A proper transmembrane Ca²⁺ gradient is essential for the higher enzymatic activity of adenylate cyclase

Adenylate cyclase from bovine brain cortex was reconstituted into liposomes with (1000 fold) or without transmembrane Ca²⁺ gradient. The highest enzyme activity (the active center of enzyme exposing outside) was observed in the vesicles with lower Ca²⁺ concentration outside (≈10^-6 M, similar to physiological condition). If the transmembrane Ca²⁺ gradient was in the inverse direction (i.e. higher Ca²⁺ concentration outside, 1 mM), a lowest enzyme activity would appear. Such a difference could be diminished following addition of A23187. Obtained results showed that a proper transmembrane Ca²⁺ gradient is essential for the optimal fluidity of phospholipid bilayer, favouring the formation of suitable conformation of adenylate cyclase with higher enzyme activity.