Tubulin does not bind glycerol irreversibly

Isolation of microtubules by assembly/disassembly methods

The microtubule-isolation procedures described here are based on the ability of the investigator to control the dimer-polymer distribution of tubulin by varying the temperature of the extract. In general, the extract is warmed to induce microtubule assembly, the polymer is collected by centrifugation, cooled to induce disassembly, clarified by centrifugation, and then warmed again to produce polymer. As long as the GTP supply is sufficient, the microtubules that result can be taken through numerous rounds of this in vitro assembly and disassembly reaction. Many microtubule-associated proteins (MAPs) associate with microtubules assembled in vitro. Some reagents can skew the equilibrium of assembly and disassembly toward formation of polymer. The inclusion of glycerol, for instance, promotes microtubule assembly by disrupting the hydration shell around the tubulin dimers. The result is a greater yield of tubulin per gram of starting material. However, the ratio of MAPs to tubulin is slightly lower, presumably because the glycerol also decreases the binding of MAPs to tubulin. Two methods are described here: The first uses buffer lacking assembly-promoting components, and the second uses buffer containing glycerol. These procedures are most efficient with vertebrate brain tissue, where the soluble protein can be up to 15%-20% tubulin. The first produces satisfactory yields when using chick or pig brain; the second is recommended for calf or cow brain. The second procedure may also be useful for studies of nonneuronal tissues where the relative concentration of tubulin per gram of wet weight is considerably lower than that of brain.

Evidence for separate modes of action in thermal radiosensitization and direct thermal cell death

It is not known whether heat potentiation of radiation damage and direct heat death are mediated by the same or by different heat lesions within the cell. In this study, three types of experiments were performed on BP-8 murine sarcoma cells to provide evidence against a common mode of action: (1)Evaluation of the kinetics of cell death from heat, radiation, and combined heat-radiation treatments: direct heat death is rapid and essentially complete within 24-48 hours after heat exposure, whereas radiation death develops only after a delay period of several days. Radiosensitization by heat affects only the delayed component of cell death, that is, the radiation component of death. (2)Thermal radiosensitization and direct heat death as a function of heating time: thermal radiosensitization requires only short exposures to heat; prolonged heating does not further enhance this effect. In contrast, direct heat death increases with increasing duration of heat exposure. (3)Independent modification of radiosensitization and thermal death: addition of 5% glycerol to the incubation medium protects cells against direct heat death, but not against thermal radiosensitization. In combination, these findings suggest that heat potentiation of radiation lethality and direct heat death are two distinct phenomena mediated by different cellular mechanisms.