Activation of skeletal muscle protein breakdown following consumption of soyabean protein in pigs

Diets with protein of inferior quality may increase protein breakdown in sk eletal muscle but the experimental results are inconsistent. To elucidate t he relationship, pigs were fed isoenergetic and isonitrogenous diets based on soyabean-protein isolate or casein for 15 weeks, with four to six animal s per group. A higher plasma level of urea (2.5-fold the casein group value , P = 0.01), higher urinary N excretion (2.1-fold the casein group value, P = 0.01), a postabsorptive rise in the plasma levels of urea, 3-methylhisti dine and isoleucine in soyabean protein-fed pigs suggested recruitment of c irculatory amino acids by protein breakdown in peripheral tissues. Signific ant differences between dietary groups were detected in lysosomal and ATP-d ependent proteolytic activities in the semimembranosus muscle of food-depri ved pigs. A higher concentration of cathepsin B protein was found, correspo nding to a rise in the cathepsin B activity, in response to dietary soyabea n protein. Muscle ATP-stimulated proteolytical activity was 1.6-fold the ca sein group value (P = 0.03). A transient rise in the level of cortisol (2.9 -times the casein group value, P = 0.02) occurred in the postprandial phase only in the soyabean group. These data suggest that the inferior quality o f dietary soyabean protein induces hormonally-mediated upregulation of musc le protein breakdown for recruitment of circulatory amino acids in a postab sorptive state.