Yl. Shi et al., PHOSPHOLAMBAN-DEPENDENT EFFECTS OF C(12)E(18) ON CALCIUM-TRANSPORT AND MOLECULAR-DYNAMICS IN CARDIAC SARCOPLASMIC-RETICULUM, Biochemistry, 35(41), 1996, pp. 13393-13399
We have studied the effects of the nonionic detergent C(12)E(8) On Ca-
ATPase enzymatic activity and oligomeric state (detected by time-resol
ved phosphorescence anisotropy, TPA) in skeletal and cardiac sarcoplas
mic reticulum (SR). In skeletal SR, C(12)E(8) inhibits the Ca-ATPase,
both at high (micromolar and above) and low (submicromolar) Ca. In car
diac SR, C(12)E(8) inhibits at high Ca but activates at low Ca. Thus C
(12)E(8) activates enzymatic activity only in cardiac SR and only unde
r conditions (submicromolar Ca) where phospholamban (PLB) regulates (i
nhibits) the enzyme [Lu, Y.-Z., & Kirchberger, M. A. (1994) Biochemist
ry 33, 5056-5062]. TPA of skeletal SR at low and high Ca demonstrates
that C(12)E(8) induces aggregation of ATPase monomers and small oligom
ers. C(12)E(8) also aggregates the Ca-ATPase in cardiac SR at high Ca.
In cardiac SR at low Ca, the Ca-ATPase is already highly aggregated,
and C(12)E(8) partially dissociates these aggregates. Thus the TPA res
ults provide a simple physical explanation for the functional effects:
C(12)E(8) inhibits the ATPase when it aggregates the enzyme (skeletal
SR at high and low Ca; cardiac SR at high Ca), and the detergent acti
vates when it dissociates ATPase oligomers (cardiac SR at low Ca). C(1
2)E(8) stabilizes the E2P conformation of the Ca-ATPase with respect t
o the E2 conformation, and this stabilization is PLB-dependent. Both t
he physical and the functional effects of C(12)E(8) On the Ca-ATPase a
re PLB-dependent, with C(12)E(8) reversing the effects of PLB. The res
ults provide insight into the mechanism by which PLB regulates the Ca-
ATPase in cardiac SR.