On the short carbonyl bond in bis[mu-1,2-benzisothiazol-3(2H)-one 1,1-dioxido-kappa2N:O]bis[[1,2-benzisothiazol-3(2H)-one 1,1-dioxido-kappaN]bis(imidazole)copper(II)]

The short carbonyl bond in the title compound, [Cu(2)(C(7)H(4)NO(3)S)(4)(C(3)H(4)N(2))(4)] [Liu, Huang, Li & Lin (1991). Acta Cryst. C47, 41-43], is an artifact of disorder in the isothiazol-3(2H)-one 1,1-dioxide part of the 1,2-benzisothiazol-3(2H)-one entity. In the present redetermination, all bond dimensions in the centrosymmetric dinuclear molecule are normal. The five-coordinate Cu atom shows trigonal-bipyramidal coordination. Hydrogen bonds from the imidazole donor ligand link adjacent molecules into a two-dimensional layer structure.

Tumor localization of anti-CEA single-chain Fvs: improved targeting by non-covalent dimers

BACKGROUND

Genetic engineering can produce novel antibody fragments with improved properties for applications such as tumor targeting in vivo.

OBJECTIVES

To produce stable monomeric (27 kDa) and dimeric (55 kDa) forms of a single-chain Fv (scFv) from the anti-carcinoembryonic antigen (anti-CEA) antibody T84.66, and assess the targeting and biodistribution properties in an animal model.

STUDY DESIGN

ScFv were constructed with either a 28 or 14 amino acid connecting peptide and expressed by secretion from E. coli. Following affinity purification, proteins were characterized by gel electrophoresis and mass spectrometry. Binding properties were assessed by size exclusion HPLC after incubation with antigen, and affinities determined by surface plasmon resonance. The shorter linker favored formation of dimers (and higher multimers) which showed unusual stability. ScFv were radiolabeled with 125I for tumor targeting and biodistribution studies of monomeric or dimeric forms were conducted in athymic mice bearing LS174T human colorectal carcinoma xenografts.

RESULTS

125I-scFv monomers and dimers targeted exhibited rapid clearance kinetics in tumor-bearing mice. Nevertheless, the anti-CEA scFvs targeted very well to xenografts, leading to high tumor: normal organ ratios (greater than 20:1 at 24 h) for both forms. Tumor localization of the non-covalent dimers was much higher than monomers, reaching 10-15% injected dose per gram at 1 h.

CONCLUSION

Non-covalent dimers of scFv (also known as diabodies) are stable, easy to produce and show excellent targeting as compared to monomeric scFv, probably due to increased mass and valency.

DNA primase from KB cells. Evidence for a novel model of primase catalysis by a highly purified primase/polymerase-alpha complex

The oligonucleotide primers synthesized by a highly purified KB fraction containing DNA primase and DNA polymerase-alpha activities display reproducible alterations of structure and composition in response to dNTPs. These observations are sufficiently explained by a novel model of primase catalysis that defines the primase as a complex enzyme with distinct catalytic properties that are regulated by mechanisms exquisitely sensitive to dNTP concentration. The enzyme performs the template-directed, de novo synthesis of 1 unit of oligoribonucleotide (canonical priming reaction) and then elongates that moiety to a limited extent by several cycles of addition of short tracts of homogeneous oligoribonucleotide or oligodeoxynucleotide. The oligomeric units have modal lengths of approximately 11- to 14- nucleotides that are postulated to reflect the inherent processivity of the catalytic mechanisms. Elongation is accomplished by two catalytic centers, or conformers of a single center, that are synchronously coupled, mutually exclusive, and extremely stringent for respective rNTP and dNTP substrates. Transitions between these two catalytic modes are regulated by dNTPs and demonstrable at dNTP:rNTP concentrations of less than or equal to 10(-4). In the absence of dNTPs, the primase produces a family of oligoribonucleotides, approximately 24- to 36- nucleotides long; at dNTP levels between 0.08 and 0.80 micron, the primase synthesizes mixed oligomers composed of strictly alternating tandem arrays of oligoribo- and oligodeoxynucleotide units; and at dNTP levels greater than or equal to 4.8 micron, the primase becomes stabilized in the deoxy mode after the initial oligoribo leads to oligodeoxy transition and products contain only a single 5' -terminal unit of oligoribonucleotide. The model predicts that the physiologically significant primer for DNA polymerase-alpha is a mixed 5' -oligoribo-3' -oligodeoxynucleotide and the signal which governs the switch from RNA leads to DNA synthesis is intrinsic in the primase mechanism and is generated by ambient dNTPs.

DNA primase from KB cells. Characterization of a primase activity tightly associated with immunoaffinity-purified DNA polymerase-alpha

A very highly purified fraction of KB cell DNA polymerase-alpha, prepared with a monoclonal antibody, contains DNA primase activity. The primase synthesizes oligonucleotide chains initiated with ATP in a reaction that is resistant to alpha-amanitin and strictly dependent on added template and ribonucleoside triphosphates (rNTPs). In the presence of added dNTPs and M13 DNA template, the primase produces a uniform population of oligoribonucleotides, predominantly hexamers to decamers, that are extended by polymerase-alpha into DNA chains up to 3000 nucleotides long. There is no evidence for nucleotide preferences at RNA/DNA junctions. In the absence of added dNTPs, the oligomeric products are heterogeneous in size and composition and susceptible to cleavage by pancreatic DNase I due to their content of short oligodeoxynucleotide tracts synthesized by primase from trace contaminant dNTPs in the rNTP substrates. The primase and polymerase-alpha activities are distinguishable by several physical and chemical criteria, and the primase reaction is only partially sensitive to two potent, independent monoclonal antibodies that neutralize polymerase-alpha. Although the presence of both primase and polymerase-alpha activities in a highly purified immune complex prepared with a monoclonal antibody argues for their tight physical association, the chemical, physical, and immunological discriminations indicate the two catalytic entities are functionally and structurally distinct.

Preparation and preliminary characterization of monoclonal antibodies against human DNA polymerase alpha

We report the successful establishment of 16 stable murine hybridoma monoclones that produce homogeneous antibodies against KB cell DNA polymerase alpha. All of the antibodies exhibit specific binding of polymerase alpha activity, and 3 of them possess anti-polymerase alpha neutralizing activity. None of the antibodies interacts detectably with KB cell DNA polymerases beta or gamma. All of the 5 antibodies so far examined demonstrate linear Scatchard binding plots and very high binding affinities, with equilibrium dissociation constants (Kd) ranging between 3.2 x 10(-9) and 3.4 x 10(-10) M. These monoclonal antibodies comprise a set of powerful and specific reagents that should facilitate the development and application of novel approaches to the complex biochemical mechanisms of mammalian DNA replication.

Intracellular localization of human DNA polymerase alpha with monoclonal antibodies

We have successfully established 16 stable murine hybridomas that secrete monoclonal antibodies specific for human DNA polymerase alpha. The results of immunocytochemical studies, using 4 of these monoclonal antibodies and immunoperoxidase detection methods, document the exclusively intranuclear localization of DNA polymerase alpha in three separate lines of cultured human cells. By light microscopy, the immunoperoxidase reaction product exhibits a diffuse pattern of distribution within the nucleoplasm, but nucleoli are clearly negative. In cultures of the transformed lines, KB nd BeWo, more than 955 of the cells are positive, suggesting that intranuclear DNA polymerase alpha antigens persist throughout the mitotic cycle. In striking contrast, in the normal diploid fibroblast line, WI-38, a smaller fraction of the cultured cells is positive, and there is no detectable polymerase alpha antigen in the closely apposed cells of microcolonies that are presumed to be contact-arrested and no longer mitotically cycling. In cells in mitosis that have dissolved their nuclear envelopes (and are thus transiently anucleate), the anti-polymerase alpha reaction continues to be strongly positive, and in this single circumstance the reaction product is diffusely distributed throughout the cell cytoplasm. Initial electron microscopic examination of KB cells confirms and extends these observations. The immunoperoxidase reaction product is essentially limited to the nuclear compartment and is predominantly distributed in the midzonal region of the nucleoplasm between centrally disposed nucleoli and peripherally located blocks of condensed chromatin.