Evolutionary characterization of recent human H3N2 influenza A isolates from Japan and China: novel changes in the receptor binding domain

Recent human H3N2 influenza viruses isolated in Japan and China were characterised from an evolutionary point of view. They appeared to have divided into three minor branch clusters, including 1992-1993, 1993-1994 and 1994-1995 isolates. It was of particular interest to reveal that in addition to amino acid substitutions in the antigenic sites of the HA molecule, amino acid changes occurred at position 226 of the receptor binding site from lysine or glutamine to isoleucine in all strains belonging to the 1994-1995 branch cluster. This is the first evidence of human H3N2 influenza isolates, or any other influenza HA serotypes, to contain a conserved amino acid residue other than lysine or glutamine at this key position.

L-NA-sensitive rCBF augmentation during vibrissal stimulation in type III nitric oxide synthase mutant mice

Regional cerebral blood flow (rCBF) was studied in type III nitric oxide (NO) synthase (endothelial, eNOS) mutant and wild type mice during mechanical whisker stimulation before and after nitro-L-arginine (L-NA) superfusion using the closed cranial window technique. rCBF increased equally in cortical barrel fields in both strains during stimulation, as measured by laser Doppler-flowmetry, and was inhibited by L-NA superfusion (1 mM) in both groups. Hence, coupling of blood flow and metabolism appears neuronal NOS-(nNOS) but not eNOS-dependent in cortical barrel fields of the mouse.

[The effect of mitomycin C on activities of carbonic anhydrase and ATPase of the ciliary body epithelium in rabbit]

PURPOSE

Enzymohistochemical methods were performed to observe the effect of mitomycin C(MMC) on the activities of carbonic anhydrase (CA) and ATPase of the ciliary body epithelium in rabbits after using 0.2mg/ml mitomycin C during sclerectomy for the further study on the mechanism of the decrease of intraocular pressure postoperatively.

METHODS

0.2mg/ml mitomycin C was topically applicated during the sclerectomy in rabbit eyes. 7 days postoperatively, Hasson's method and plumbum nitrate's method were used to stain the CA and ATPase of the ciliary body epithelium. The grey value of the enzymes was quantitated by Image Analysor.

RESULTS

MMC inhibited the activities of CA and ATPase of the ciliary body epithelium underlying the site of MMC application, but the difference of enzymatic activities of the region 90 degree apart from the surgical site and that of surgical site in normal saline control group was not statistically obvious (P < 0.05).

CONCLUSIONS

MMC could inhibit the activities of CA and ATPase of ciliary body epithelium, thus leading to the decreased secretion of humor aqueous and the lowerdown of intraocular pressure even persistent hypotony.

Functional expression of the cystic fibrosis transmembrane conductance regulator in yeast

Recombinant human cystic fibrosis transmembrane conductance regulator (CFTR) has been produced in a Saccharomyces cerevisiae expression system used previously to produce transport ATPases with high yields. The arrangement of the bases in the region immediately upstream from the ATG start codon of the CFTR is extremely important for high expression levels. The maximal CFTR expression level is about 5-10% of that in Sf9 insect cells as judged by comparison of immunoblots. Upon sucrose gradient centrifugation, the majority of the CFTR is found in a light vesicle fraction separated from the yeast plasma membrane in a heavier fraction. It thus appears that most of expressed CFTR is not directed to the plasma membrane in this system. CFTR expressed in yeast has the same mobility (ca. 140 kDa) as recombinant CFTR produced in Sf9 cells in a high resolution SDS-PAGE gel before and after N-glycosidase F treatment, suggesting that it is not glycosylated. The channel function of the expressed CFTR was measured by an isotope flux assay in isolated yeast membrane vesicles and single channel recording following reconstitution into planar lipid bilayers. In the isotope flux assay, protein kinase A (PKA) increased the rate of 125I- uptake by about 30% in membrane vesicles containing the CFTR, but not in control membranes. The single channel recordings showed that a PKA-activated small conductance anion channel (8 pS) with a linear I-V relationship was present in the CFTR membranes, but not in control membranes. These results show that the human CFTR has been expressed in functional form in yeast. With the reasonably high yield and the ability to grow massive quantities of yeast at low cost, this CFTR expression system may provide a valuable new source of starting material for purification of large quantities of the CFTR for biochemical studies.

Influence of fludarabine on pharmacokinetics and pharmacodynamics of cytarabine: implications for a continuous infusion schedule

Arabinosylcytosine (ara-C) is a cytotoxic agent with major activity against acute leukemias. To exert this effect, it must first be phosphorylated to its active 5'-triphosphate, ara-CTP, which is incorporated into DNA. Our previous studies demonstrated that preincubation with arabinosyl-2-fluoroadenine (F-ara-A) increased the rate of ara-CTP accumulation in leukemia cells when incubated with 10 microM ara-C. Such concentrations of ara-C are readily obtained during intermittent bolus infusions of ara-C, and clinical trials were conducted using fludarabine in combination with 2-h infusions of intermediate-dose ara-C. During continuous infusion of ara-C, however, serum ara-C levels are <10 microM. Because the effectiveness of ara-C depends on the levels of intracellular ara-CTP and its incorporation into DNA, we sought to investigate the influence of fludarabine on pharmacodynamics of ara-C at concentrations of ara-C achieved during continuous infusion. Using the K562 human leukemic cell line, we established that incubation with 30 microM F-ara-A was able to modulate intracellular dNTP pools and achieve maximum enhancement of ara-CTP levels at all concentrations of ara-C tested (0.3-10.0 microM). The relative enhancement of ara-CTP concentrations ranged from 2.2- to 2.8-fold. Combination of F-ara-A with 1.0 and 3.0 microM ara-C also increased the incorporation of ara-CTP into DNA. To model the influence of F-ara-A on continuous infusion ara-C, cells were incubated with 1 microM ara-C alone or in combination with F-ara-A. The F-ara-A-incubated cells accumulated effective intracellular concentrations of F-ara-ATP, which resulted in greatly increased intracellular ara-CTP levels. These studies demonstrate the capacity of clinically attainable concentrations of F-ara-ATP to enhance the formation of ara-CTP at concentrations of ara-C that are achieved during a continuous infusion schedule. Given the important role intracellular ara-CTP concentrations and ara-CMP incorporation into DNA have on the ultimate cytotoxic capacity of ara-C against acute myelogenous leukemia blasts, these studies suggest a promising pharmacological model for improving the efficacy of the continuous infusion ara-C regimen.

The effect of whole-body irradiation of nude mice on the tumor transplantability and control probability of a human soft tissue sarcoma xenograft

This study has evaluated the impact of the suppression and recovery of the residual immunity in NCr/Sed nude (nu/nu) mice after whole-body irradiation using xenotransplantability and tumor control probability as the end points. For this investigation the xenograft was a human soft tissue sarcoma (HSTS26T). Two assays, the TD50 (the number of tumor cells required to induce a tumor in 50% of the recipients) and the TCD50 (the radiation dose required to control 50% of tumors) were used. For TD50 assays, tumor cells were injected subcutaneously (sc) into the legs of control and whole-body- irradiated nude mice at 1 day or 4, 8 or 12 weeks after irradiation. For TCD50 assays, tumors were transplanted sc into the legs of nude mice which had not been irradiated or which had been given whole-body irradiation at 1 day or 12 weeks prior to transplantation. The tumors were given single-dose irradiation when they reached 6 mm mean diameter under clamp-hypoxic conditions. The results show that the TD50's of mice receiving the injection 1 day and 4 and 8 weeks after whole-body irradiation were 3.6 to >100 times lower than that of unirradiated mice. Two groups which showed a statistically significant difference in TD50 's were those which received the injection 1 day and 8 weeks after whole-body irradiation (P < 0.01 and P < 0.05, respectively). No difference was found in TD50 values between mice that received injection 12 weeks after whole-body irradiation and those which were not irradiated. The TCD50 values of tumors in nonirradiated mice and in mice which had received whole-body irradiation 1 day and 12 weeks prior to to transplantation were 26.8, 44.1 and 33.9 Gy, respectively. Significantly lower TCD50 values were found in groups of nonirradiated mice or mice which received whole-body irradiation 12 weeks prior to transplantation in comparison with the group of mice that received whole-body irradiation on day 1 (both P < 0.05). No significant difference was found between the TCD50 values of the group of mice that received whole-body irradiation 12 weeks prior to transplantation and those for nonirradiated controls. Our conclusion is that the whole-body irradiation can enhance the transplantability of the HSTS26T tumor in nude mice significantly; this enhancing effect will decrease to the pre-irradiation level by 12 weeks after whole-body irradiation. Also, the suppression and recovery of residual immunity after whole-body irradiation can influence the TCD50 values of the same tumor xenografts in nude mice significantly. The changes in TD50 and TCD50 values correlate with the depletion and recovery of the total splenic lymphoid cell number, and especially in natural killer cell activity. We recommend that further immunosuppression in nude mice is necessary when using this model system for studies of human tumors.

[Restoration of external auditory canal by filling mastoid cavity with carbon pellet]

To prevent infection of an open mastoid cavity after mastoidectomy and improve hearing, the external auditory canal had been maintained on 72 cases (73 ears) by filling mastoid cavity with carbon pellet. The average duration of dry ear after operation was 25 days with a dry ear rate being 98.6%. No phenomenon of rejection and falling out appeared within 1-7 years follow-up. Carbon pellet has advantages of chemical stability, nontoxicity, good histocompatability, anti-fatigue characteristic in body fluid, and anti-corrosive effect against acid, base and salt.

[Study on the principles of base change in 4-fold degenerate sites of protein coding genes]

The relationship of transition and transversion of base change in 4-fold degenerate sites of coding genes of 6 proteins, which showed 70% homology in wmposition investigated. Taking into account of the effects of base composition on base substitution, it was found that the transition-transversion bias was evident but not as pronounced as in mitochondrial DNA. Comparison of different kinds of transition or transversion indicated that they happened with rather equal vate, with 0.65-0.73 for different kinds of transversion and 0.99 for A-G transition assuming that T-C transition was 1. The causes for transition-transversion bias is discussed and it is proposed that mutagens may play a role in it.

Preclinical characteristics of gemcitabine

Gemcitabine (2',2'-difluorodeoxycytidine, dFdC) is a nucleoside analogue of deoxycytidine in which two fluorine atoms have been inserted into the deoxyribofuranosyl ring. Once inside the cell gemcitabine is rapidly phosphorylated by deoxycytidine kinase, the rate-limiting enzyme for the formation of the active metabolites gemcitabine diphosphate (dFdCDP) and gemcitabine triphosphate (dFdCTP). Gemcitabine diphosphate inhibits ribonucleotide reductase, which is responsible for producing the deoxynucleotides required for DNA synthesis and repair. The subsequent decrease in cellular deoxynucleotides (particularly dCTP) favours gemcitabine triphosphate in its competition with dCTP for incorporation into DNA. Reduction in cellular dCTP is an important self-potentiating mechanism resulting in increased gemcitabine nucleotide incorporation into DNA. Other self-potentiating mechanisms of gemcitabine include increased formation of active gemcitabine di- and triphosphates, and decreased elimination of gemcitabine nucleotides. After gemcitabine nucleotide is incorporated on the end of the elongating DNA strand, one more deoxynucleotide is added, and thereafter the DNA polymerases are unable to proceed. This action, termed "masked chain termination", appears to lock the drug into DNA because proof-reading exonucleases are unable to remove gemcitabine nucleotide from this penultimate position. Incorporation of gemcitabine triphosphate into DNA is strongly correlated with the inhibition of further DNA synthesis. Compared with ara-C, gemcitabine serves as a better transport substrate, is phosphorylated more efficiently, and is eliminated more slowly. These differences, together with self-potentiation, masked chain termination and the inhibition of ribonucleotide reductase, which are not seen with ara-C, may explain why gemcitabine is, and ara-C is not, active in solid tumours. This unique combination of metabolic properties and mechanistic characteristics suggests that gemcitabine is likely to be synergistic with other drugs that damage DNA, and also with other modalities such as radiation.

Construction, expression and characterization of tissue-type plasminogen activator mutants

Three tissue-type plasminogen activator (t-PA) mutants were constructed by recombinant and site-directed mutagenesis techniques. They are del(296-302) with deletion of PAI-1 binding site, N117Q/N184Q with deglycosylation of K1 and K2 domains, and their combination mutant designated as GGI. Then these three mutants were successfully transiently expressed in COS-7 cells, and GGI was further stably expressed in CHO cells. The biological characterization of the expression products indicated that del(296-302) and GGI possessed the resistance to inhibition by PAI-1. In addition, the specific activity of GGI was increased by about 46%, the plasma half-life was prolonged by about one fold, while its affinity for fibrin was not affected.

Inhibitory effect of 3'-untranslated region (3'-UTR) of human tissue-plasminogen activator (ht-PA) mRNA on its expression

A series of t-PA cDNA mutants containing different parts of 3'-UTR sequences have been constructed. In vitro translation of t-PA transcripts in rabbit reticulocyte lysates and its expression in COS-7 cells show that the 3'-UTR sequence has a very strong inhibitory effect on t-PA translation. The deletion of 3'-UTR results in 3-8-fold increase of t-PA expression. Further study shows that an AU-rich sequence of some 200 nt at 3' end of 3'-UTR is responsible for the translational inhibition. RNA stability experiment reveals that the AU-rich segment leads to a 3-fold decrease of t-PA mRNA stability. The insertion of this segment into the 3'-UTR of luciferase gene results in an obvious inhibition of Luc expression. A model is proposed for the regulation of t-PA expression.

The first reported outbreak of diarrheal illness associated with Cyclospora in the United States

OBJECTIVE

To investigate and characterize the epidemiology of a diarrheal outbreak associated with a potentially new pathogen, Cyclospora species (previously referred to as Cyanobacteria [blue-green algae]-like bodies).

DESIGN

Three retrospective cohort studies supported by laboratory studies, environmental investigation, and community surveillance.

SETTING

A hospital in Chicago.

PARTICIPANTS

Housestaff physicians and hospital administrative staff.

MEASUREMENTS

Identification of clinical features associated with illness and potential risks for acquisition of infection.

RESULTS

Illness was characterized by watery diarrhea, abdominal cramping, decreased appetite, and low-grade fever. Symptoms typically occurred in a distinctive cycle of remissions and exacerbations lasting up to several weeks. Stool cultures and examinations for known ova and parasites were negative. Microscopic examination of stool specimens from 11 ill persons showed many spherical bodies, 8 to 10 microns in diameter, that were identified as Cyclospora organisms. The organisms disappeared by 9 weeks after onset of illness in the 7 patients from whom follow-up specimens were obtained. Epidemiologic studies implicated tap water from a physicians' dormitory as the most likely source of the outbreak. Environmental investigation suggested that stagnant water in a storage tank may have contaminated the water supply after a pump failure.

CONCLUSIONS

This is the first reported outbreak of diarrhea associated with Cyclospora in the United States. Cyclospora may be a human enteric pathogen able to produce bouts of acute and relapsing diarrhea, and it should be considered in assessments of patients with unexplained, prolonged diarrheal illness.

High molecular weight DNA fragmentation: a critical event in nucleoside analogue-induced apoptosis in leukemia cells

Cleavage of DNA into internucleosomal fragments is one of the characteristics of apoptosis. However, searches for in vivo evidence of nucleosomal DNA fragmentation in leukemia cells freshly obtained from patients during chemotherapy frequently failed to reveal nucleosomal multimers (DNA ladders). It is not clear whether this type of DNA cleavage is an essential event in drug-induced apoptosis and thus a denominator of cell killing, or whether the internucleosomal DNA fragments are merely the by-products of the apoptotic process. Here, we report our investigation into the role of DNA fragmentation in apoptotic cell death induced by anticancer nucleoside analogues, both in cell culture and in leukemia patients undergoing chemotherapy. Using a 5'-end DNA-labeling technique and pulsed field gel electrophoresis, we detected fragmentation of DNA in two distinct size classes, internucleosomal and high molecular weight (predominantly 50 kb) DNA fragments, in a human leukemia cell line exposed to the nucleoside analogues fludarabine and gemcitabine. We further demonstrated that the two types of DNA fragmentation were separate events, distinguishable by their requirements for Ca2+ and responses to phorbol ester treatment. The drug-treated cells underwent morphological changes of apoptosis even after internucleosomal DNA fragmentation was selectively inhibited by intracellular Ca2+ chelation, or by treatment with phorbol ester. In contrast, neither apoptotic morphology nor internucleosomal DNA fragmentation was observed when the high molecular weight DNA fragmentation was blocked by inhibition of nucleoside analogue incorporation into DNA. These results suggest that cleavage of DNA into large fragments may be an initial event that is critical for drug-induced apoptosis, whereas activation of a Ca2+-dependent endonuclease to cleave DNA at internucleosomal sites is not an absolute requirement for the execution of the apoptotic cell death program. Further studies of leukemic lymphocytes obtained from 9 patients with chronic lymphocytic leukemia during therapy with fludarabine revealed high molecular weight DNA fragmentation, which was correlated with a decrease of peripheral lymphocytes in 6 patients, whereas only 1 of the 15 patients evaluated for nucleosomal DNA fragments showed the DNA ladders. These results indicate that high molecular weight DNA fragmentation occurs in vivo, and may be correlated with the cytotoxic action of the anticancer drugs. Further study of the association of high molecular weight DNA fragmentation with clinical response to chemotherapy is warranted.

Induction of apoptosis by gemcitabine

Inhibition of cellular DNA synthesis is the major action of gemcitabine. In cells, this drug is converted to its triphosphate (dFdCTP), which is incorporated into DNA and terminates DNA strand elongation. After incorporation of gemcitabine nucleotide into the DNA strand, one more deoxynucleotide is incorporated, and thereafter the DNA polymerases are unable to proceed ("masked chain termination"). Gemcitabine also inhibits DNA synthesis indirectly by decreasing cellular dNTP pools via inhibition of ribonucleotide reductase. Incubation of human leukemia cells (CEM) with gemcitabine leads to apoptotic cell death. Two types of DNA fragmentation were observed in the gemcitabine-treated cells: (1) large-sized double-stranded DNA fragments range from 5 kb to 500 kb with the majority of the fragments located at 50 kb, and (2) nucleosomal-sized DNA fragments. Both types of drug-induced DNA fragmentation were detected in exponentially growing cells and were much more prominent in cells synchronized at S phase. The gemcitabine-induced DNA fragmentation in either synchronized or nonsynchronized cells was inhibited by the DNA synthesis inhibitor, aphidicolin. Thus, incorporation of gemcitabine into DNA is essential to induce DNA fragmentation. The intracellular calcium chelator BAPTA-AM inhibited the drug-induced nucleosomal DNA fragmentation but did not prevent the large-sized DNA fragmentation, suggesting that the nucleosomal DNA fragmentation is a calcium-dependent event, whereas the large-sized DNA fragmentation is independent of calcium. Furthermore, BAPTA-AM did not prevent the morphologic appearance of apoptotic bodies in cells incubated with gemcitabine, indicating that degradation of DNA to nucleosomal fragments is not an essential element of the apoptotic process. Phorbol 12-myristate 13-acetate also inhibited drug-induced nucleosomal DNA fragmentation, but prevented neither large-sized DNA fragmentation nor formation of apoptotic bodies. In contrast, aphidicolin inhibited both types of DNA fragmentation and blocked the formation of apoptotic bodies in the presence of gemcitabine. These data suggest that the generation of large-sized DNA fragments caused by incorporated gemcitabine monophosphate in DNA is critical in gemcitabine-induced apoptosis, whereas nucleosomal DNA fragmentation is not a requirement in this cell death process.

Growth and metastatic behavior of five human glioblastomas compared with nine other histological types of human tumor xenografts in SCID mice

The growth and metastatic behavior of five human glioblastoma multiforme xenografts and nine human xenografts of various histological types were compared in severe combined immunodeficient (SCID) mice. The results demonstrate that the metastatic behavior of the human glioblastoma multiforme xenografts did not differ significantly from a variety of other histological xenografts when evaluated at the same transplantation site in the SCID model. These results are consistent with the hypothesis that the site of glioblastoma multiforme growth influences the extraneural metastatic spread of this disease and lead the authors to suggest that the clinical rarity of distant metastasis is not a fundamental property of these cells. A total of 340 male 7- to 8-week-old SCID mice received subcutaneous transplantation of tumor fragments (21-25 mice per tumor type). The tumor-bearing leg was amputated when the tumor reached a volume of 500 mm3; mice were observed for up to 5 months. There was a trend for a lower take rate, longer latent period, longer volume doubling time (VDT) and growth time (GT) in glioblastoma multiforme as opposed to carcinoma and soft tissue sarcoma xenografts. The highest local recurrence rates (78% and 68%) were observed in two glioblastomas multiforme. Both the glioblastoma multiforme and the other histological xenografts exhibited a widely varying metastatic rate: no correlation was demonstrated between VDT, GT, local control/recurrence, and distant metastasis. These findings show SCID mice to be an attractive model for further biological and preclinical studies of human glioblastoma multiforme.

Gemcitabine: metabolism, mechanisms of action, and self-potentiation

Gemcitabine (dFdC) is a new anticancer nucleoside that is an analog of deoxycytidine. It is a pro-drug and, once transported into the cell, must be phosphorylated by deoxycytidine kinase to an active form. Both gemcitabine diphosphate (dFdCTP) and gemcitabine triphosphate (dFdCTP) inhibit processes required for DNA synthesis. Incorporation of dFdCTP into DNA is most likely the major mechanism by which gemcitabine causes cell death. After incorporation of gemcitabine nucleotide on the end of the elongating DNA strand, one more deoxynucleotide is added and thereafter, the DNA polymerases are unable to proceed. This action ("masked termination") apparently locks the drug into DNA as the proofreading enzymes are unable to remove gemcitabine from this position. Furthermore, the unique actions that gemcitabine metabolites exert on cellular regulatory processes serve to enhance the overall inhibitory activities on cell growth. This interaction is termed "self-potentiation" and is evidenced in very few other anticancer drugs.

Difluorodeoxyguanosine: cytotoxicity, metabolism, and actions on DNA synthesis in human leukemia cells

The success of gemcitabine (2',2'-difluorodeoxycytidine; dFdC) resulted in new interest in its purine congeners. Based on the structure-activity relationship studies of catabolism and anabolism, 2',2'-difluorodeoxyguanosine (dFdG) emerged as a lead candidate among the difluoropurine analogs. The cytotoxicity, metabolism, and actions of dFdG on DNA synthesis were studied in the human leukemia lymphoblastoid line CCRF-CEM. The IC50 values of dFdG after a 72-hour continuous incubation were 0.01, 0.03, and 0.28 mumol/L for CCRF-CEM, K562, and HL-60 cells, respectively. A cell line deficient in dCyd kinase was equally sensitive to dFdG, suggesting that, in contrast to dFdC, dFdG may be activated by other deoxynucleoside kinase(s). Consistent with these data, coincubation with dGuo spared the dFdG-mediated toxicity; however, up to 500 mumol/L dCyd failed to reverse the toxicity of dFdG. These observations indicated that dGuo kinase, which phosphorylates arabinosylguanine, also appears to play a major role in activating dFdG. CCRF-CEM cells incubated with varying concentrations of [3H]dFdG accumulated dFdGTP in a dose-dependent manner; a 3-hour incubation with 1 mmol/L dFdG resulted in more than 600 mumol/L intracellular dFdGTP. This is in contrast to the gemcitabine triphosphate accumulation, which is saturated at 10 to 20 mumol/L of exogenous dFdC. dFdG metabolites affected ribonucleotide reductase, resulting in a lowering of the dCTP pool; this is in agreement with the effect of dFdC on dNTP pools in leukemia cell lines. The major effect of dFdG on macromolecular synthesis was inhibition of DNA synthesis. DNA primer extension over a defined template revealed that dFdGTP was a good substrate for DNA polymerase alpha and incorporated opposite C sites of the template. Unlike arabinosyl analogs, but similar to gemcitabine triphosphate, dFdGTP incorporation caused DNA polymerase to pause after one normal deoxynucleotide was incorporated beyond the analog. The unique activation requirements of dFdG, its novel mode of inhibition of DNA synthesis, and its potent toxicity to human leukemia cells make it a promising new antimetabolite.

An overview of hospital accreditation in Taiwan, Republic of China

This paper examines the hospital accreditation system in Taiwan, Republic of China. The paper describes the historical evolution of accreditation; preliminary teaching hospital accreditation; hospital and teaching hospital accreditation; issues and problems; and perspectives and challenges.

Redox-dependent changes in beta-sheet and loop structures of Cu,Zn superoxide dismutase in solution observed by infrared spectroscopy

Redox-dependent conformational changes of bovine Cu,Zn superoxide dismutase in 20 mM phosphate buffer (pH 7.4) were studied at 20 degrees C using Fourier transform infrared spectroscopy. Amide I spectra provide evidence that conformational changes in the protein accompany a change in the oxidation state of copper at the active site. Quantitative analysis of these spectra indicates that both reduced (CuI,ZnII) and oxidized (CuII,ZnII) enzymes are composed of about 35% antiparallel beta-sheet, 45% unordered/loop, and 20% beta-turn structures. Significant redox-dependent changes occur in regions ascribed to beta-sheet and unordered/loop structures that are consistent with an active channel structure wherein the copper ion bonds to imidazolate side chains of His 44, 46, and 118 within the beta-sheet structure and also to the imidazolate side chain of His 61 associated with unordered/loop structure. This study provides the first experimental evidence that an unordered structure can exhibit bands in more than one region, one near 1658 cm-1 and another near 1648 cm-1 in both H2O and D2O solutions. The detected changes in protein conformation are expected to be critical to the catalytic function of this enzyme.

The effect of combining recombinant human tumor necrosis factor-alpha with local radiation on tumor control probability of a human glioblastoma multiforme xenograft in nude mice

PURPOSE

To evaluate the antitumor activity of recombinant human tumor necrosis factor-alpha (rHuTNF-alpha) on a human glioblastoma multiforme (U87) xenograft in nude mice, and to study the effect of combining rHuTNF-alpha with local radiation on the tumor control probability of this tumor model.

METHODS AND MATERIALS

U87 xenograft was transplanted SC into the right hindleg of NCr/Sed nude mice (7-8 weeks old, male). When tumors reached a volume of about 110 mm3, mice were randomly assigned to treatment: rHuTNF-alpha alone compared with normal saline control; or local radiation plus rHuTNF-alpha vs. local radiation plus normal saline. Parameters of growth delay, volume doubling time, percentage of necrosis, and cell loss factor were used to assess the antitumor effects of rHuTNF-alpha on this tumor. The TCD50 (tumor control dose 50%) was used as an endpoint to determine the effect of combining rHuTNF-alpha with local radiation.

RESULTS

Tumor growth in mice treated with a dose of 150 micrograms/kg body weight rHuTNF-alpha, IP injection daily for 7 consecutive days, was delayed about 8 days compared to that in controls. Tumors in the treatment group had a significantly longer volume doubling time, and were smaller in volume and more necrotic than matched tumors in control group. rHuTNF-alpha also induced a 2.3 times increase of cell loss factor. The administration of the above-mentioned dose of rHuTNF-alpha starting 24 h after single doses of localized irradiation under hypoxic condition, resulted in a significant reduction in TCD50 from the control value of 60.9 Gy to 50.5 Gy (p < 0.01).

CONCLUSION

rHuTNF-alpha exhibits an antitumor effect against U87 xenograft in nude mice, as evidenced by an increased delay in tumor growth as well as cell loss factor. Also, there was an augmentation of tumor curability when given in combination with radiotherapy, resulting in a significantly lower TCD50 value in the treatment vs. the control groups.

The effect of the overall treatment time of fractionated irradiation on the tumor control probability of a human soft tissue sarcoma xenograft in nude mice

PURPOSE

To study the impact of the overall treatment time of fractionated irradiation on the tumor control probability (TCP) of a human soft tissue sarcoma xenograft growing in nude mice, as well as to compare the pretreatment potential doubling time (Tpot) of this tumor to the effective doubling time (Teff) derived from three different schedules of irradiation using the same total number of fractions with different overall treatment times.

METHODS AND MATERIALS

The TCP was assessed using the TCD50 value (the 50% tumor control dose) as an end point. A total of 240 male nude mice, 7-8 weeks old were used in three experimental groups that received the same total number of fractions (30 fractions) with different overall treatment times. In group 1, the animals received three equal fractions/day for 10 consecutive days, in group 2 they received two equal fractions/day for 15 consecutive days, and in group 3 one fraction/day for 30 consecutive days. All irradiations were given under normal blood flow conditions to air breathing animals. The mean tumor diameter at the start of irradiation was 7-8 mm. The mean interfraction intervals were from 8-24 h. The Tpot was measured using Iododeoxyuridine (IudR) labeling and flow cytometry and was compared to Teff.

RESULTS

The TCD50 values of the three different treatment schedules were 58.8 Gy, 63.2 Gy, and 75.6 Gy for groups 1, 2, and 3, respectively. This difference in TCD50 values was significant (p < 0.05) between groups 1 and 2 (30 fractions/10 days and 30 fractions/15 days) vs. group 3 (30 fractions/30 days). The loss in TCP due to the prolongation of the overall treatment time from 10 days to 30 days was found to be 1.35-1.4 Gy/day. The pretreatment Tpot (2.4 days) was longer than the calculated Teff in groups 2 and 3 (1.35 days).

CONCLUSION

Our data show a significant loss in TCP with prolongation of the overall treatment time. This is most probably due to an accelerated repopulation of tumor clonogens. The pretreatment Tpot of this tumor model does not reflect the actual doubling of the clonogens in a protracted regimen.

Cytotoxicity, metabolism, and mechanisms of action of 2',2'-difluorodeoxyguanosine in Chinese hamster ovary cells

The emerging clinical success of gemcitabine (2',2'-difluorodeoxycytidine) stimulated interest in the synthesis and evaluation of purine congeners. The cytotoxicity, metabolism, and mechanisms of action of the lead candidate, 2',2'-difluorodeoxyguanosine (dFdGuo), were studied in Chinese hamster ovary cells. Unlike the natural nucleoside deoxyguanosine (dGuo), dFdGuo was not a substrate for purine nucleoside phosphorylase. Wild-type Chinese hamster ovary cells and a mutant line deficient in deoxycytidine (dCyd) kinase were similarly affected by dFdGuo (50% inhibitory concentration, 7.5 and 6.5 microM, respectively), suggesting that unlike gemcitabine, dCyd kinase was not responsible for activation of dFdGuo. This was further confirmed by separation of nucleoside kinases (adenosine kinase, dGuo kinase, and dCyd kinase) of Chinese hamster ovary cells on DEAE-cellulose column chromatography. The kinase activity that phosphorylated dGuo also converted dFdGuo to its monophosphate, suggesting that dGuo kinase activated dFdGuo. Consistent with this result, coincubation with dGuo spared the dFdGuo-mediated toxicity; however, addition of up to 10 mM dCyd did not reverse the toxicity of dFdGuo. Intracellularly, dFdGuo was phosphorylated to its mono-, di-, and triphosphates; dFdGuo triphosphate (dFdGTP) was the major metabolite and accumulated to 45 microM after a 6-h incubation with 30 microM dFdGuo. The elimination of dFdGTP was monophasic with a t1/2 of about 6 h. Deoxynucleotides were decreased in cells incubated with dFdGuo, suggesting that ribonucleotide reductase was inhibited. dATP, which decreased 78% after a 4-h incubation with 30 microM dFdGuo, was most affected. dFdGuo was a potent inhibitor of DNA synthesis. Extension of a DNA primer over a defined template in the presence of dFdGTP revealed that dFdGTP was a good substrate for incorporation opposite C sites of the template by DNA polymerase alpha. dFdGTP incorporation caused DNA polymerase alpha to pause after the polymerization of one additional deoxynucleotide. This pattern of inhibition, which is shared by gemcitabine, distinguishes 2',2'-difluoronucleosides from arabinosylnucleosides which halt primer extension at the incorporation site. dGTP competed effectively with dFdGTP for incorporation by DNA polymerase alpha. The unique activation requirements and patterns of inhibition of DNA synthesis distinguish this promising new antimetabolite from other nucleoside analogues.

Quantitative comparison of xenotransplantation of a human soft tissue sarcoma into the subcutaneous tissue of normal, postincision, and postincision plus indomethacin-treated nude mice

The purpose of this study was to test the hypotheses that (1) surgical wounding can enhance the xenotransplantability of a human soft tissue sarcoma (HSTS26T) into subcutaneous (s.c.) tissue of nude mice, and (2) Indomethacin may reduce the xenotransplantability of this human tumor in the surgical wounding animal model by suppressing angiogenesis. The experimental method was to employ the quantitative transplantation assays (TD50, the number of tumor cells that, on average, would be expected to induce a tumor in 50% of the recipients). After an incisional wound (1.0-1.2 cm long) was made on the right leg of each experimental mouse, tumor cells were inoculated into the surgical wound, or into the contralateral leg at 24 and 72 hr postincision, and in another group tumor cells were inoculated into the wound at 72 hr postincision, plus daily s.c. injection of indomethacin, 2 mg/kg body weight for 8 consecutive days in a separate experiment. Nonincisional mice received the same inoculation as the control groups. The TD50s of surgically wounded groups were 3.5-10.7 times lower than that of the control groups. Significantly lower TD50 values were found in groups of cells inoculated into the surgical wound at 72 hr postincision (P < 0.05 or P < 0.01) and into the contralateral leg at 24 hr postincision (P = 0.05). No significant difference was found between the TD50 values in mice that received cells inoculated at 72 hr postincision plus indomethacin treatment, and those with no wound controls. Our conclusion is that the surgical wound can enhance the xenotransplantability of HSTS26T in nude mice. Indomethacin can decrease this enhancing effect level similar to that in no-wound controls and may prevent tumor recurrence in a surgical wound.

Effects of dimethyl sulfoxide, glycerol, and ethylene glycol on secondary structures of cytochrome c and lysozyme as observed by infrared spectroscopy

Effects of 10-30% (v/v) of dimethyl sulfoxide, glycerol, and ethylene glycol on the H-O-H bending vibration of water and the amide I bands of horse heart cytochrome c and chicken egg white lysozyme in 25 mM sodium phosphate buffer (pH 7.4) were examined at 20 degrees C by Fourier transform infrared spectroscopy. The H-O-H bending mode of water was strongly affected by these cryoprotectant solvents. Increasing the concentration of cryosolvents from 0 to 30% shifts the water bending band maximum from 1645 to about 1650 cm-1. Second-derivative analysis reveals significant changes in conformation-sensitive amide I regions of lysozyme ascribed to alpha-helix (1657 cm-1), turn (1674 cm-1), and unordered (1646 cm-1) structures; each cryosolvent increases the intensity of the 1657 cm-1 band at the expense of bands at 1674 and 1646 cm-1. No changes in spectra deemed significant were observed for cytochrome c under the same conditions. There is no spectral evidence of structural randomization of proteins due to the presence of these cryosolvents. Cryosolvent-induced changes in secondary structure of proteins may result from changes in water structure which, in turn, perturb the structure of the protein and/or from direct interactions between cryosolvent and protein.

Interaction of an amphiphilic peptide with a phospholipid bilayer surface by molecular dynamics simulation study

Corticotropin-releasing factor (CRF) is the principal neuroregulator of adrenocorticotropic hormone (ACTH) secretion. Previous experiments have demonstrated that CRF binds avidly to the surface of single egg phosphatidylcholine vesicles and its amphiphilic secondary structure might play an important role in the function. In this study, the interaction of the residues 13-41 in human CRF with the surface of a DOPC bilayer was investigated by molecular dynamics (MD) simulation in order to understand the role of the membrane surface in the formation of the amphiphilic alpha helix as well as to determine the effects of the peptide on the lipid bilayer. The model used included 60 DOPC molecules, 1 helical peptide (CRF13-41) on the bilayer surface, and explicit waters of solvation in the lipid polar head group regions, together with constant-volume periodic boundary conditions in three dimensions. The MD simulation was carried out for 510 ps. In addition, CRF13-41, initially in a helical form, was simulated in vacuo as a control. The results indicate that while it was completely unstable in vacuo, the peptide helical form was generally maintained on the bilayer surface, but with distortions near the terminal ends. The peptide was confined to the bilayer headgroup/water region, similar to that reported from neutron diffraction measurement of tripeptides bound to the phosphatidylcholine bilayer surface (Ref 1). The amphiphilicity of the peptide matched that of the bilayer headgroup environment, with the hydrophilic side oriented toward water and the hydrophobic side making contact with the bilayer hydrocarbon core. These results support the hypothesis that the amphiphilic environment of a membrane surface is important in the induction of peptide amphiphilic alpha-helical secondary structure. Two major effects of the peptide on the lipids were found: the first CH2 segment in the lipid chains was significantly disordered and the lipid headgroup distribution was broadened towards the water region.

Infrared analysis of ligand- and oxidation-induced conformational changes in hemoglobins and myoglobins

Effects of the binding of O2 and CO to heme iron (II) of deoxy forms and of the oxidation of deoxy forms to aquoiron (III) complexes on the infrared spectra of hemoglobins and myoglobins have been examined. Spectra were measured for aqueous solutions 3-4 mM in heme of human, bovine, and equine hemoglobins and sperm whale, bovine, and equine myoglobins in 10 mM sodium phosphate buffer, pH 7.4, at 20 degrees C. All ligand binding and oxidation reactions resulted in similar spectral shifts in the region 1665 to 1670 cm-1, a portion of the amide I region assignable to beta-turn structure. There were no other significant changes in the amide I region, a finding consistent with no other alterations in secondary structure. The major bands near 1655 cm-1 associated with alpha-helices were consistently at 2 cm-1 lower wavenumber for myoglobins than for hemoglobins. The changes in solution infrared spectra observed in this study may result at least in part from conformational changes at the FG corner associated with movements of F and E helices that have been noted previously in crystal structures.

Molecular dynamics simulation of anesthetic-phospholipid bilayer interactions

To probe the hypothesis of a lipid-mediated mechanism of general anesthetic action on a molecular level, and to help elucidate the nature of the interactions of bioactive compounds with membranes, the effects of trichloroethylene (TCE), an inhalational general anesthetic, on a dioleoylphosphatidylcholine (DOPC) lipid bilayer have been investigated by molecular dynamics (MD) simulations at 37 degrees C and 1 atm and the results compared with 31P and 2H NMR experimental studies (Ref 1). The model used included a single TCE molecule embedded in a lipid bilayer consisting of 24 DOPC molecules and an 8 A layer of explicit water of solvation in each polar head group region of the bilayer, together with constant-pressure periodic boundary conditions in three dimensions. A comparison of the bilayer properties calculated in the presence and absence of the anesthetic led to the detection of three major perturbations of the bilayer caused by the anesthetic at 1 atm: i) an increase in the ratio of the effective areas of hydrocarbon tails and the head group per lipid, predicting the tendency of lipids near the anesthetic site of action to form a hexagonal phase (HII); ii) a slight increase in the frequency of chain dihedral angles found in the gauche conformation; and iii) a significant increase in the lateral mean-square displacement of lipid molecules, an indication of increased lipid lateral diffusion and membrane fluidity. The pressure antagonism of these effects was also studied by MD simulations at pressures of 200 and 400 atm. The study of the pressure reversibility of these effects at 200 and 400 atm indicated that they were partially prevented at 200 atm and essentially blocked at 400 atm, suggesting their probable relevance to the pressure reversal effect seen with general anesthesia. These results may thus provide insights into the interaction between general anesthetics and similar small organic molecules with membranes.

Functional compartmentation of dCTP pools. Preferential utilization of salvaged deoxycytidine for DNA repair in human lymphoblasts

The utilization of dCTP derived from de novo synthesis through ribonucleotide reductase in exponentially growing CCRF-CEM cells was compared with the metabolic fate of dCTP produced by the salvage pathway. Exogenous dCyd was not effectively incorporated into replicating DNA; instead, dCTP derived from ribonucleotide reductase (labeled by [5-3H]Cyd) was the main precursor for that purpose, apparently because of functional compartmentation of the dCTP pool in these cells. Studies of the metabolic route of incorporation of exogenous [5-3H]dCyd into DNA of growing CCRF-CEM cells demonstrated that it was mainly incorporated through the DNA repair pathway. Incorporation of [5-3H]dCyd into DNA of synchronized cell populations was maximal in G1 cells, whereas [3H]dThd incorporation occurred predominantly in S phase cells. When cellular DNA was density labeled by incubation with BrdUrd, repaired DNA, which was less dense than replicated DNA, was preferentially labeled by [5-3H]dCyd. In contrast, replicated DNA was labeled by both [3H]dThd and [5-3H]Cyd. The DNA-damaging agents methylmethanesulfonate, ultraviolet irradiation, and gamma-irradiation inhibited [3H]dThd incorporation, whereas they stimulated the accumulation of [5-3H]dCyd in DNA. Based on these results, we propose that the dCTP pool is functionally compartmentalized in growing CCRF-CEM cells. dCTP derived from the salvage pathway is utilized predominantly for DNA repair, whereas the de novo pathway supplies dCTP for DNA replication.

Fludarabine- and gemcitabine-induced apoptosis: incorporation of analogs into DNA is a critical event

The nucleoside analogs fludarabine and gemcitabine inhibit cellular DNA synthesis by two different mechanisms: (1) direct termination of DNA strand elongation after the triphosphate of each drug is incorporation into DNA; and (2) indirect inhibition of DNA synthesis by decreasing cellular dNTPs through inhibition of ribonucleotide reductase. The present study demonstrated that incorporation of the analogs into DNA is critical for the cytotoxic action of these drugs in human T lymphoblastoid CEM cells. S phase cells, which actively incorporated the analogs into DNA, were most sensitive to the cytotoxic action of these compounds. A relatively short-term (5-24 h) cessation of cellular DNA synthesis without analog incorporation was not sufficient to cause cell death. The drug-treated cells died through apoptosis characterized by generation of internucleosomal DNA fragmentation and apoptotic morphology. Induction of high molecular mass (50-500 kb) DNA fragmentation was also observed in cells undergoing apoptosis; this type of DNA degradation was strongly correlated with the analog-induced cell death process. Inhibition of the analog incorporation into DNA by aphidicolin blocked both types of DNA fragmentation and apoptotic morphology, indicating the essential role of analog incorporation into DNA in drug-induced cell death.

Characterization of sites occupied by the anesthetic nitrous oxide within proteins by infrared spectroscopy

We report here a comprehensive infrared spectroscopic study of the interactions between the anesthetic nitrous oxide (N2O) and six proteins: lysozyme, cytochrome c, myoglobin, hemoglobin, serum albumin, and cytochrome c oxidase. Sites occupied by N2O molecules within these proteins were characterized. Three types of hydrophobic sites were found within the proteins. One with nu 3 near 2225 cm-1 is likely to be near peptide bond carbonyls; one with nu 3 near 2219 cm-1 may be near a benzene-like structure such as the side chains of phenylalanine and tyrosine; and the other with nu 3 near 2215 cm-1 is likely to be in a nonpolar alkane-like environment provided by the side chains of Leu, Ile, and Val residues. The amount of N2O molecules bound to myoglobin increases as the pH decreases from 9.2 to 5.2. N2O-protein interactions produced no detectable changes in the ligand-binding pockets of myoglobin, hemoglobin, and cytochrome c oxidase. N2O-induced secondary structure changes were detected only in the fully reduced cytochrome c oxidase, not in the fully oxidized oxidase and the other five proteins. N2O-induced conformational changes in the alpha beta-interface of hemoglobin and the h2 and h3 alpha-helices of human serum albumin were detected by monitoring the S-H stretch vibrations of cysteine residues. These findings provide direct evidence that anesthetic N2O interacts with proteins and occupies sites in the interior of the proteins.

Molecular dynamics simulations of phospholipid bilayers

Molecular dynamics (MD) simulations at 37 degrees C have been performed on three phospholipid bilayer systems composed of the lipids DLPE, DOPE, and DOPC. The model used included 24 explicit lipid molecules and explicit waters of solvation in the polar head group regions, together with constant-pressure periodic boundary conditions in three dimensions. Using this model, a MD simulation samples part of an infinite planar lipid bilayer. The lipid dynamics and packing behavior were characterized. Furthermore, using the results of the simulations, a number of diverse properties including bilayer structural parameters, hydrocarbon chain order parameters, dihedral conformations, electron density profile, hydration per lipid, and water distribution along the bilayer normal were calculated. Many of these properties are available for the three lipid systems chosen, making them well suited for evaluating the model and protocols used in these simulations by direct comparisons with experimental data. The calculated MD behavior, chain disorder, and lipid packing parameter, i.e. the ratio of the effective areas of hydrocarbon tails and head group per lipid (a(t)/ah), correctly predict the aggregation preferences of the three lipids observed experimentally at 37 degrees C, namely: a gel bilayer for DLPE, a hexagonal tube for DOPE, and a liquid crystalline bilayer for DOPC. In addition, the model and conditions used in the MD simulations led to good agreement of the calculated properties of the bilayers with available experimental results, demonstrating the reliability of the simulations. The effects of the cis unsaturation in the hydrocarbon chains of DOPE and DOPC, compared to the fully saturated one in DLPE, as well as the effects of the different polar head groups of PC and PE with the same unsaturated chains on the lipid packing and bilayer structure have been investigated. The results of these studies indicate the ability of MD methods to provide molecular-level insights into the structure and dynamics of lipid assemblies.

Fludarabine inhibits DNA replication: a rationale for its use in the treatment of acute leukemias

Fludarabine is a prodrug that must enter cells and be phosphorylated to the nucleoside triphosphate, F-ara-ATP, to elicit biological activity. F-ara-ATP serves as an inhibitory alternative substrate in several key processes involved in DNA synthesis. The enzymes required in DNA synthesis and affected by F-ara-ATP are ribonucleotide reductase, DNA primase, DNA polymerases, 3'-5' exonuclease activity of DNA polymerases delta and epsilon, and DNA ligase I. The action of fludarabine on DNA replication provides a compelling rationale to use this agent for leukemias where target cells are actively synthesizing DNA, for example acute myelogenous leukemia. Additionally, the role of F-ara-ATP to potentiate the activity of deoxycytidine kinase makes it an appropriate candidate to use in combination with other nucleoside analogs which require deoxycytidine kinase for their activation. The present article reviews the effect of fludarabine on enzymes involved in DNA synthesis and the role of fludarabine in combination with arabinosylcytosine for the treatment of diseases other than indolent leukemias.

[Rifampicin: rabbits sensitization, two rifampicin-protein conjugates and detection of rifampicin specific IgG]

The presence of rifampicin (RFP) specific IgG in the sera of rabbits immunized with RFP or with two RFP-protein conjugates (RFP-bovine serum albumin, RFP-BSA and RFP-rabbit serum albumin RFP-RSA) was measured by ELISA. The way of conjugation and the molecular conjugation rate were different between RFP-BSA and RFP-RSA. The titre of RFP specific IgG was 1:10-1:100 in RFP immunized rabbits, 1:100-1:1,000 in RFP-RSA immunized rabbits, and over 1:1000 in RFP-BSA immunized rabbits. The binding properties of the sera to RFP were demonstrated by inhibition assay. These are the models of RFP sensitization first established by immunization animals with RFP itself or RFP-protein conjugates.

Fludarabine: pharmacokinetics, mechanisms of action, and rationales for combination therapies

Knowledge of the pharmacokinetics of a drug is essential to the optimal design of the dose and schedule of chemotherapy protocols. As an extension, an understanding of the mechanism of drug action is necessary to construct the optimal strategy for combination chemotherapy. Nucleoside antimetabolites such as fludarabine are pro-drugs that must enter cells and be phosphorylated to the nucleoside triphosphate before they can elicit biologic activity. Thus, knowledge of the pharmacokinetics of the triphosphate in target cells and an understanding of the mechanisms by which this active form of the drug act are indispensable to the rational design of treatment protocols. This article reviews the essential elements of the pharmacokinetics and mechanisms of action of fludarabine to provide a rationale for combinations of fludarabine with other chemotherapeutic agents and anti-cancer modalities.

Solution structure of the exocyclic 1,N2-propanodeoxyguanosine adduct opposite deoxyadenosine in a DNA nonamer duplex at pH 8.9. model of pH-dependent conformational transition

The solution structure of the complementary d(C1-A2-T3-G4-X5-G6-T7-A8-C9).d(G10-T11-A12-C13-A14-C15-A16-T17-G18) DNA duplex (designated X.A 9-mer), which contains a 1,N2-propanodeoxyguanosine exocyclic adduct X5 opposite deoxyadenosine A14 at the center, is pH dependent [Kouchakdjian, M., Eisenberg, M., Live, D., Marinelli, E., Grollman, A., & Patel, D.J. (1990) Biochemistry 29, 4456-4465]. In our previous paper [Huang, P., & Eisenberg, M. (1992) Biochemistry 31, 6518-6532] we established the three-dimensional structure of this X.A 9-mer duplex at pH 5.8 by use of restrained molecular dynamics followed by NOE-based back-calculation refinement. The present paper discusses the structure at pH 8.9 and the pH-dependent conformational transition between the structures at pH 5.8 and at pH 8.9. The structure at pH 8.9 is calculated starting from five different conformations. The final structures converge and agree well with the experimental NOE intensities. These structures are essentially B-type DNA (with X5 and A14 in the BII conformation while the other residues are in the most commonly described BI conformation) and display an approximate 27 degrees kink at the center of the helix. At the kink site, X5 is positioned in the major groove with the exocyclic ring directed toward the G6.C13 base pair, unstacked from the flanking base G6 and exposed to the solvent. A14, opposite the lesion, remains stacked with its neighbor C15, but not with C13. The kinked helix can accommodate the rotation of the bulky X5 about its glycosidic bond. We propose here a model for the pH-dependent transition. Our model explains the conformational change, which includes the anti and syn rotation of the bulky adduct around its glycosidic bond, with a minimal energy barrier and with an overall kink of the DNA helix. These new findings, fully consistent with the NMR experimental data, were revealed only after restrained dynamics refinement. Distance-restrained energy minimization by itself was insufficient, as shown by the previous NMR study.

Stimulus-associated protein in gastric parietal cell detected using antimelittin antibody

The bee venom polypeptide melittin binds to and inhibits the gastric hydrogen-potassium-adenosinetriphosphatase (H(+)-K(+)-ATPase). A search for parietal cell proteins with a melittin-like structure was carried out. A 67-kDa (doublet) protein, which reacted with a polyclonal antimelittin antibody, was found in purified rabbit parietal cells. The protein exhibited reversible stimulus-dependent redistribution from cytosol to (total) membranes. It was also found to be associated with H(+)-K(+)-ATPase-containing membranes when isolated from the gastric mucosae of rabbits treated with histamine, but not with cimetidine. The presence of the protein correlated with the ability of the membrane preparations to exhibit ionophore-independent HCl accumulation, a characteristic of gastric membranes from histamine-stimulated animals. The 67-kDa melittin-like protein may play a role in the functional changes in the gastric parietal cell that are involved in stimulation of HCl secretion.

Isolation and characterization of a gene encoding DNA topoisomerase I in Drosophila melanogaster

We synthesized a DNA probe specific for the gene encoding eucaryotic DNA topoisomerase I by the polymerase chain reaction. The sequences of the primers for this reaction were deduced from the regions with extensive homology among the enzymes from the fission and budding yeasts, and the human. From the clones isolated by screening a Drosophila cDNA library with this DNA probe, two cDNA clones of 3.8 and 5.2 kb were characterized and completely sequenced. Both cDNA sequences contain an identical open reading frame for 972 amino acid residues. The 3.8 kb messenger RNA is likely generated by using a polyadenylation site 5' upstream to that used in generating the 5.2 kb mRNA. The predicted amino acid sequence shows that a segment of 420 amino acid residues at the amino terminus is hydrophilic, similar to the amino terminal 200 residues in the yeast and human enzymes. Furthermore, the Drosophila enzyme is unique in that the amino terminal 200 residues are enriched in serine and histidine residues; most of them are present in clusters. The rest of the Drosophila sequence is highly homologous to those from yeast and human enzymes. The evolutionarily conserved residues are identified and are likely the critical elements for the structure and function of this enzyme. A plasmid vector containing the cloned cDNA was constructed for the expression of Drosophila protein in Escherichia coli. The enzymatic and immunochemical analysis of the polypeptide produced in this heterologous expression system demonstrated that the expressed protein shares similar enzymatic properties and antigenic epitopes with DNA topoisomerase I purified from Drosophila embryos or tissue culture cells, thus establishing the bacterial expression system being useful for the future structure/function analysis of the Drosophila enzyme.

Anti-HIV plant proteins catalyze topological changes of DNA into inactive forms

GAP 31, DAP 32 and DAP 30 comprise a new class of plant proteins with potent anti-HIV activity and insignificant cytotoxicity. We report here the identification and characterization of a new DNA enzyme activity in these three proteins. They irreversibly relax and decatenate supercoiled DNA, as well as catalyze double-stranded breakage to form linear DNA. The relaxed molecules are topologically inactive and no longer serve as substrates for DNA gyrase to form supercoils, phenomena similar to those of cellular topoisomerases in the presence of topoisomerase poisons. The ability of these anti-HIV agents to interrupt essential topological interconversions of DNA may provide a novel mechanism for their antiviral and antitumor actions. The presence of this new DNA topological enzyme activity in these plant proteins also suggests that their anti-HIV activity may not be merely a consequence of ribosome inactivation previously recognized.

A quantitative assay for fragmented DNA in apoptotic cells

Cleavage of cellular chromatin at internucleosomal sites is a characteristic change of DNA integrity in cells undergoing apoptosis. We have developed an assay for quantitation of internucleosomal DNA fragmentation in apoptotic cells. This technique involves purification of cellular DNA, dephosphorylation of the DNA ends, labeling of DNA with 32P at the 5'-end, gel electrophoresis through agarose, and quantitation of the radioactivity in DNA bands. This assay, which is about 1000- to 2000-fold more sensitive than visualization of DNA bands by ethidium staining, allows the detection of DNA fragments at picogram levels. A method for quantitatively determining the number of fragmented DNA strands is also described. Application of this new assay to evaluate the time course of internucleosomal DNA fragmentation was demonstrated in apoptotic cells induced by an anticancer nucleoside analogue.

The three-dimensional structure in solution (pH 5.8) of a DNA 9-mer duplex containing 1,N2-propanodeoxyguanosine opposite deoxyadenosine. Restrained molecular dynamics and NOE-based refinement calculations

The solution structure of the complementary d(C1-A2-T3-G4-X5-G6-T7-A8-C9).d(G10-T11-A12-C13-A14-C15-A 16-T17-G18) nonanucleotide duplex (designated X.A 9-mer) that contains a 1,N2-propanodeoxyguanosine exocyclic adduct, X5, opposite deoxyadenosine A14 in the center of the helix at pH 5.8 is investigated by use of restrained molecular dynamics followed by NOE-based back-calculation refinement. The molecular dynamics calculation is based on 91 interresidue and 97 intraresidue interproton distance restraints derived from two-dimensional nuclear Overhauser enhancement data on the X.A 9-mer at mixing times of 50 and 250 ms [Kouchakdjian, M., Marinelli, E., Gao, X., Johnson, F., Grollman, A., & Patel, D.J. (1989) Biochemistry 28, 5647-5657]. Separate runs start from classical A and B DNA and converge to essentially identical structures (atomic root mean square difference of 0.69 A). Both structures are B-type DNA in character and satisfy the experimental distance restraints with the rms difference of only 0.001 A between the calculated and experimental interproton distances. The dynamics behavior of the A----B DNA transition is monitored and analyzed. Our results clearly indicate that the driving force of the convergence is the experimental interproton distance restraints. The molecular dynamics structures are further refined by a back-calculation dynamics which directly minimizes the difference between the observed 2D NOE intensities and those calculated by the full relaxation matrix approach. The fit of the refined structures to the NOE intensities is measured by the NOE R value, which is analogous to the crystallographic residual index. These R values of the final structures are only 0.17. The refined structures are generally B type, and their convergency improves slightly to an atomic root mean square difference of 0.64 A, despite relatively large structural shifts (approximately 1 A) which occur during the back-calculation refinement in both cases. These results suggest that the converged refined structures represent reasonable approximations of the solution structure.