The Ca2+-ATPase (SERCA1) Is Inhibited by 4-Aminoquinoline Derivatives through Interference with Catalytic Activation by Ca2+, Whereas the ATPase E2 State Remains Functional

Several clotrimazole (CLT) and 4-aminoquinoline derivatives were synthesized and found to exhibit in vitro antiplasmodial activity with IC50 ranging from nM to mu M values. We report here that some of these compounds produce inhibition of rabbit sarcoplasmic reticulum Ca2+-ATPase (SERCA1) with IC50 values in the mu M range. The highest affinity for the Ca2+-ATPase was observed with NF1442 (N-((3-chlorophenyl)(4-((4-(7-chloroquinolin-4-yl) piperazin-1-yl)methyl)phenyl)methyl)-7-chloro-4-aminoquinoline) and NF1058 (N-((3-chlorophenyl)(4-(pyrrolidin-1-ylmethyl)phenyl)methyl)-7-chloro-4-aminoquinoline), yielding IC50 values of 1.3 and 8.0 mu M as demonstrated by measurements of steady state ATPase activity as well as single cycle charge transfer. Characterization of sequential reactions comprising the ATPase catalytic and transport cycle then demonstrated that NF1058, and similarly CLT, interferes with the mechanism of Ca2+ binding and Ca2+-dependent enzyme activation (E-2 to E-1.Ca-2 transition) required for formation of phosphorylated intermediate by ATP utilization. On the other hand, Ca2+ independent phosphoenzyme formation by utilization of P-i (i.e. reverse of the hydrolytic reaction in the absence of Ca2+) was not inhibited by NF1058 or CLT. Comparative experiments showed that the high affinity inhibitor thapsigargin interferes not only with Ca2+ binding and phosphoenzyme formation with ATPbut also with phosphoenzyme formation by utilization of P-i even though this reaction does not require Ca2+. It is concluded that NF1058 and CLT inhibit SERCA by stabilization of an E-2 state that, as opposed to that obtained with thapsigargin, retains the functional ability to form E-2-P by reacting with P-i.