Metabolism and action of fludarabine phosphate

Fludara I.V. (fludarabine phosphate) (9-beta-D-arabinosyl-2-fluoroadenine, F-ara-A) is an adenine nucleoside analogue resistant to adenosine deaminase that shows promising therapeutic activity in the clinical treatment of lymphocytic hematologic malignancies. F-ara-A is transported into cells, where it is converted to its 5'-triphosphate (F-ara-ATP), the principal active metabolite. Deoxycytidine kinase is the enzyme responsible for the initial step of this activation metabolism. The differential transport and phosphorylation of F-ara-A and accumulation of F-ara-ATP by normal and cancer cells may constitute the metabolic basis of its positive therapeutic index. The major action of F-ara-A is the inhibition of DNA synthesis. F-ara-ATP competes with deoxyadenosine triphosphate for incorporation into the A sites of the elongating DNA strand by DNA polymerases and terminates DNA synthesis at the incorporation sites. That action is potentiated by the decrease of cellular dATP that results from inhibition of ribonucleotide reductase by F-ara-ATP. In vitro experiments demonstrated that DNA polymerase delta is able to excise the incorporated F-ara-AMP residues from DNA with its 3' to 5' exonuclease activity. The terminal incorporation of F-ara-AMP into DNA results in deletion of genetic material. That mechanism may be responsible for the observed mutagenicity of Fludara I.V., and ultimately its cytotoxic action.

Termination of DNA synthesis by 9-beta-D-arabinofuranosyl-2-fluoroadenine. A mechanism for cytotoxicity

The action of 9-beta-D-arabinofuranosyl-2-fluoroadenine (F-ara-A) on DNA synthesis was evaluated both in whole cells and in vitro. 9-beta-D-Arabinofuranosyl-2-fluoroadenine was converted to its 5'-triphosphate 9-beta-D-arabinofuranosyl-2-fluoroadenine 5'-triphosphate (F-ara-ATP) in cells and then incorporated into DNA in a self-limiting manner. More than 94% of the analogue was incorporated into DNA at the 3' termini, indicating a chain termination action. In vitro DNA primer extension experiments further revealed that F-ara-ATP compared with dATP for incorporation into the A site of the extending DNA strand. The incorporation of F-ara-AMP into DNA resulted in termination of DNA strand elongation. Human DNA polymerase alpha incorporated more F-ara-AMP into DNA than polymerase epsilon (proliferating cell nuclear antigen-independent DNA polymerase delta) and was more sensitive to inhibition by F-ara-ATP. On the other hand, DNA polymerase epsilon was able to excise the incorporated F-ara-AMP from DNA in vitro. The incorporation of F-ara-AMP into DNA was linearly correlated both with inhibition of DNA synthesis and with loss of clonogenicity; thus it may be the mechanism of cytotoxicity.

Selective action of 3'-azido-3'-deoxythymidine 5'-triphosphate on viral reverse transcriptases and human DNA polymerases

The action of 3'-azido-3'-deoxythymidine 5'-triphosphate (N3dTTP) on DNA strand elongation catalyzed by human immunodeficiency virus type 1 reverse transcriptase was evaluated in comparison with human DNA polymerase alpha and proliferating cell nuclear antigen-independent DNA polymerase delta. Sequencing gel analysis demonstrated that the human immunodeficiency virus 1 reverse transcriptase preferentially incorporated N3dTTP into the T sites of the growing DNA strands and caused chain termination in a dose-dependent manner. This effect was observed even when the N3dTTP concentration was 0.3 microM, 100-fold less than dTTP. Studies with reverse transcriptases from avian myeloblastosis virus and Moloney murine leukemia virus showed that N3dTTP was also efficiently incorporated into DNA by these enzymes and terminated DNA strand elongation. In contrast, human DNA polymerases alpha and delta did not incorporate detectable amounts of N3dTTP into the DNA and were not inhibited by 300 microM N3dTTP. The selective incorporation of the chain-terminating nucleotide by the viral reverse transcriptases appears to be a molecular basis for the positive therapeutic index of 3'-azido-3'-deoxythymidine.

Protein secondary structures in water from second-derivative amide I infrared spectra

Infrared spectra have been obtained for 12 globular proteins in aqueous solution at 20 degrees C. The proteins studied, which vary widely in the relative amounts of different secondary structures present, include myoglobin, hemoglobin, immunoglobulin G, concanavalin A, lysozyme, cytochrome c, alpha-chymotrypsin, trypsin, ribonuclease A, alcohol dehydrogenase, beta 2-microglobulin, and human class I major histocompatibility complex antigen A2. Criteria for evaluating how successfully the spectra due to liquid and gaseous water are subtracted from the observed spectrum in the amide I region were developed. Comparisons of second-derivative amide I spectra with available crystal structure data provide both qualitative and quantitative support for assignments of infrared bands to secondary structures. Band frequency assignments assigned to alpha-helix, beta-sheet, unordered, and turn structures are highly consistent among all proteins and agree closely with predictions from theory. alpha-Helix and unordered structures can each be assigned to only one band whereas multiple bands are associated with beta-sheets and turns. These findings demonstrate a method of analysis of second-derivative amide I spectra whereby the frequencies of bands due to different secondary structures can be obtained. Furthermore, the band intensities obtained provide a useful method for estimating the relative amounts of different structures.

Gene deletion, a mechanism of induced mutation by arabinosyl nucleosides

9-beta-D-Arabinofuranosyl-2-fluoroadenine (F-ara-A) and 9-beta-D-arabinofuranosyladenine (ara-A) are purine nucleoside analogues which are incorporated into nucleic acids. This study demonstrates the mutagenic properties of F-ara-A and ara-A and provides evidence for mechanisms by which the arabinosyl nucleosides induce mutation. At the drug dosages that evoked exponential cell killing, F-ara-A and ara-A caused a significant increase in the number of 6-thioguanine-resistant mutants in Chinese hamster ovary cells. Southern analyses showed that 15 of 16 drug-induced mutants had lost all or part of the HPRT gene, whereas no loss of the gene was found in 4 spontaneous mutants. We conclude that both F-ara-A and ara-A induced mutation predominantly by causing deletion of genetic material. The remarkable frequency of gene deletion among these drug-induced mutations is discussed with respect to possible mechanisms of action of arabinosyl nucleosides in mutational studies.

Insulin-receptor and apolipoprotein genes contribute to development of NIDDM in Chinese Americans

The frequencies of restriction-fragment-length polymorphism (RFLP) alleles as well as RFLP haplotypes at six genetic loci responsible for carbohydrate and lipid metabolism [insulin/insulin-like growth factor II complex, insulin receptor (INSR), HepG2/erythrocyte-type glucose transporter, apolipoprotein A-II, apolipoprotein B (APOB), and the apolipoprotein A-I/C-III/A-IV cluster (APOA1/C3/A4)] were compared between nondiabetic and diabetic Chinese Americans. The disease-association data suggest that genetic variation at the INSR, APOB, and APOA1/C3/A4 loci contributes to the development of non-insulin-dependent diabetes mellitus (NIDDM). The analysis of the INSR locus revealed "protective" haplotypes, and it may be possible to use two of the INSR haplotypes as genetic markers to identify individuals having a very low probability of developing NIDDM regardless of the presence of other genes conferring susceptibility to this disorder. The APOB and APOA1/C3/A4 loci appear to contribute to the development of NIDDM in individuals who are of lean/normal weight and overweight, respectively. The APOA1/C3/A4 locus may account for approximately 8% of the difference between baseline and total possible risk of NIDDM in overweight individuals.

Acute respiratory effects of short-term exposures to nitrogen dioxide

Eleven study subjects with asthma and 12 normal controls were monitored for 5 days with a portable continuous nitrogen dioxide (NO2) monitoring instrument held at breathing level before, during, and after cooking dinner on a gas cooking range. Forced expiratory volume in 1 sec (FEV1.0), forced expiratory volume (FEV25-75), peak expiratory flow, and a tracing of the entire flow curve was monitored before the gas stove was turned on, during a break in cooking, immediately after, and 1 hr after cooking was completed. Study design for detection of health effects of the acute exposures to NO2 described above and a method of validating findings in an exposure chamber are discussed in this paper.

Prediction of traumatic aortic rupture from plain chest film findings using stepwise logistic regression

We reviewed initial chest radiographs of 21 patients with, and 26 without, aortic rupture, and examined the presence or absence of individual signs previously cited to be associated with aortic rupture. Using stepwise logistic regression, the three most significant signs associated with rupture were loss of aortic contour, tracheal deviation, and mediastinal-to-chest ratio. A formula calculating the probability of aortic rupture (P) using these three variables was derived. Using a low cutoff point, this equation would approach 100% sensitivity in detecting aortic rupture while reducing the number of negative aortographs. The reliability of this equation and the optimal cutoff point must be determined in a prospective study before being used to make clinical decisions.

Phosphorolytic cleavage of 2-fluoroadenine from 9-beta-D-arabinofuranosyl-2-fluoroadenine by Escherichia coli. A pathway for 2-fluoro-ATP production

2-Fluoroadenine (F-Ade) is a metabolite of 9-beta-D-arabinofuranosyl-2-fluoroadenine (F-ara-A) that may be involved in the development of toxic side effects from this anticancer drug. The liberation of F-Ade from F-ara-A has been examined in different biological systems. Extracts of Escherichia coli but not mammalian cells or tissues catalyzed the conversion of F-ara-A to F-Ade with apparent Km and Vmax values of 1350 microM and 7.7 nmol/min/mg protein respectively. This reaction depended on the presence of phosphate and was inhibited by purine nucleosides in a competitive manner, indicating that the enzyme responsible for the conversion is purine nucleoside phosphorylase. After incubation of intact bacteria with 100 microM [3H]F-ara-A, [3H]F-Ade was the same percentage of cellular radioactivity as in the medium, but it was only one-tenth the concentration of F-ara-A in the cells. In contrast, the cellular concentration of 2-fluoro-ATP was 20-fold greater than that of F-ara-A-5'-triphosphate. These results suggest that F-ara-A entered the bacteria intact and was phosphorolytically cleaved to liberate F-Ade, which would have been either anabolized to the toxic triphosphate or excreted. The latter pathway would provide a route by which F-Ade might be absorbed into the host circulation.

Origin, transmission, and segregation of mitochondrial DNA dimers in mouse hybrid and cybrid cell lines

Hybrid and cybrid progeny lines were constructed from mouse LA9 cells which contain almost exclusively mtDNA monomers and LDTK cells which contain only unicircular mtDNA dimers. The proportion of mtDNA monomers and dimers in the progeny lines was determined both as a function of the number of population doublings since fusion and of selection for expression of a mutant phenotype encoded on one of the parental mtDNAs. There was no preferential segregation of either parental mtDNA in early-passage progeny lines, irrespective of whether or not selection was applied. In marked contrast, there was an accumulation of mtDNA dimers in late-passage progeny lines maintained in the absence of selection for a drug-resistance marker carried by the parental mtDNA monomers. When such selection was applied, roughly equal mass proportions of both parental mtDNAs were maintained in most lines. However, in several progeny lines, new types of mtDNA dimers carrying the selected resistance marker initially encoded in the monomeric mtDNA were present. In some of these latter lines, the new mtDNA dimers apparently arose from LA9 monomers, possibly by recombination. It is hypothesized that mammalian mitochondria normally have a recombination system which maintains low steady-state levels of mtDNA unicircular oligomers by preferentially resolving dimers into two monomers.

Mammalian mitochondrial mutants selected for resistance to the cytochrome b inhibitors HQNO or myxothiazol

Mouse LA9 cell lines were selected for increased resistance to either HQNO or myxothiazol, inhibitors of electron transport which bind to the mitochondrial cytochrome b protein. Two phenotypically distinguishable HQNO-resistant mutants were recovered while the myxothiazol-resistant isolates had a common phenotype. All three mutant phenotypes were transmitted cytoplasmically in cybrid crosses. Biochemical studies further established that for all three mutant types, resistance at the cellular level was paralleled by an increase in inhibitor resistance of mitochondrial succinate-cytochrome c oxidoreductase, the respiratory complex containing cytochrome b. As with the previously described mitochondrial antimycin-resistant mutant, the initial biochemical and genetic studies indicated that these mutations occur within the mitochondrial cytochrome b gene. This conclusion was strongly supported by the results of mtDNA restriction fragment analyses in which it was found that one HQNO-resistant mutant had undergone a small insertion or duplication in the apocytochrome b gene. Finally, all four mitochondrial cytochrome b mutants have been analyzed in both cell plating studies and succinate-cytochrome c oxidoreductase assays to determine the pattern of cross-resistance to inhibitors of cytochrome b other than the one used for selection.

Mitochondrial genetics of mammalian cells: a mouse antimycin-resistant mutant with a probable alteration of cytochrome b

Mouse LA9 antimycin-resistant mutants (ANT-R) were isolated and characterized. Genetic analyses established that this phenotype is encoded within the mtDNA: (1) the ANT-R phenotype showed frequent mitotic segregation and reassortment in hybrid clonal lines; (2) it was transmitted directly in cybrid crosses; and (3) it was cotransmitted in cybrid crosses with the mitochondrial CAP-R marker. Furthermore, the genetic studies suggested that the LA9 CAP-R ANT-R cells were heteroplasmic and contained at least two mtDNA genotypes, cap-r ant-s and cap-s ant-r. Cellular respiration of the ANT-R mutant was markedly more resistant to inhibition by antimycin than that of the parental ANT-S cells. The increased resistance of cellular respiration was entirely accounted for by an increase in the resistance of mitochondrial succinate-cytochrome c oxidoreductase to antimycin inhibition. There was no detectable change in the specific activity of the oxidoreductase in mitochondria of resistant ANT-R cells nor in the sensitivity of the complex to three other specific inhibitors of the complex: TTFA, myxothiazol, and HQNO. Taken together, these studies indicate that the ANT-R phenotype is most likely encoded within the mitochondrial cytochrome b gene and, more specifically, within an antimycin binding domain.

Sonographic demonstration of fibromuscular hyperplasia of the cervical internal carotid artery

Ultrasound examination of the carotid artery has recently become an accepted procedure in screening patients with transient ischemic attacks. We report a patient with fibromuscular hyperplasia of the carotid artery diagnosed successfully with digital gray scale contact ultrasonic scanning and confirmed with arteriography.