Electrolyte balance was examined in ascites tumor cells treated with the surface-combining mitotic inhibitor gum tragacanth. Potassium and sodium levels remained normal during treatment with physiological doses of the agent, thus precluding imbalance of these ions as an explanation of the tumor-inhibitory effect. Incorporation of amino acids into protein also remained unaffected.

Washing and centrifugation of treated cells resulted in a dose-dependent loss of potassium and gain of H2O, indicating that tragacanth sensitizes the membrane to mechanical damage. Both this membrane alteration and the tumor-inhibitory effect are observed whether or not ascitic fluid is present in the treatment medium. In contrast, the nature of the reaction at the cell surface, as revealed by periodic acid-Schiff staining, is strongly dependent on ascitic fluid protein; in its absence the agent apparently penetrates into the peripheral area of the cell, and in a form which no longer distinguishes native inhibitory tragacanth from its heat-deactivated control.

Fractionation of phosphate-buffered saline suspensions of the gum by centrifugation showed the tumor-inhibitory component to be located exclusively in the soluble portion, but both portions affected membrane function. Further purification of the soluble portion on DEAE-cellulose showed tumor-inhibitory activity to be related to negative charge.

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This work was supported by grants from the Medical Research Council and Cancer Research Campaign (to the late E. M. F. R.) and from the Medical Research Council of Ireland and the Irish Cancer Society (H. S.).

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