Thermodynamic analysis of the effect of D2O on mitotic spindles in developing sea urchin eggs.

Abstract

D2O increases the volume of the mitotic spindle and its bire-fringence. This effect is completely reversible and repeatable within an optimum range of temperature and D2O concentration. We made a thermodynamic analysis of the D2O effect on the first mitotic spindle in dividing sea urchin eggs at metaphase because of the possibility of a dynamic equilibrium existing between cytoplasmic tubulin molecules and the oriented microtubules in the spindle.D2O has been thought to promote hydrophobic interactions between tubulin molecules. Thermodynamic factors such as changes in enthalpy or in entropy should, therefore, be increased by an application of D2O. No significant changes in the thermodynamic factors, however, were found in our experiments. Our data indicate that D20 increases the available pool of polymerizable tubulin molecules in the cell. This is evidence that a pool of unpolymerizable tubulin molecules is present in the dividing eukaryotic cell.