STABILIZING AND DESTABILIZING EFFECTS ON PLASMA-MEMBRANE CA2-ATPASE ACTIVITY()

We have examined the temperature-dependent effects of several organic compounds on the activity of the purified Ca2+-ATPase of erythrocytes. The monomeric enzyme was activated either by interaction with calmodu lin or by oligomerization in the absence of calmodulin. Of the four ho mologous solute series studied including polyols, alkanols, aprotic so lvents, and N-methyl derivatives of formamide and acetamide only polyo ls stabilized the enzyme over a broad range of concentration and tempe rature. Similarity of Ca2+-ATPase activity patterns at 25 and 37 degre es C and in the presence of glycerol is in agreement with indirect, st abilizing interactions. Glycerol also protected the Ca2+-ATPase from t hermal denaturation at 45 degrees C. Within each homologous series, in hibitory effects increased with increasing solute concentration and wi th increasing structural similarity to detergents, indicating that dir ect destabilizing interactions are responsible for the observed inhibi tion. These were comparable to the destabilizing effect of urea. Oligo mers were more resistant to all inhibitory solutes as compared to calm odulin-activated monomers suggesting that the nonpolar patches of the oligomerized enzyme are less accessible to solutes.