EFFECT OF DIVALENT-CATIONS ON NA-ATPASE OBTAINED FROM HUMAN PLACENTA(,K+)

Circular dichroism (CD) and acrylamide quenching studies of Na+,K+-ATP ase from human placenta showed that its incorporation into phosphatidy lcholine vesicles increased the enzymic activity by 55%. Moreover, bot h with the purified and the vesicle-reconstituted protein, Ca2+ and Mg 2+ increased the activity, the effect being more pronounced after prei ncubation of the protein with Mg2+. CD data suggest that this activity increase may be linked to a change in the secondary structure of the ATPase, in particular beta-turn, beta-sheet and random coil. Acrylamid e quenching studies suggest that ions could primarily interact with ph ospholipid head groups, but not directly with the protein.