EFFECTS OF MAGNESIUM PLUS VANADATE ON PARTIAL REACTIONS OF THE CA2-ATPASE FROM HUMAN RED-CELL MEMBRANES()

Under conditions in which pretreatment with Mg2+ plus vanadate activat e the Ca2+-ATPase, the initial rate of phosphorylation of the enzyme i ncreased from 141 to 259 pmol/mg protein per s while the steady-state level of phosphoenzyme lowered from 1.9 to 1.1 pmol/mg protein. The dr op in phosphoenzyme level was caused by incubation and washing during treatment rather than by vanadate. The data allowed to estimate a turn over number for the enzyme that raised by 170% after pretreatment. The results show that the activation of the Ca2+-ATPase by Mg2+ plus vana date is due to changes in the kinetic properties of the enzyme.