DERIVATIVES OF THAPSIGARGIN AS PROBES OF ITS BINDING-SITE ON ENDOPLASMIC-RETICULUM CA2- STEREOSELECTIVITY AND IMPORTANT FUNCTIONAL-GROUPS( ATPASE )

The naturally occurring sesquiterpene lactone thapsigargin is a potent and selective inhibitor of SERCA ATPases, a family of Ca2+-pumppng AT Pases present in the endoplasmic reticulum of all mammalian cells. We have studied some of the molecular features of thapsigargin responsibl e for its inhibitory action towards these Ca2+ ATPases. A series of th apsigargin analogues were synthesised and their inhibitory potencies d etermined using the uptake of Ca-45(2+) in bovine cerebellar microsome s as a sensitive marker of Ca2+ ATPase activity. An attenuation of the inhibitory potency relative to the parent compound was found ranging from slight to over 3 orders of magnitude. The inhibitory activity sho wed a very strong configuration dependence, a major contribution from the ester groups at C3 and C10, and an apparently minor contribution f rom the lactone ring substituents. The data are consistent with thapsi gargin fitting to a sterically discriminating cleft involving the hydr ophobic transmembrane region of the ATPase, and is compatible with ava ilable kinetic evidence of thapsigargin-mediated inhibition.