EFFECT OF TRANSMEMBRANE Ca^(2+) GRADIENT ON CONFORMATION AND ENZYME ACTIVITY OF RECONSTITUTED ADENYLATE CYCLASE

Adenylate cyclase from bovine brain cortex was reconstituted into asolectin liposomes with (500-fold) or without transmembrane Ca^(2+) gradient. The enzyme activity of four types of proteoliposomes (the active center of enzyme exposing outside) was compared. The highest adenylate cyclase activity was observed in the vesicles with outside lower Ca^(2+)concentration (≈10^(-6) mol/L, similar to thephysiological condition). If the transmembrane Ca^(2+) gradient was in the inverse direction (i.e. outside higher Ca^(2+) concentration, 0.5 mmol/L), a lowest enzymatic activity would appear. The difference in enzymatic activity between the two types of proteoliposomes could be diminished following the addition of Ca^(2+) ionophore A23187. Proteoliposomes without transmembrane Ca^(2+) gradient exhibited intermediate activities.The conformation difference of adenylatecyclases in the above-mentioned proteoliposomes was also detected by measuring intrinsic fluorescence and fluorescence quenching with KI.