The action of a muscle proteinase on the myofibrillar proteins of bovine muscle

Myofibrillar proteins from bovine muscle have been treated with a Ca 2+ activated muscle proteinase and the consequent changes in these proteins have been examined by various techniques. Tropomyosin, α‐actinin and troponin were substrates for the enzyme, the last losing its property of inhibiting actomyosin ATPase in the absence of Ca 2+ ions. Actin and actomyosin were apparently not digested but the Mg 2+ ‐activated ATPase activity of actomyosin was less after treatment whereas the Ca 2+ ‐activated ATPase was unaffected. It is suggested that the observed destruction of the Z‐bands of the myofibrils by this proteinase is due to its digestion of the α‐actinin, rather than the actin component.