Endogenous proteolytic enzymes in chicken muscles. Differences among strains with different growth rates and protein efficiencies.

The theory that net muscle growth is, at least partly, regulated by catabolic factors has been tested in order to set up an animal model to study meat aging and post-mortem tenderization. Male and female chickens of a layer strain (White Leghorn), a commercial broiler strain (Ross), and two experimental broiler lines (designated GL and FC) were used to estimate differences in proteolytic enzyme activities in the breast muscles. The GL and the FC lines were selected for high body weight gain and high feed efficiency, respectively. At 6 wk of age the birds were slaughtered and the activities of endogenous proteinases and their specific inhibitors in breast muscles measured. The Leghorns showed significant differences in all traits compared with the three broiler genotypes. Within the broiler types, FC birds tended in the direction of the Leghorns and GL birds in the opposite direction. Ross birds were intermediate between FC and GL birds. All types and sexes differed significantly in slaughtering weight. Feed conversion ratio and protein conversion ratio were highest for Leghorns. The FC birds showed the lowest feed conversion. Ross and GL birds showed intermediate values. The Leghorns showed higher calpain activities and lower calpastatin activity than the three broiler genotypes. The FC broilers showed intermediate calpain and calpastatin activities but higher cathepsin H and total cystatin values. The GL broilers showed lower cathepsin B, D, and H activities. In all cases the Ross broilers showed intermediate values. From these figures it is concluded that the strains of birds used in this study can be used as a natural source of variability to study the mechanisms involved in post-mortem proteolytic degradation and thus in the study of muscle tenderization and meat aging. It is also concluded that it could be very interesting to study the behavior of the different proteolytic systems more carefully in relation to muscular growth characteristics and compare them to anabolic factors involved in muscle growth.