Reexamination of the amino acid compositions of proteins from penaeid shrimp: failure to find uncommonly-high tryptophan levels

1. 1. A soluble fraction and an insoluble fraction were isolated from cuticle-free tail tissue of Penaeus vannamei and their amino acid compositions compared to analogous fractions previously isolated from Penaeus setiferus by Thompson & Thompson (Comp. Biochem. Physiol. 27, 127-132 (1968); 35, 471-477 (1970)) in which tryptophan contents of 119.3 and 190.4 residues/1000 were reported. 2. 2. The previously-reported tryptophan values are surprisingly high, because tryptophan is generally the least abundant amino acid in proteins. In this study, a maximum of 9.7 tryptophan residues/1000 were found. 3. 3. Extraction with 1 M NaCl followed by 0.5 M acetic acid solubilized 94% of the total protein but no hydroxyproline, indicating that shrimp collagen is uniformly insoluble, perhaps because of extensive cross-linking.