Steady state kinetics of the (Ca2+ + Mg2+)-dependent P-nitrophenylphosphatase activity of sarcoplasmic reticulum vessicles.

The true substrate of the (Ca2+ + Mg2+)-dependent p-nitrophenylphosphatase activity of sarcoplasmic reticulum vesicles is the Mg. p-nitrophenylphosphate complex. The activity displays a hyperbolic dependence for the substrate (Km = 1.51 mM). The Ca2+ dependence is sigmoidal, with a K0.5 of 0.52 microM, which is the same as the observed for the ATPase activity determined in similar experimental conditions. However, the value of the Hill coefficient (3.0) is about twice as that observed for ATP hydrolysis. Simultaneous measurement of the hydrolysis of p-nitrophenylphosphate and ATP or acetylphosphate showed that all three substrates share a common catalytic site, while ATP is the only one of them which interacts also with a regulatory site, what explains the complex dependence of the ATPase activity on the substrate. Permeabilization of the vesicles to Ca2+, or the addition of K+ to the assay media, do not activate the hydrolysis of p-nitrophenilphosphate, contrarily to what is observed when ATP is the substrate.