Evaluation of Five Partially Purified Proteins by Nitrogen Balance in Mature Dogs, including a Study of the Antitryptic Activity of Egg White

Five partially purified proteins which had been previously evaluated for efficiency in promoting the growth of cocker spaniel dogs were examined by the nitrogen balance method, with 6 young adult dogs of the same strain as subjects. At a 7% level in the diet the true digestibilities were: beef muscle 98, casein 97, whole egg 96, peanut flour 93, egg albumin 84. The biological values were: egg albumin 89, whole egg 86, casein 68, peanut flour 61, beef muscle 57. At a 40% level in the diet the true digestibility of the egg albumin, tested with two fully mature dogs, was found to be 54%. Since in the young dogs the corresponding value was 49%, no improvement with age in ability to denature the antitrypsin of egg white can be said to have occurred. Vigorous beating in a blender of egg white suspensions did not decrease their antitryptic activity, nor did heating the suspensions at 73° for 10 minutes. Dry heating at 80° or 100°C. for two to 18 hours produced insignificant changes in the antitryptic activity, but heating at 150° for one, two and 18 hours destroyed 59, 73 and 100% of this activity. Complete denaturation and partial decomposition of the anti-tryptic protein appeared ncessary for prevention of the inhibitory effect. The vulnerability of the dog to the antitryptic action of egg white is contrasted with the excellent utilization of this protein by young and adult rats and by adult humans. The low value of beef muscle protein for maintenance of adult dogs as compared with its superior value for growth may be due to changes in amino acid requirements from those preponderantly for muscle and organ growth to those for protein in hair and secretions.