THE HYDROPHILIC DOMAIN OF PHOSPHOLAMBAN INHIBITS THE CA2-ATPASE - THEIMPORTANCE OF THE METHOD OF ASSAY()

The peptide MEKVQYLTRSAIRRASTIEMPQQAR-Cys representing residues 1-25 o f phospholamban (PLN) decreases by 40 % the maximal state rate of ATP hydrolysis by the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulu m (SR), measured at saturating concentrations of Ca2+. The pattern of Ca2+ uptake by SR vesicles in the presence of oxalate is complex, with an initial fast phase being followed by a lag phase and a second, slo wer phase of Ca2+ accumulation. PLN(1-25) reduces the rate of the slow er phase of Ca2+ accumulation by 30 %. However, if the level of accumu lation of Ca2+ is measured after 2 min., the effect of PLN(1-25) is mu ch less marked. It is concluded that PLN(1-25) inhibits the ATPase, bu t that the effects of this inhibition are not apparent under some assa y conditions. (C) 1995 Academic Press, Inc.