Comparison of the protein-bound and free amino acid contents of two northern adapted soybean cultivars

The total protein and the protein-bound and free amino acid contents of two northern adapted soybean cultivars, Maple Arrow and AC Proteus, were compared as potentially useful and practical indices for evaluating their protein quality. As the content of total protein was increased by 10.1% in AC Proteus by breeding, the concentration of protein-bound arginine, aspartic acid, and histidine increased, while the levels of threonine, tryptophan, and methionine decreased. The free amino acid content of AC Proteus increased from 2.4 to 3.7% of the dry weight, and glutamic and aspartic acids and arginine represented 52.8% of the total. Both cultivars contained an excellent balance of essential amino acids (EAA) limited only in methionine, followed by tryptophan. Compared to the FAO/WHO reference value of 33.9%, mean values for total EAA ranged from 46.1 to 46.5%, and both cultivars had mean protein efficiency ratio values of 2.7. The 4-hydroxyproline-rich glycoproteins found in the extracellular matrices of soybean seeds ranged from 2.12 to 2.36 g/kg of total protein in Maple Arrow and AC Proteus, respectively