Changes in the activity of skeletal muscle calcium-activated neutral proteinase (EC 3.4.22.17) and its specific inhibitor in chickens grown at different rates in response to graded levels of dietary protein.

1. Two experiments are reported in which the effect of alteration in growth rate on the levels of avian skeletal muscle calcium-activated neutral proteinase (EC 3.4.22.17) (CANP or calpain) and its specific inhibitor (calpastatin), a system thought to be implicated in myofibrillar catabolism, was studied by means of manipulation of dietary protein concentration. 2. In Expt 1 broiler chicks were given free access to diets containing 105, 149, 197 and 212 g protein/kg for 20 d. In Expt 2 the four dietary treatments were 119, 141, 182 and 227 g protein/kg diet given for 16 d. Chick growth rate and total leg skeletal muscle weight significantly increased (P less than 0.001) with increasing dietary protein concentration in both experiments. Total skeletal muscle protein increased with the level of dietary protein, the effect being significant (P less than 0.01 and P less than 0.001 in Expts 1 and 2 respectively). 3. Minced leg muscle was homogenized in low-salt buffers, and the extract chromatographed on DEAE-cellulose to separate proteinase and inhibitor activity. The partially purified CANP enzyme and inhibitor proteins were present at a concentration broadly consistent with literature reports, and their elution characteristics and Ca2+ concentration dependence were not varied by dietary protein concentration. 4. Both the muscle CANP and CANP inhibitor activities (units/kg muscle) exhibited upward trends with growth rate and increased muscle weight. However, these differences were not statistically significant (P greater than 0.05) and were not present at all when the results were expressed as units/g muscle protein.