The hydrolysis of
p-nitrophenyl phosphate
catalyzed by the erythro
cyte membrane Ca
2+-ATPaseis stimulated by low
con
centrations of the
compound 4,4'-diisothio
cyanatostilbene-2,2'-disulfoni
c a
cid(DIDS), a
classi
c inhibitor of anion transport. Enhan
cement of the phosphatase a
ctivity varies from 2- to6-fold, depending on the Ca
2+ and
calmodulin
con
centrations used. Maximum stimulation of the pNPPasea
ctivity in ghosts is rea
ched at 4-5
M DIDS. Under the same
conditions, but with ATP rather thanpNPP as the substrate, the Ca
2+-ATPase a
ctivity is strongly inhibited. A
ctivation of pNPP hydrolysis byDIDS is equally effe
ctive for both ghosts and purified enzyme, and therefore is independent of its effe
ctas an anion transport inhibitor. Binding of the a
ctivator does not
change the Ca
2+ dependen
ce of thepNPPase a
ctivity. Stimulation is partially additive to the a
ctivation of the pNPPase a
ctivity eli
cited by
calmodulin and appears to involve a strong affinity binding or
covalent binding to sulfhydryl groups ofthe enzyme, sin
ce a
ctivation is reversed by addition of dithiothreitol but not by washing. The degree ofa
ctivation of pNPP hydrolysis is greater at alkaline pH values. DIDS de
creases the apparent affinity ofthe enzyme for pNPP whether in the presen
ce of Ca
2+ alone or Ca
2+ and
calmodulin or in the absen
ce ofCa
2+ (with 5
M DIDS the observed
Km shifts from 4.8 ± 1.4 to 10.1 ± 2.6, from 3.8 ± 0.4 to 7.0 ± 0.8,and from 9.3 ± 0.7 to 15.5 ± 1.1 mM, respe
ctively). However, the pNPPase rate is always in
creased (asabove, from 3.6 ± 0.6 to 11.2 ± 1.7, from 4.4 ± 0.5 to 11.4 ± 0.9, and from 2.6 ± 0.6 to 18.6 ± 3.9nmol mg
-1 min
-1, in the presen
ce of Ca
2+ alone or Ca
2+ and
calmodulin or in the absen
ce of Ca
2+,respe
ctively). ATP inhibits the pNPPase a
ctivity in the absen
ce of Ca
2+, both in the presen
ce and in theabsen
ce of DIDS. Therefore, kineti
c eviden
ce indi
cates that DIDS does more than shift the enzyme to theE
2 conformation. We propose that the transition from E
2 to E
1 is de
creased and a new enzyme
conformer,denoted E
2*, is a
ccumulated in the presen
ce of DIDS.