Phosphoenzyme decomposition in sarcoplasmic reticulum isolated from cat caudofemoralis, tibialis and soleus

Decomposition of phosphoenzyme (E approximately P) in sarcoplasmic reticulum isolated from caudofemoralis, tibialis and soleus of cat hind leg skeletal muscles was studied under various conditions of monovalent cations. In the presence of Li+, Na+, and K+ chosen for E approximately P formation and decomposition after quenching of E approximately P with EGTA, E approximately P in the caudofemoralis and tibialis sarcoplasmic reticulum decomposed faster than that in the soleus sarcoplasmic reticulum. Quenching the E approximately P formation with EGTA and ADP revealed that 30-40% of the total E approximately P formed in all types of sarcoplasmic reticulum was 'ADP sensitive'. Decomposition of the remaining E approximately P in caudofemoralis and tibialis sarcoplasmic reticulum was enhanced by ADP, which resulted in a multiphasic decomposition pattern. A larger portion of the remaining E approximately P in the soleus sarcoplasmic reticulum, on the other hand decomposed in a monophasic manner and was not significantly influenced by ADP. The data on E approximately P decomposition clearly differentiate between the fast and slow muscle types.