Olsnes S, Sandvig K
J Biol Chem. 1986 Feb 5;261(4):1553-61.
When cells with surface-bound diphtheria toxin were exposed to pH 4.5, the toxin became shielded against lactoperoxidase-catalyzed radioiodination, indicating that the toxin was inserted into the membrane. Cells thus treated had strongly reduced ability to take up 36Cl-, 35SO4(2-), and [14C]SCN-. The reduction of chloride uptake was strongest at neutral pH, whereas that of sulfate was strongest at acidic pH. Lineweaver-Burk plots indicated that the toxin treatment reduced the Jmax but not the Km for the anions. The toxin also inhibited the NaCl-stimulated efflux of 35SO4(2-), indicating that the toxin inhibits the antiporter. No inhibition was found when toxin-treated cells were not exposed to low pH, whereas exposure to pH 4.5 for 20 s induced close to maximal inhibition. Half-maximal inhibition was obtained after exposure to pH 5.4. The concentration of diphtheria toxin required to obtain maximal inhibition (0.3 micrograms/ml) was sufficient to ensure close to maximal toxin binding to the cells. Even in ATP-depleted cells and in the absence of permeant anions, low pH induced inhibition of anion antiport in toxin-treated Vero cells. There was no measurable inhibition of anion antiport in cells with little or no ability to bind the toxin.
当表面结合白喉毒素的细胞暴露于pH 4.5时，毒素受到保护，免受乳过氧化物酶催化的放射性碘化作用，这表明毒素已插入膜中。经如此处理的细胞摄取³⁶Cl⁻、³⁵SO₄²⁻和[¹⁴C]SCN⁻的能力大幅降低。氯化物摄取的减少在中性pH时最强，而硫酸盐摄取的减少在酸性pH时最强。Lineweaver-Burk图表明，毒素处理降低了阴离子的Jmax，但未改变Km。毒素还抑制了NaCl刺激的³⁵SO₄²⁻外流，表明毒素抑制了反向转运体。当毒素处理的细胞未暴露于低pH时未发现抑制作用，而暴露于pH 4.5 20秒可诱导接近最大抑制。暴露于pH 5.4后可获得半数最大抑制。获得最大抑制所需的白喉毒素浓度（0.3微克/毫升）足以确保毒素与细胞的结合接近最大程度。即使在ATP耗尽的细胞中且不存在渗透性阴离子时，低pH也会诱导毒素处理的Vero细胞中阴离子反向转运受到抑制。在几乎没有或没有结合毒素能力的细胞中，未检测到阴离子反向转运受到抑制。
