INTERACTIONS OF THE REGULATORY LIGANDS MG2-) WITH THE RENAL PLASMA-MEMBRANE CA2+-ATPASE - EFFECTS OF OSMOLYTES THAT STABILIZE OR DESTABILIZE PROTEIN-STRUCTURE( AND MGATP(2)

In this report we analyze the kinetics of activation of the plasma mem brane Ca2+-ATPase from kidney proximal tubules by the regulatory ligan ds Mg2+ and MgATp2-, and we examine modifications in the effects of th ese ligands that are promoted by organic solutes of natural occurrence that stabilize or destabilize protein structure and function. The sol utes tested were trimethylamine-N-oxide (TMA-O), sucrose and urea. TMA -O and sucrose were chosen as representative of the different methylam ines and polyols, respectively, that accumulate iii living organisms. The results lead to the conclusion that free Mg2+ and the MgATp2- comp lex both activate the rate-determining E2 --> E1 transition during the catalytic cycle of the enzyme, by binding to nonidentical and indepen dent regulatory sites. They also indicate that TMA-O, sucrose and urea not only promote global modifications in the enzyme structure, but al so modify specific interactions of the ligands Mg2+ and MgATp2- at the ir regulatory sites.