Sources of amino acids for animals Dietary proteins are digested into amino acids in the gastrointestinal〔胃 i)tract via the action of pepsin, trypsin, chymotrypsin, carboxypeptidases and aminopeptidasesDietary proteins are digested into amino acids in the gastrointestinal(胃 肠) tract via the action of pepsin, trypsin, chymotrypsin, carboxypeptidases and aminopeptidases. Sources of amino acids for animals