Active site mutations in DNA topoisomerase I distinguish the cytotoxic activities of camptothecin and the indolocarbazole, rebeccamycin.

DNA topoisomerase I (Top1p) catalyzes topological changes in DNA and is the cellular target of the antitumor agent camptothecin (CPT). Non-CPT drugs that target Top1p, such as indolocarbazoles, are under clinical development. However, whether the cytotoxicity of indolocarbazoles derives from Top1p poisoning remains unclear. To further investigate indolocarbazole mechanism, rebeccamycin R-3 activity was examined in vitro and in yeast. Using a series of Top1p mutants, where substitution of residues around the active site tyrosine has well-defined effects on enzyme catalysis, we show that catalytically active, CPT-resistant enzymes remain sensitive to R-3. This indolocarbazole did not inhibit yeast Top1p activity, yet was effective in stabilizing Top1p-DNA complexes. Similar results were obtained with human Top1p, when Ser or His were substituted for Asn-722. The mutations altered enzyme function and sensitivity to CPT, yet R-3 poisoning of Top1p was unaffected. Moreover, top1delta, rad52delta yeast cells expressing human Top1p, but not catalytically inactive Top1Y723Fp, were sensitive to R-3. These data support hTop1p as the cellular target of R-3 and indicate that distinct drug-enzyme interactions at the active site are required for efficient poisoning by R-3 or CPT. Furthermore, resistance to one poison may potentiate cell sensitivity to structurally distinct compounds that also target Top1p.     

Dermatan sulphate proteoglycan from human articular cartilage. Variation in its content with age and its structural comparison with a small chondroitin sulphate proteoglycan from pig laryngeal cartilage.

Low molecular mass proteoglycans (PG) were isolated from human articular cartilage and from pig laryngeal cartilage, which contained protein cores of similar size (Mr 40-44 kDa). However, the PG from human articular cartilage contained dermatan sulphate (DS) chains (50% chondroitinase AC resistant), whereas chains from pig laryngeal PG were longer and contained only chondroitin sulphate (CS). Disaccharide analysis after chondroitinase ABC digestion showed that the human DS-PG contained more 6-sulphated residues (34%) than the pig CS-PG (6%) and both contained fewer 6-sulphated residues than the corresponding high Mr aggregating CS-PGs from these tissues (86% and 20% from human and pig respectively). Cross-reaction of both proteoglycans with antibodies to bovine bone and skin DS-PG-II and human fibroblasts DS-PG suggested that the isolated proteoglycans were the humans DS-PG-II and pigs CS-PG-II homologues of the cloned and sequenced bovine proteoglycan. Polyclonal antibodies raised against the pig CS-PG-II were shown to cross-react with human DS-PG-II. SDS/polyacrylamide-gel analysis and immunoblotting of pig and human cartilage extracts showed that some free core protein was present in the tissues in addition to the intact proteoglycan. The antibodies were used in a competitive radioimmunoassay to determine the content of this low Mr proteoglycan in human cartilage extracts. Analysis of samples from 5-80 year-old humans showed highest content (approximately 4 mg/g wet wt.) in those from 15-25 year-olds and lower content (approximately 1 mg/g wet wt.) in older tissue (greater than 55 years). These changes in content may be related to the deposition and maintenance of the collagen fibre network with which this class of small proteoglycan has been shown to interact.     

Genotoxicity studies with four organophosphorus insecticides using the unstable white-zeste system of Drosophila melanogaster

The present study was carried out to evaluate the suitability of the unstable white-zeste system in Drosophila melanogaster by testing 4 organophosphorus insecticides for potential genotoxic activity: dimethoate, fenitrothion, malathion, and methyl parathion. In view of the high sensitivity to insecticides of the unstable zeste strain used in this assay and the negative results obtained in this work, the white-zeste system does not appear to be sufficiently accurate for the evaluation of the mutagenic potential of specifically toxic chemicals, like insecticides.     

Testing of chloroquine and quinacrine for mutagenicity in Drosophila melanogaster

To extend the data on the mutagenic effects of intercalating agents in Drosophila melanogaster, chloroquine and quinacrine were tested for the induction of genetic damage in D. melanogaster males. Sex-linked recessive lethals and sex-chromosome loss induction were studied following treatment of adult males using a feeding technique. Our results show that both intercalating compounds increase significantly the frequency of sex-linked recessive lethals, but are unable to induce sex-chromosome loss in male germ cells under the conditions of testing.     

Trypanosoma cruzi: inhibition of host cell uptake of infective bloodstream forms by alpha-2-macroglobulin

The infection of murine macrophages and fibroblasts by recently isolated infective bloodstream trypomastigotes of Trypanosoma cruzi is inhibited by the addition of human plasma protease inhibitor alpha-2-macroglobulin (alpha 2M) or of soybean trypsin inhibitor. The ingestion of the non-infective epimastigotes by macrophages is not affected by the physiological protease inhibitor. Incubation of bloodstream trypomastigotes for 20 h in a serum-free axenic medium enhances their ability to infect macrophages in a process influenced by the temperature and sensitive to alpha 2M. After this period the infectivity of the parasites to cells was not sensitive to alpha 2M. These observations suggest that proteases located on the surface and/or secreted by the bloodstream trypomastigote form of T. cruzi may modulate its ability to infect host cells.     

Molecular control mechanisms of lysine and threonine biosynthesis in amino acid-producing corynebacteria: Redirecting carbon flow

Abstract Threonine and lysine are two of the economically most important essential amino acids. They are produced industrially by species of the genera Corynebacterium and Brevibacterium . The branched biosynthetic pathway of these amino acids in corynebacteria is unusual in gene organization and in the control of key enzymatic steps with respect to other microorganisms. This article reviews the molecular control mechanisms of the biosynthetic pathways leading to threonine and lysine in corynebacteria, and their implications in the production of these amino acids. Carbon flux can be redirected at branch points by gene disruption of the competing pathways for lysine or threonine. Removal of bottlenecks has been achieved by amplification of genes which encode feedback resistant aspartokinase and homoserine dehydrogenase (obtained by in vitro directed mutagenesis).     

Complete sequence of a new HLA-B35 allele found in a tribe of Mapuche Indians in the south of Argentina

The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession number U17107. The nameB*3509 was officially assigned by the WHO Nomenclature Committee in December 1994     

Solvent effects on the catalytic activity of subtilisin suspended in compressed gases

We studied a model transesterification reaction catalyzed by subtilisin Carlsberg suspended in carbon dioxide, propane, and mixtures of these solvents under pressure. To account for solvent effects due to differences in water partitioning between the enzyme and the bulk solvents, we measured water sorption isotherms for the enzyme in each solvent. We measured catalytic activity as a function of enzyme hydration and obtained bell-shaped curves with maxima at the same enzyme hydration (12%) in all the solvents. However, the activity maxima were different in all media, being much higher in propane than in either CO(2) or the mixtures with 50 and 10% CO(2). Considerations based on the solvation ability of the solvents did not offer an explanation for the differences in catalytic activity observed. Our results suggest that CO(2) has a direct adverse effect on the catalytic activity of subtilisin. (c) 1996 John Wiley & Sons, Inc.     

Solvent effects on the catalytic activity of subtilisin suspended in organic solvents

We studied a model transesterification reaction catalyzed by subtilisin Carlsberg suspended in toluene, n-hexane, diisopropyl ether, and mixtures of these solvents. To account for solvent effects due to differences in water partitioning between the enzyme and the bulk solvents, we measured water sorption isotherms for the enzyme in each solvent. We measured catalytic activity as a function of enzyme hydration and obtained bell-shaped curves with maxima at the same enzyme hydration in all the solvents. However, the activity maxima were different in all the media, being the lowest in toluene. Differences in the partitioning of substrates and product between the bulk solvent phase and the enzyme active site were accounted for but could not explain the lower catalytic activity observed in toluene. The fact that toluene is very similar to one of the substrates suggested the possibility of competitive inhibition by this solvent. We derived a model allowing for differences in solvation of the substrates, by using thermodynamic activities instead of concentrations, as well as for competitive inhibition by toluene. The model fit the experimental data well, confirming that toluene had a direct adverse effect on the catalytic activity of the enzyme. (c) 1996 John Wiley & Sons, Inc.