Cathepsin B, D and H activities in muscles of chicks of fast and slow growing strains: effect of age and diet

1. Activities of cathepsins B, D and H were measured in leg and breast muscles of fast growing (broiler) and slow growing (layer) chicks at eight time intervals between 1 and 29 days of age. 2. These enzyme activities were also measured in muscles from fast and slow growing chicks given a low protein (125 g/kg crude protein) diet between the ages of 17 and 24 days. 3. Activities of none of these cathepsins differed greatly between muscle type or strain of chick. However in both strains of chick cathepsin D and H in muscles significantly decreased with increasing age (muscle size) of the chick. Cathepsin D activity also increased when muscle proteolytic rates were increased by feeding a low protein diet. This latter effect was significant only in the muscles of fast growing chicks. 4. The results suggest that lysosomal proteases are not responsible for the differences in muscle protein degradation and growth between fast and slow growing strains of chicks, or between muscle types in the chick.