Protease activity of rat gastrocnemius myofibrillar fraction, optimally active at
the alkaline pH range, was investigated. The activity was inhibited by soybean trypsin
inhibitor, tranexamic acid and other inhibitors of proteases, as well as by 2- to
lo-mm solutions of FeCl? , FeCL , and ZnCl~ . CaC12 , MgC12 , and MnCls at the same
concentration range did not affect the activity of the enzyme. Treating the myofibrillar
fraction with 1 M KCNS or KC1 solubilized the enzyme. The protease activity
was increased after 4 days of fasting, and on Day 6 was five times higher than
in nonfasted rats. This increase in specific activity was significant when expressed in
terms of total protein, noncollagen protein, or muscle DNA content. Prot’ease activity
also increased in rats after glucocorticoid administration, in rats with alloxan
diabetes, in aminonucleoside-nephrotic rats, and in rats with hypoproteinemia induced
by plasmapheresis. These findings suggest that the activity of the myofibrillar
protease is adaptive, and increases whenever catabolism of muscle proteins and
mobilization of amino acids to the liver is required.