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Abstract

The rate of ATP-splitting in the sarcosomes and small granules of rabbit muscle is compared with that in the granules of other tissues. 2. Neither in the small granules nor in the sarcosomes does a phosphate exchange between ATP and inorganic phosphate take place. In the presence of Mg2+, however, in both kinds of granules the phosphate exchange between ATP and ADP is 12 times as rapid as in the granules of other tissues. Without Mg2+ the rate of exchange is still considerably higher than the maximum speed in other tissues. Only in the presence of Ca2+ does the rate of exchange drop to the values similar to those known sofar. 3. The exchange rate in sarcosomes is quantitatively identical with the exchange rate in the small granules of rabbit muscle under all conditions of activation, inhibition, and poisoning. 4. Neither in the sarcosomes nor in the small granules is a latent ATPase revealed by dinitrophenol. On the other hand, the ATPase activity of the sarcosomes and small granules increases by aging and heating. 5. In the presence of Mg2+ the ATPase activity of sarcosomes is 2.5 times as great as the ATPase activity of the small granules. In the absence of Mg2+ and in the presence of Ca2+ the ATPase activity of both kinds of granules is identical. 6. Both kinds of granules split ATP optimally and exchange phosphate optimally at pH 7.7. Exchange and splitting are more rapid at an ionic strength of 0.4 μ than at an ionic strength of 0.1 μ. 7. With rising concentration of free Mg2+ the ATPase activity and the phosphate exchange increase steeply. In an excess of Mg2+, however, the ATPase activity decreases, while the phosphate exchange does not decrease. 8. With Ca2+ the ATPase is less activated than with Mg2+. The activation is preserved in an excess of Ca2+. On the other hand, the phosphate exchange continuously decreases with rising Ca2+ concentration up to the highest investigated values (10−2M). In Ca2+Mg2+ mixtures exchange and splitting are determined chiefly by the Ca2+ ions. Only when the Mg2+ proportion is very high, the influence of Mg2+ prevails. Hence, there is no summation of Ca2+ and Mg2+ activities.