Interaction of dibucaine with the transmembrane domain of the Ca(2+)-ATPase of sarcoplasmic reticulum

Abstract

The site of interaction of dibucaine with the Ca(2+)-ATPase of rabbit sarcoplasmic reticulum, an ion-transporting membrane protein, was investigated by determining the effect of dibucaine on the denaturation of the transmembrane domain and the aqueous domain containing, respectively, the high-affinity Ca2+ binding sites and the site of ATP hydrolysis. In the absence of Ca2+, a single irreversible denaturation transition with Tm approximately equal to 49 degrees C is observed for the Ca(2+)-ATPase by differential scanning calorimetry (DSC). In the presence of Ca2+, but not Mg2+, Sr2+, or Ba2+, a new high-temperature transition is observed that has been shown to be due to stabilization of the transmembrane region [Lepock, J. R., Rodahl, A. M., Zhang, C., Heynen, M. L., Waters, B., & Cheng, K. H. (1990) Biochemistry 29, 681-689]. The maximum stabilization corresponds to a shift in Tm of 13.8 degrees C, and Hill analysis indicates that the Ca2+ binding site yielding stabilization has a Kd = 2.5 x 10(-4) M with a cooperativity (n) of 1. Thus, stabilization is due to Ca2+ binding not to the high-affinity sites but to one of the previously observed sites of low or intermediate affinity, which must be located in the transmembrane or stalk subdomains. Dibucaine has little effect on the Tm of the aqueous domain, but it decreases the Tm of the transmembrane domain with Kd approximately equal to 4.1 x 10(-4) M and a cooperativity of approximately 1.6, implying that destabilization is due to the binding of dibucaine to sites of intermediate or moderately high affinity.(ABSTRACT TRUNCATED AT 250 WORDS)