into complete remission with surgical excision of the primary tumor and conventional induction chemotherapy with daunorubicin and cytosine arabinoside. He was then treated with high dose cytosine arabinoside, fractionated total body irradiation, and allogeneic marrow transplantation from his 22-month-old brother who was completely matched at the major histocompatibility complex. Methotrexate was given following the transplant to prevent graft-versus-host disease (GVHD). His post-transplantation course was complicated by a transient autoimmune hemolytic anemia related to an ABO blood group incompatibility and hepatic fungal microabscesses which responded to Amphotericin therapy. Four years following the transplant the patient remains in complete remission. The prognosis for patients with granulocytic sarcoma has been poor although, perhaps, improved over the past decade. This is the first published case report of successful treatment of a granulocytic sarcoma of the ileum by allogeneic marrow transplantation.

Antineoplastic Combined Chemotherapy Protocols↗
PubMed1991

Impact of chromosomal translocations on prognosis in childhood acute lymphoblastic leukemia.

The presence of a chromosomal translocation in the leukemic cells at diagnosis of acute lymphoblastic leukemia (ALL) in children is associated with a high risk for treatment failure. We have reexamined the relationship between translocations and prognosis in 146 children with ALL who received risk-based therapy such that high-risk patients were treated with intensive drug schedules. In univariate analysis, multiple factors were associated with a relatively poor event-free survival (EFS) including age less than 2 years or greater than 10 years (combined group), WBC count greater than 10 x 10(9)/L, French-American-British (FAB) morphologic classification L2, absence of common ALL antigen (CALLA, CD10) expression, absence of hyperdiploidy with a chromosome number of 50 to 60, and presence of the specific translocations t(4; 11)(q21;q23) or t(9;22)(q34;q11) (combined group). However, there was no disadvantage with respect to EFS in patients with translocations compared with those who lacked translocations (73% at 4 years in both groups). Furthermore, when patients with specific cytogenetic abnormalities for which the prognostic significance has been well established (hyperdiploid 50 to 60, t(4;11), and t(9;22] were removed from the analysis, the remaining group with other translocations had a better EFS than the remaining group lacking translocations, although this was not statistically significant (81% v 65% at 4 years, P = .24). In a multivariate analysis, a model including WBC count and FAB classification was the strongest predictor of EFS. The presence or absence of translocations was not an independent predictor of EFS and did not contribute to the ability of any model to predict EFS. In conclusion, when effective intensive therapy is used to treat childhood ALL with high-risk clinical features, categorization of patients on the basis of chromosomal translocations without attention to the specific abnormality is not useful as a prognostic factor.

Adolescent↗
PubMed1990

Open-wedge testicular biopsy in childhood acute lymphoblastic leukemia after two years of maintenance therapy: diagnostic accuracy and influence on outcome--a report from Children's Cancer Study Group.

Bilateral testicular biopsies were performed on 708 males with acute lymphoblastic leukemia completing 24 to 30 months of maintenance chemotherapy in continuous remission. The 73 patients (10.3%) with occult testicular leukemia (TL) had a significantly increased risk of subsequent relapse (P = .0001) and death (P less than .0001) when compared with patients with negative biopsies. Protocol-specified therapy for occult TL included reinduction therapy with concurrent bilateral testicular radiation, and 2 years of maintenance therapy. Four-year event-free survival for patients with negative biopsies was 78.2% +/- 4% versus 65% +/- 14% for patients with occult TL who received protocol-specified therapy (P = .05). This study suggests that (1) occult TL occurs in 10% of males completing 2 years of maintenance therapy; (2) occult TL significantly increases risk for subsequent relapse and death; (3) treatment results for occult TL and isolated overt off therapy TL (no previous biopsy) are similar; and (4) given current therapy, documentation of occult TL after 2 years of therapy does not improve disease-free survival.

Antineoplastic Agents↗
PubMed1990

Crystal structure of two covalent nucleoside derivatives of ribonuclease A.

Crystal structures of two forms of ribonuclease A with deoxynucleosides covalently bound to respectively His 12 and His 119 have been solved. One form, T-H12-RNase, has a deoxythymidine bound to N epsilon 2 of His 12, while the other one, U-H119-RNase, has a deoxyuridine bound to N delta 1 of His 119. The two crystal forms are nearly isomorphous, with two molecules in the asymmetric unit. However, the modified ribonucleases differ both in their enzymatic activities and in the conformation of the catalytic site and of the deoxynucleoside-histidine moiety. T-H12-RNase is characterized by complete loss of enzymatic activity; in this form the deoxynucleoside completely blocks the catalytic site and forms intramolecular contacts with residues associated with both the B1 and B2 sites. U-H119-RNase retains 1% of the activity of the unmodified enzyme, and in this form His 119 adopts a different orientation, corresponding to the alternate conformation reported for this residue; the deoxynucleoside-histidine moiety points out of the active site and does not form any contacts with the rest of the protein, thus allowing partial access to the catalytic site. On the basis of these structures, we propose possible mechanisms for the reactions of bromoacetamido nucleosides with ribonuclease A.

Binding Sites↗