At enzyme concentrations above 40 nM the configuration of the purified
plasma membrane Ca2+-ATPase is that of calmodulin-insensitive dimers,
Dilution of the enzyme generates progressively higher proportions of
calmodulin-sensitive monomers with lower V-max and Ca2+ sensitivity th
an the dimeric enzyme, Dimerization from monomeric state had not been
documented before. We investigated whether concentration by volume exc
lusion, obtained by addition of a large molecular weight dextran to a
monomeric Ca2+-ATPase would elicit dimer-like behavior, Dextran induce
d self-association of monomers, as monitored by fluorescence energy tr
ansfer, but the Ca2+ sensitivity of the re-associated monomers aas low
er than that of the native dimers, These results suggest that the self
-association reaction is structurally but not functionally reversible,
and also document the existence of a hitherto unknown kinetic state o
f the oligomerized Ca2+-ATPase, with high V-max but low Ca2+-sensitivi
ty.