Specific association of bromocresol purple anions with a magnesium complex of a phosphorylated intermediate during steady-state hydrolysis of ATP by the Mg2+ + Ca2+-dependent ATPase of sarcoplasmic reticulum.
The mechanism of dimeric binding of bromocresol purple (BCP) anions to Mg2+ + Ca2+-ATPase of the sarcoplasmic reticulum (SR) and the resulting partial inhibition of the ATPase activity were studied. BCP anions in three states, free monomer, bound monomer, and bound dimer, were spectrophotometrically calculated by solving simultaneous equations, delta A lambda 1-lambda 2 = sigma delta ai (epsilon i lambda 1-epsilon i lambda 2), and concentration changes of these states were analyzed. The addition of ATP caused an increase in the bound dimer and a decrease in the free monomer, but the change of the bound monomer was slight. The decrease in delta A (decrease phase) on the addition of ATP on dual-wavelength spectrophotometry at 585-610 nm was related to an increase in the amount of dimer bound to the SR membranes. The magnitude of the decrease phase increased with an increase in Mg2+ concentration and decreased with an increase in the concentration of Ca2+. BCP anions at the probe concentration partially inhibited the ATPase activity, and brought about a decrease in the ADP-sensitive E-P (E1P) and an increase in the ADP-insensitive E-P (E2P), though BCP anions did not affect the amount of total E-P. On elimination of Mg2+ at the steady-state E-P level both E2P and E2P . (BCP)2 were decomposed, suggesting that the enzyme form binding the BCP dimer was Mg . E-P. An increase in Mg2+ concentration increased E2P but an increase in Ca2+ concentration decreased E2P. Decomposition of E2P to P1 was inhibited by BCP anions. The following simple scheme was suggested to explain the partial inhibition of the ATPase activity, (Formula: see text). Application of BCP anions was discussed for use as a probe for Mg . E-P in the steady-state ATP hydrolysis.[1]References
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