ENHANCED CALMQDULIN-DEPENDENT (Ca2++ Mg2+)-ATPASE OF HYPOTHYROID RAT ERYTHROCYTE MEMBRANES

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Abstract
Erythrocyte membranes obtained from chronically hypothyroid rats (propylthiouracil-treated), prepared after disruption of cells in iso-osmotic imidazole buffer, showed 1.5- to 2- fold higher (Ca2+ + Mg2+)-ATPase activity as compared to similar preparations obtained from normal rats. These differences disappeared when the membrane-bound activity was assayed in presence of trifluoperazine, an inhibitor of calmodulin-mediated alterations of enzymatic activities. Membranes prepared from hypo-osmotically lysed erythrocytes showed levels of activity comparable to those of trifluoperazine-suppressed iso-osmoticallv obtained preparations and no difference was observed between normal and hypothyroid membranes. (Ca2+ + Mg2+)-ATPase activities of hypo-osmotically prepared erythcocyte membranes from normal and hypothyroid rats were comparably stimulated by increased concentrations of Ca alone, and reached the same maximum level of activity on addition of a calmodulin preparation obtained from rat brain. In contrast, Na+ + K+-ATPase activity of chronically hypothyroid rat erythrocyte membranes was about 40% lower than that of normal rats, was not affected by trifluoperazine, nor influenced by the osmolarity of the disrupting medium. The data suggest that the enhanced (Ca2+ + Mg2+)-ATPase of erythrocyte membranes of chronically hypothyroid rats is attributable to an increase in the calmodulin-dependent fraction of the total activity, and that this results from greater association of activated calmodulin with the membrane, rather than from an increase in the quantity of enzyme.