Rat myofibrillar protease: Enzyme properties and adaptive changes in conditions of muscle protein degradation

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 10-m m solutions of FeCl 2 , FeCl 3 , and ZnCl 2 . CaCl 2 , MgCl 2 , and MnCl 2 at the same concentration range did not affect the activity of the enzyme. Treating the myofibrillar fraction with 1 m KCNS or KCl 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. Protease 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.