Gel to liquid crystalline phase transition promotes a conformational reorganization of Ca2+, Mg2+-ATPase from sarcoplasmic reticulum in dimyristoylphosphatidylcholine reconstituted systems.

Sarcoplasmic reticulum Ca2+, Mg2+-ATPase has been reconstituted in membranes highly enriched in dimyristoylphosphatidylcholine. According to electron microscopy data these membranes form vesicles of an average diameter of 1000 +/- 200 A. These reconstituted membranes show hysteretic behavior in some physical-chemical properties, such as light scattering and fluorescence when labeled with iodoacetamidofluorescein and with N-iodoacetyl-N'-(5-sulfo-1-naphthyl) ethylenediamine. Hysteretic behavior in catalytic activity can also be inferred from the kinetic data presented in this paper, because the temperature dependence of the Ca2+, Mg2+-ATPase activity is altered by a mild thermal pretreatment of the samples. Furthermore, it was noticed that the Ca2+-dependent ATPase activity of these complexes, when assayed above the phase transition temperature (Tc) of the lipid matrix, showed a lag phase in the minute time scale range. On the basis of these findings, it is suggested that the gel-to-liquid crystalline phase transition of the lipid is able to shift the conformational equilibrium E----E* of Ca2+, Mg2+-ATPase. The fact that the -SH reactivity against 5,5'-dithio-bis-nitrobenzoic acid of these complexes is also altered by preincubation above Tc for several minutes also supports that lipid melting induces a conformational change in Ca2+, Mg2+-ATPase.