Summary. We investigated whole blood coagulation by thrombelastography (TEG) employing activation with minute amounts of tissue factor (TF). Continuous raw data captured were transformed into novel parameters, such as the maximum velocity (MaxVel) and the time to maximum velocity (t,MaxVel) of whole blood clot formation. The courses of the whole blood clot development were very similar to thrombin generation curves reported in plasma. In this assay healthy women (n ¼ 30) showed an earlier onset and an increased coagulation velocity compared to healthy men (n ¼ 30). In patients with severe hemophilia, and persons undergoing thromboprophylaxis, distinctly abnormal coagulation profiles were observed with a decrease in the MaxVel, as well as a prolonged t,MaxVel. Changes appeared to be dependent on the nature and severity of the hemostatic deficit. Preliminary studies in patients substituted with recombinant factor VIIa demonstrated a marked change in the coagulation profile, in which the MaxVel and t,MaxVel shifted towards normal in a dose-dependent way. Data suggest that the whole blood coagulation TEG profile, following activation with minute amounts of TF, may reflect the hemostatic potential in patients suspected of impaired hemostasis.

Topoisomerases are involved in many aspects of DNA metabolism such as replication and transcription reactions. Camptothecins, which stabilize the covalent intermediate of topoisomerase I and DNA are effective, though toxic, drugs for cancer therapy. In this study, a new class of topoisomerase I inhibitors was identified, and their mode of action was characterized using recombinant human topoisomerase I preparations and human HL-60 leukemic cells. Quercetin and the related natural flavones, acacetin, apigenin, kaempferol, and morin, inhibit topoisomerase I-catalyzed DNA religation. In contrast to camptothecin, these compounds do not act directly on the catalytic intermediate and also do not interfere with DNA cleavage. However, formation of a ternary complex with topoisomerase I and DNA during the cleavage reaction inhibits the following DNA religation step. 3,3,4,7-Tetrahydroxy-substituted flavones stabilize the covalent topoisomerase I-DNA intermediate most efficiently. Enhanced formation of covalent topoisomerase I-DNA complexes was also demonstrated in human HL-60 cells. In contrast, synthetic 3,5-dibromo-4-hydroxy-3-methylflavones bind selectively to topoisomerase I in its non-DNA-bound form and block the following DNA binding step. As a consequence, these synthetic flavonoids are capable of counteracting topoisomerase I-directed effects of camptothecin. Inhibition of DNA binding is obtained by voluminous hydrophobic substituents in 6-position of the flavone structure. Our data show that selective inhibitors of both half-reactions of topoisomerase I can be derived from the flavone structure.

No abstract

The DNA cleavage reaction of eukaryotic topoisomerase II produces nicked DNA along with linear nucleic acid products. Therefore, relationships between the enzyme's DNA nicking and double-stranded cleavage reactions were determined. This was accomplished by altering the pH at which assays were performed. At pH 5.0 Drosophila melanogaster topoisomerase II generated predominantly (greater than 90%) single-stranded breaks in duplex DNA. With increasing pH, less single-stranded and more double-stranded cleavage was observed, regardless of the buffer or the divalent cation employed. As has been shown for double-stranded DNA cleavage, topoisomerase II was covalently bound to nicked DNA products, and enzyme-mediated single-stranded cleavage was salt reversible. Moreover, sites of single-stranded DNA breaks were identical with those mapped for double-stranded breaks. To further characterize the enzyme's cleavage mechanism, electron microscopy studies were performed. These experiments revealed that separate polypeptide chains were complexed with both ends of linear DNA molecules generated during cleavage reactions. Finally, by use of a novel religation assay [Osheroff, N., & Zechiedrich, E. L. (1987) Biochemistry 26, 4303-4309], it was shown that nicked DNA is an obligatory kinetic intermediate in the topoisomerase II mediated reunion of double-stranded breaks. Under the conditions employed, the apparent first-order rate constant for the religation of the first break was approximately 6-fold faster than that for the religation of the second break. The above results indicate that topoisomerase II carries out double-stranded DNA cleavage/religation by making two sequential single-stranded breaks in the nucleic acid backbone, each of which is mediated by a separate subunit of the homodimeric enzyme.

No abstract