Chapter 8, Problem 8.10PS

QUANTITATIVE The Calcium Pump of the Sarcoplasmic Reticulum. Muscle cells use calcium ions to regulate the contractile process. Calcium is both released and taken up by the sarcoplasmic reticulum (SR). Release of calcium from the SR activates muscle contraction, and ATP-driven calcium uptake causes the muscle cell to relax afterward. When muscle tissue is disrupted by homogenization, the SR forms small vesicles called microsomes that maintain their ability to take up calcium. To obtain the data shown in Figure 8-17, a reaction medium was prepared to contain 5 mM ATP and 0.1 M KCl at pH 7.5. An aliquot of SR microsomes containing 1.0 mg protein was added to 1 mL of the reaction mixture, followed by 0.4 mmol of calcium. Two minutes later, a calcium ionophore was added. (An ionophore is a substance that facilitates the movement of an ion across a membrane.) ATPase activity was monitored during the additions, with the results shown in the figure.

Figure 8-17 Calcium Uptake by the Sarcoplasmic Reticulum. See Problem 8-10.

1. (a) What is the ATPase activity, calculated as micromoles of ATP hydrolyzed per milligram of protein per minute?
2. (b) The ATPase is calcium-activated, as shown by the increase in ATP hydrolysis when the calcium was added and the decrease in hydrolysis when all the added calcium was taken up into the vesicles 1 minute after it was added. How many calcium ions are taken up for each ATP hydrolyzed?
3. (c) The final addition is an ionophore that carries calcium ions across membranes. Why does ATP hydrolysis begin again?