An investigation of functional similarities between the sarcoplasmic reticulum and platelet calcium-dependent adenosinetriphosphatases with the inhibitors quercetin and calmidazolium. Biochemistry 1987 Dec 01;26(24):8024-30
Date
12/01/1987Pubmed ID
2962642DOI
10.1021/bi00398a070Scopus ID
2-s2.0-0023462024 (requires institutional sign-in at Scopus site) 30 CitationsAbstract
The platelet and skeletal sarcoplasmic reticulum calcium-dependent adenosinetriphosphatases (Ca2+-ATPases) were functionally compared with respect to substrate activation by steady-state kinetic methods using the inhibitors quercetin and calmidazolium. Quercetin inhibited platelet and sarcoplasmic reticulum Ca2+-ATPase activities in a dose-dependent manner with IC50 values of 25 and 10 microM, respectively. Calmidazolium also inhibited platelet and sarcoplasmic reticulum Ca2+-ATPase activities, with half-maximal inhibition measured at 5 and 4 microM, respectively. Both inhibitors also affected the calcium transport activity of intact platelet microsomes at concentrations similar to those which reduced Ca2+-ATPase activity. These inhibitors were then used to examine substrate ligation by the platelet and sarcoplasmic reticulum calcium pump proteins. For both Ca2+-ATPase proteins, quercetin has an affinity for the E-Ca2 (fully ligated with respect to calcium at the exterior high-affinity calcium binding sites, unligated with respect to ATP) conformational state of the protein that is approximately 10-fold greater than for other conformational states in the hydrolytic cycle. Quercetin can thus be considered a competitive inhibitor of the calcium pump proteins with respect to ATP. In contrast to the effect of quercetin, calmidazolium interacts with the platelet and sarcoplasmic reticulum Ca2+-ATPases in an uncompetitive manner. The dissociation constants for this inhibitor for the different conformational states of the calcium pump proteins were similar, indicating that calmidazolium has equal affinity for all of the reaction intermediates probed. These observations indicate that the substrate ligation processes are similar for the two pump proteins. This supports the concept that the hydrolytic cycles of the two proteins are comparable.
Author List
Fischer TH, Campbell KP, White GC 2ndAuthor
Gilbert C. White MD Professor in the Medicine department at Medical College of WisconsinMESH terms used to index this publication - Major topics in bold
AnimalsBlood Platelets
Calcium-Transporting ATPases
Enzyme Activation
Enzyme Inhibitors
Flavonoids
Humans
Imidazoles
Kinetics
Mathematics
Microsomes
Models, Theoretical
Muscles
Quercetin
Rabbits
Sarcoplasmic Reticulum
