Unexpected mortality from the use of E. coli L-asparaginase during remission induction therapy for childhood acute lymphoblastic leukemia: a report from the Taiwan Pediatric Oncology Group

The relative efficacy and toxicity of E. coli L-asparaginase and epidoxorubicin used in remission induction therapy for childhood acute lymphoblastic leukemia (ALL) were assessed in a randomized trial conducted in Taiwan. All patients had standard-risk ALL, defined as a leukocyte count <10 x 10(9)/l and were aged between 1 and 2 or 7 and 10 years, or a leukocyte count <50 x 10(9)/l and were aged between 2 and 7 years, without evidence of a T cell or mature B cell immunophenotype, central nervous system leukemia or expression of two or more myeloid-associated antigens. Ninety-three patients were randomized to receive E. coli L-asparaginase at 10,000 IU/m2 thrice weekly for nine doses and 108 to receive epidoxorubicin at 20 mg/m2 weekly for two doses during remission induction with daily prednisolone, weekly vincristine and, on day 22, a dose of etoposide plus cytarabine. Patients treated with L-asparaginase had a significantly higher rate of fatal infection with or without hemorrhage than did those who received epidoxorubicin during remission induction (six of 93 vs none of 108, P = 0.009), resulting in a lower rate of complete remission in the former group (93.6 vs 99.1%, P = 0.05). In addition, patients treated with L-asparaginase had a higher frequency of hyperglycemia and hypoalbuminemia. The overall rate of event-free survival was lower in patients treated with L-asparaginase than in other patients (P = 0.06); estimated 3-year rates were 72% (95% confidence interval, 55-89%) and 87.2% (78-96%), respectively. We conclude that L-asparaginase (Leunase) given at 10,000 IU/m2 for nine doses was poorly tolerated and resulted in excessive toxicity, both through its effects as a single agent and possibly through potentiation of etoposide.

Operative repair of symptomatic spondylolysis following a positive response to diagnostic pars injection

From 1989 to 1996, 275 patients (245 male and 20 female) with back pain symptoms and spondylolysis of the lumbar spine were reviewed. All patients were evaluated by a protocol that included nonoperative treatment, bone scan, and pars injection. Only those whose symptoms failed nonoperative measures, showing negative bone scan and positive pars injection, were regarded as candidates for surgical management. Pars injection with Marcaine was done, and there were 93 cases with a positive response of reproducing symptoms and symptom relief. Patients then received autogenous bone grafting and internal fixation of the pars interarticularis defect. The internal fixation devices used included translaminar screws (AO 3.5 cancellous screw) for the most frequent level of L5, hook screws for levels above L4, and augmentation with wire for cases with concomitant spinal bifida occulta. The average age of the surgical group (85 male and 8 female) was 23 years (range: 19-35 years). After a follow-up averaging 30.4 months (range: 24-48 months), fusion results were 87%. Clinical results of 85 cases (91.3%) were excellent to good; 8 cases were fair; there were no poor cases. Direct repair of the pars defect by internal fixation and bone grafting was done to preserve involved motion segment and to prevent abnormal stresses at adjacent levels. These procedures seemed to be clinically effective.

Quantification of bacterial lipopolysaccharides by the purpald assay: measuring formaldehyde generated from 2-keto-3-deoxyoctonate and heptose at the inner core by periodate oxidation

We have adapted the purpald assay (M. S. Quesenberry and Y. C. Lee, Anal. Biochem. 234, 50-55, 1996) to quantify lipopolysaccharide (LPS) content in solution in 96-well microtiter plates at room temperature. This method employs the oxidation of unsubstituted terminal vicinal glycol groups in 2-keto-3-deoxyoctonate (Kdo) and l-(or d-)glycero-d-manno-heptose of LPS molecules by periodate to release formaldehyde. The formaldehyde is quantified at 550 nm (or 530-570 nm) by reacting with purpald reagent followed by oxidation with NaIO4. The sensitivity of the purpald assay is comparable to that of the Kdo assay for LPS determination. However, the purpald assay is superior to the Kdo assay because: (i) No acid hydrolysis of the samples and no boiling in the assay process are required; thus, it can be directly carried out with microtiter plates for a large number of samples at room temperature. (ii) The purpald assay can detect many types of LPS from various bacteria since LPS contains Kdo and heptose which possess unsubstituted terminal vicinal glycol in its structure, while the Kdo assay cannot detect LPS from certain bacteria (e.g., Haemophilus influenzae, Bordetella pertussis, and Vibrio cholerae) due to substitution at the C-4 and C-5 positions of Kdo.

Emodin, an anthraquinone derivative isolated from the rhizomes of Rheum palmatum, selectively inhibits the activity of casein kinase II as a competitive inhibitor

Ser/Thr protein kinases play important roles in signal transduction pathways that control the proliferation and differentiation of eukaryotic cells. In this paper, we present evidence that emodin, an anthraquinone derivative, selectively inhibits casein kinase II (CKII), a Ser/Thr kinase, as a competitive inhibitor. The results with ethyl acetate extracts of the rhizomes of Rheum palmatum showed that emodin significantly inhibited the activity of cyclin B/cdc2 protein kinase (cdc2). We measured IC50 values for emodin on the activities of several Ser/Thr protein kinases, including cAMP-dependent protein kinase (PKA), protein kinase C (PKC), cdc2, casein kinases I (CKI) and CKII. Interestingly, emodin inhibited CKII activity with an IC50 value of 2 microM, which was two to three orders of magnitude lower than those against the other kinases. Enzyme kinetic assays showed that emodin inhibited CKII activity as a competitive inhibitor against ATP with a Ki value of 7.2 microM. Collectively, we suggest that emodin is a selective CKII inhibitor, whose action mechanism is mediated through competitively binding to the ATP binding site.

Superoxide production of neutrophils after severe injury: impact of subsequent surgery and sepsis

To evaluate the early variations of superoxide production of neutrophils (SPN) in injured patients, SPN was serially measured on the first, third, and seventh day after severe injury (injury severity score of >16). For patients receiving subsequent surgery, SPN was measured again on the first postoperative day. Eighteen patients were studied. Six had subsequent surgery within 1 day (early operation); 6 had surgery 3 days after injury (late operation); 6 did not have surgery (nonoperation). SPN increased on the first day and recovered from the third day after injury in all three groups. In patients who had surgery, SPN did not significantly increase on the first postoperative day. Eight patients developed sepsis, 4 of whom had early multiple organ dysfunction (EMOD). On the last measurement, mean SPN was suppressed in septic patients with EMOD, whereas it was elevated in septic patients without EMOD. Patients with EMOD also had a higher injury severity score. In conclusion, subsequent surgery after injury has no effect on the priming of neutrophils. While late priming of neutrophils in injured patients coincides with the development of sepsis, suppression of SPN is found in septic patients with EMOD that frequently results from severe injury.

Minimum error rate training for PHMM-based text recognition

In this work, discriminative training is studied to improve the performance of our pseudo two-dimensional (2-D) hidden Markov model (PHMM) based text recognition system. The aim of this discriminative training is to adjust model parameters to directly minimize the classification error rate. Experimental results have shown great reduction in recognition error rate even for PHMMs already well-trained using conventional maximum likelihood (ML) approaches.

Pylephlebitis associated with appendicitis

Pylephlebitis usually occurs secondary to infection in the region drained by the portal venous system. A most common antesecent focus of infection is diverticulitis and the most common blood isolate is E. coli (54%), followed by Proteus mirabilis (23%). Overall mortality is 32% and most of the patients who had died had severe sepsis prior to the initiation of antibiotic therapy. We describe a case of pylephlebitis which had appendicitis and consequent septic thrombosis of the portal vein and its branches, with dissemination of infection to the liver. The patient had recovered due to timely antibiotic treatment alone and resulted in complete resolution. Early diagnosis and treatment are basic to a favorable clinical course.

Effects of IL-1 beta, TNF-alpha, and TGF-beta on ciliary beat frequency of human nasal ciliated epithelial cells in vitro

Previous reports suggested that several cytokines may influence the ciliary beat of the airway ciliated epithelial cells. The aim of this study is to determine the effects of cytokines including IL-1 beta, TNF-alpha, and TGF-beta on ciliary beat frequency (CBF) of human nasal ciliated epithelial cells. CBF of cultured human nasal ciliated epithelial cells was measured 24 hours after incubating with concentrations of 0.01 ng/mL, 0.1 ng/mL, 1 ng/mL, 10 ng/mL, and 100 ng/mL of each recombinant human (rh) cytokine including rhIL-1 beta, rhTNF-alpha, and rhTGF-beta. CBF was measured with time at concentrations of 1 ng/mL of rhIL-1 beta, 10 ng/mL of TNF-alpha, and 1 ng/mL of TGF-beta solutions. CBF of the human nasal ciliated epithelial cells increased after addition of rhIL-1 beta and rhTNF-alpha. Maximum CBF was observed at 1 ng/mL of rhIL-1 beta and at 10 ng/mL of rhTNF-alpha. CBF increased progressively to 4 hours after addition of rhIL-1 beta and rhTNF-alpha. Increased CBF sustained for 24 hours and decreased by 2 days. However, no variation of CBF was observed after addition of rhTGF-beta, regardless of concentrations and time. The results of this study suggest that during acute inflammation, IL-1 beta and TNF-alpha may have a potential role in defense mechanism of human nasal epithelium by regulating CBF of the nasal ciliated epithelial cells.

A case of pancytopenia secondary to low-dose pulse methotrexate therapy in a patient with rheumatoid arthritis and renal insufficiency

Most reports on serious MTX toxicity have focused on hepatic abnormalities, while other effects, including hematologic reactions, have not been emphasized. We experienced a case of pancytopenia secondary to MTX therapy in a patient with RA and renal insufficiency. A 67-year-old woman with a 12-year history of active seropositive RA that was a response to non-steroidal anti-inflammatory drugs, hydroxychloroquinine and intra-articular steroid injections, had been followed up and was diagnosed as early chronic renal failure in October, 1993. Recently, because of significant morning stiffness and polyarthralgia, the decision was made to institute MTX treatment. This was begun as a single oral dose of 5mg/week. After 2 doses, the patient was admitted to the hospital with general weakness. Laboratory tests showed a hemoglobin level of 7.9 g/dl, WBC count 1800/mm3 and platelet count of 64000/mm3. The serum creatinine level was 6.1 mEq/dl and the BUN level was 82 mEq/dl. Liver function test results were normal, but the serum albumin level was 2.7 g/dl. The patient subsequently developed fever and blood transfusions, granulocyte colony stimulating factor (G-CSF) and intravenous prophylactic antibiotic therapy were required. Her condition was improved. In summary, Low-dose MTX-related adverse hematologic side effects, including fatal pancytopenia, are rare but are a cause of increasing concern in patients with RA and renal insufficiency. Close monitoring of associated risk factors, particularly impaired renal function, should be mandatory for all patients who are receiving MTX therapy.

Spontaneous variability of arterial oxygenation in critically ill mechanically ventilated patients

OBJECTIVE

To assess the magnitude of spontaneous variability of arterial oxygenation and oxygen tension-based indices over time in medical intensive care unit (ICU) patients and to study whether high positive end-expiratory pressure (PEEP) or inverse inspiratory-to-expiratory (I:E) ratio ventilation (IRV) results in a greater variability than low PEEP with conventional I:E ratio ventilation.

DESIGN

Prospective study.

SETTING

Medical ICU in a tertiary medical center.

PARTICIPANTS

23 patients requiring a pulmonary artery floating catheter for hemodynamic monitoring.

INTERVENTION

After being completely sedated, patients were randomized to receive pressure-control ventilation at setting A: high PEEP (15 cm H2O) with conventional I:E ratio (1:2) and setting B: inverse I:E ratio (2:1) with low PEEP (5 cm H2O) alternately, and then at setting C: low PEEP (5 cm H2O) with conventional I:E ratio (1:2). Each ventilation setting lasted 1 h.

MEASUREMENTS AND RESULTS

The arterial and mixed venous blood samples were measured simultaneously at baseline (time 0), and at 15, 30, 45, and 60 min thereafter. The coefficient of variation (CV) of arterial oxygen tension (PaO2) over time was 5.9 % for setting A, 7.2 % for setting B, and 6.9 % for setting C. ANOVA showed no significant differences in CVs of PaO2 between the three settings. Oxygen tension-based indices, alveolar-arterial oxygen difference (A-aDO2) and PaO2/ PAO2 (alveolar oxygen tension), displayed CVs equal to that of PaO2; the CV of A-aDO2/PaO2 was significantly greater than that of PaO2.

CONCLUSIONS

In critically ill medical ICU patients, despite sedation, the spontaneous variability in PaO2 over time is substantial. A high PEEP or IRV does not contribute to the increased variation in PaO2.

Quinoline alkaloids from the fruits of Evodia officinalis

A new quinoline derivative, 2-hydroxy-4-methoxy-3-(3'-methyl-2'-butenyl)-quinoline and five known quinoline alkaloids were isolated from the fruits of Evodia officinalis. The structure of 1 was determined by spectroscopic methods.

Cryoglobulinaemia and rheumatic manifestations in patients with hepatitis C virus infection

OBJECTIVES

To investigate the association of cryoglobulinaemia and rheumatic manifestations in Korean patients with hepatitis C virus (HCV) infection.

METHODS

Forty nine Korean patients with HCV infection were recruited. The prevalence, concentration, and type of cryoglobulin (by immunofixation), rheumatoid factor (RF), antinuclear antibody (ANA), and various rheumatological symptoms were investigated and HCV genotype was determined by polymerase chain reaction with genotype specific primer.

RESULTS

The prevalence of cryoglobulin was 59% in Korean HCV patients and the concentration of cryoglobulin was 9.8 (7.9) g/l (mean (SD)). The type of cryoglobulinaemia was identified in 23 (80%) of 29 HCV patients with cryoglobulinaemia and they were all type III. There were no differences in age, sex, history of operation and transfusion, proportion of liver cirrhosis between the patients with cryoglobulinaemia and those without cryoglobulinaemia. The frequencies of RF and ANA were 14% and 3.4% respectively in HCV patients with cryoglobulinaemia. There was no difference in HCV genotype between the patients with cryoglobulinaemia and those without cryoglobulinaemia. Clinical features of HCV patients were as follows: arthralgia/arthritis (35%), cutaneous manifestation (37%), Raynaud's phenomenon (8%), paresthesia (44%), dry eyes (22%), dry mouth (10%), oral ulcer (33%), and abdominal pain (14%). However, these rheumatological symptoms did not differ between the two groups.

CONCLUSION

Although the rheumatological symptoms were not different between HCV patients with and without cryoglobulinaemia, HCV patients showed various rheumatological manifestations. These result suggests that HCV infection could be included as one of the causes in patients with unexplained rheumatological symptoms.

Functional pancreatic islet cell tumors with liver metastasis: the role of cytoreductive surgery and transcatheter arterial chemoembolization: a report of five cases

Malignant pancreatic islet tumors are slow-growing tumors. Their relatively benign behavior makes aggressive treatment worthwhile. From January, 1987, to January, 1998, five cases of malignant pancreatic islet tumors with liver metastasis were diagnosed at the Veterans General Hospital-Taipei. Of these, three were gastrinomas and the others were vasoactive intestinal peptide (VIPoma, 1 case) and insulinoma (1 case). Four patients (3 with gastrinomas and 1 with insulinoma) had undergone cytoreductive surgery when the diagnosis of metastasis was made. All five patients underwent transcatheter arterial chemoembolization (TACE). All patients had improved symptoms after cytoreductive surgery and TACE. The survival of patients who underwent combined surgery and TACE was 38 and 17 months in the two gastrinoma cases, more than eight months in one gastrinoma case and more than 20 months in the insulinoma case (these 2 patients are still alive). One VIPoma patient who underwent TACE survived for 12 months. In conclusion, treatment for metastatic pancreatic islet cell tumors require a multidisciplinary approach. Metastasis of the tumor is not a contraindication for aggressive therapy. Combined cytoreductive surgery and TACE can relieve symptoms and are of benefit for patients with pancreatic islet cell tumors with liver metastases.

Endoscopic treatment in a case with extensive spontaneous intramural dissection of the oesophagus

Spontaneous intramural dissection of the oesophagus has been recognized as a separate clinical entity between a transmural oesophageal rupture (Boerhaave syndrome) and an oesophageal mucosal tear (Mallory-Weiss syndrome). It is important to differentiate this condition from myocardiac infarction, dissecting aneurysm and other acute surgical conditions. Conservative management is usually thought to be adequate. We report a case of spontaneous intramural oesophageal dissection, in which the symptom of dysphagia did not improve with the conservative management and an endoscopic incision of the septum between true and false lumens using a needle-type diathermy knife was done safely and effectively.

Cloning and characterization of mouse RIP140, a corepressor for nuclear orphan receptor TR2

The mouse homologue of the human receptor-interacting protein 140 (RIP140) was isolated from a mouse embryonic cDNA library in yeast two-hybrid screening experiments by using the ligand binding domain (LBD) of nuclear orphan receptor TR2 as the bait. The receptor-interacting domains of mouse RIP140 were mapped to the regions containing the LXXLL motif (where L is leucine and X is any amino acid), and the RIP140-interacting domain of TR2 was mapped to its C-terminal 10- to 20-amino-acid sequence, a putative activation function 2 (AF-2) region. In a GAL4 reporter system and a reporter driven by the proximal region of the TR2 promoter, RIP140 functioned as a corepressor for both a GAL4 DNA binding domain (BD)-TR2 fusion and the wild-type receptor. When tethered to the BD of GAL4, RIP140 exerted a trans-repressive effect on the GAL4 reporter. In addition, RIP140 suppressed the retinoic acid (RA) receptor-mediated RA induction in a dose-dependent manner. Finally, it was demonstrated that in the presence of RIP140, a cytosolic, green fluorescent protein-tagged TR2 LBD translocated into the nucleus, and TR2 and RIP140 were coimmunoprecipitated from the cell extract, indicating that the interaction between RIP140 and the LBD of TR2 occurred in vivo. The potential biological role of RIP140 in TR2-modulated transcriptional activity is discussed.

Risk factors for Enterobacter septicemia in a neonatal unit: case-control study

Thirty cases of Enterobacter aerogenes or Enterobacter cloacae septicemia diagnosed over a 32-month period in a tertiary care neonatal unit were enrolled in a case-control study. Each case patient was matched with two controls (patients occupying the cots nearest the case patient when the latter developed septicemia). Of the 32 perinatal characteristics evaluated, 11 were identified by univariate analysis to be significantly associated with the infection. These included parents being residents of the Vietnamese refugee camps, respiratory distress syndrome, necrotizing enterocolitis, umbilical arterial catheterization, umbilical venous catheterization, bladder catheterization, mechanical ventilation, antibiotic treatment, peripheral venous catheterization, nasogastric intubation, and parenteral nutrition. Multivariate analysis, however, showed that preceding bladder catheterization and ongoing parenteral nutrition were the only independent risk factors for enterobacter septicemia. Strict aseptic technique in the preparation of parenteral nutrition fluid and avoidance of bladder catheterization are measures that may reduce the risk of enterobacter sepsis for newborns.

Is homozygous alpha-thalassaemia a lethal condition in the 1990s?

Two cases of homozygous alpha-thalassaemia who received active treatment in accordance with parental wishes are reported. One infant survived and the other, although successfully weaned off mechanical respiratory support, unexpectedly developed portal vein thrombosis and died. Homozygous alpha-thalassaemia, a condition previously considered to be universally fatal, and an indication for therapeutic abortion, is now potentially curable with advances in diagnostic technology and treatment. However, active management of these cases raises serious ethical questions and has major financial implications on the health-care system. Invasive prenatal and intensive postnatal interventions should remain experimental and cannot be recommended as routine clinical practice until the questions of long-term neurodevelopmental outcome, and the morbidity and mortality associated with bone-marrow transplantation have been fully addressed. As a result of advances in information technology, more and more parents of affected foetuses are likely to request active treatment.

Inhibition of aortic wall calcification in bioprosthetic heart valves by ethanol pretreatment: biochemical and biophysical mechanisms

The effectiveness of ethanol pretreatment on preventing calcification of glutaraldehyde-fixed porcine aortic bioprosthetic heart valve (BPHV) cusps was previously demonstrated, and the mechanism of action of ethanol was attributed in part to both lipid removal and a specific collagen conformational change. In the present work, the effect of ethanol pretreatment on BPHV aortic wall calcification was investigated using both rat subdermal and sheep circulatory implants. Ethanol pretreatment significantly inhibited calcification of BPHV aortic wall, but with less than complete inhibition. The maximum inhibition of calcification of BPHV aortic wall was achieved using an 80% ethanol pretreatment; calcium levels were 71.80+/-8.45 microg/mg with 80% ethanol pretreatment compared to the control calcium level of 129.90+/-7.24 microg/mg (p = 0.001). Increasing the duration of ethanol exposure did not significantly improve the inhibitory effect of ethanol on aortic wall calcification. In the sheep circulatory implants, ethanol pretreatment partly prevented BPHV aortic wall calcification with a calcium level of 28.02+/-4.42 microg/mg compared to the control calcium level of 56.35+/-6.14 microg/mg (p = 0.004). Infrared spectroscopy (ATR-FTIR) studies of ethanol-pretreated BPHV aortic wall (vs. control) demonstrated a significant change in protein structure due to ethanol pretreatment. The water content of the aortic wall tissue and the spin-lattice relaxation times (T1) as assessed by proton nuclear magnetic resonance spectroscopy did not change significantly owing to ethanol pretreatment. The optimum condition of 80% ethanol pretreatment almost completely extracted both phospholipids and cholesterol from the aortic wall; despite this, significant calcification occurred. In conclusion, these results clearly demonstrate that ethanol pretreatment is significantly but only partially effective for inhibition of calcification of BPHV aortic wall and this effect may be due in part to lipid extraction and protein structure changes caused by ethanol. It is hypothesized that ethanol pretreatment may be of benefit for preventing bioprosthetic aortic wall calcification only in synergistic combination with another agent.

A constitutive nuclear localization signal from the second zinc-finger of orphan nuclear receptor TR2

The orphan nuclear receptor TR2 and its truncated isoform deleted in the ligand binding domain (LBD) were localized exclusively in the nuclei as revealed by two methods of detection. An anti-hemagglutinin (HA) antibody detected specific nuclear localization of HA-tagged receptors and the green fluorescent protein (GFP)-tagged receptors were found to be distributed in the nuclei of living cells. By deletion analyses, the sequence responsible for targeting this receptor into the nucleus was defined. A stretch of 20 amino acid residues (KDCVINKHHRNRCQYCRLQR) within the second zinc-finger of this receptor is required for its nuclear localization and this signal is constitutively active. No nuclear localization signal was found in the N-terminus or the LBD. The GFP-tagged receptor remained biologically active, as evidenced by its repressive activity on the reporter that carried a binding site for this receptor, a direct repeat-5 (DR5). An electrophoretic mobility shift assay was performed to characterize the binding property of TR2 and its truncated isoform. TR2 bound to the DR5 as dimers whereas its truncated isoform bound as monomers.

Increased P-glycoprotein messenger RNA stability in rat liver tumors in vivo

P-glycoproteins (Pgp) are comprised of a small family of plasma membrane proteins whose abundance in cultured cells is often associated with the multidrug resistance phenotype. Overexpression of Pgp has been observed in many types of human cancers, but the molecular basis for this overexpression has not been established. We have used primary monolayer cultures of adult rat hepatocytes and a stepwise model of rat liver carcinogenesis to study the regulation of Pgp gene expression. We observed a marked overexpression of Pgp, specifically the class II Pgp, in both systems. In addition, we observed that a number of unrelated genes including alpha-tubulin, beta-actin, gamma-actin, cytokeratin 8, cytokeratin 18, and c-myc are overexpressed in cultured hepatocytes, and they are also overexpressed during liver carcinogenesis and in transplantable tumors. Nuclear run-on assays showed no increase in the transcriptional activity of Pgp genes in transplantable liver tumors compared to normal liver. Studies of in vivo mRNA stability, however, revealed that all three Pgp mRNAs were relatively stable in transplantable liver tumors (t(1/2) > 12 h), in contrast to what was found in normal liver (t(1/2) < 2 h). In addition, mRNA for several other genes, including alpha-tubulin, c-myc, and cyclin D1, all appear to be stabilized in the tumors. These findings suggest that the overexpression of Pgp genes in rat liver tumors may be the result of a mechanism involving stabilization of a diverse group of mRNAs.

Purification and characterization of a polysome-associated endoribonuclease that degrades c-myc mRNA in vitro

The regulation of mRNA half-lives is determined by multiple factors, including the activity of the messenger RNases (mRNases) responsible for destroying mRNA molecules. Previously, we used cell-free mRNA decay assays to identify a polysome-associated endonuclease that cleaves c-myc mRNA within the coding region. A similar activity has been solubilized and partially purified from a high salt extract of adult rat liver polysomes. Based on a correlation between protein and enzyme activity, the endonuclease is tentatively identified as a approximately 39-kDa protein. It cleaves the coding region stability determinant of c-myc mRNA with considerable specificity. Cleavages occur predominantly in an A-rich segment of the RNA. The endonuclease is resistant to RNase A inhibitors, sensitive to vanadyl ribonucleoside complex, and dependent on magnesium. In these and other respects, the soluble enzyme we have purified resembles the polysome-associated c-myc mRNase.