BIOLOGICAL ACTIVITIES OF BULLFROG SKIN PROTEIN HYDROLYSATES: EFFECT OF ULTRAFILTRATION AND IN VITRO GASTROINTESTINAL DIGESTION

Bullfrog skin protein hydrolysates were prepared with pepsin, Alcalase, Protamex, Flavourzyme, and Corolase H-pH. Their antioxidant properties, α-amylase, and ACE inhibitory activity were evaluated in the crude hydrolysates, UF fractionated hydrolysate (<1 kDa and >1 kDa), and after in vitro digestion. A model system was used to evaluate the effect of heat on the stability of Corolase H-pH hydrolysate in biscuits. Corolase H-pH and Alcalase hydrolysates exhibited the highest DPPH radical scavenging and ferric reducing activities. Corolase H-pH hydrolysate is related with low concentrations of glycine amino acid. Flavourzyme hydrolysate is correlated to glycine and proline and low antioxidant activity and HPe is related to low level alanine amino acid and low antioxidant activity. The highest ACE inhibitory activity was also recorded in the Corolase H-pH hydrolysate whereas the hydrolysates prepared with pepsin and Flavourzyme exhibited the highest α-amylase inhibitory activity. The peptides with MW < 1 kDa exhibited higher radical scavenging, α-amylase inhibitory, and ACE inhibitory activities than crude hydrolysates. In vitro digestion reduced the radical scavenging activity of all hydrolysates and led to a reduction of α-amylase inhibitory activity of hydrolysates obtained with pepsin and Alcalase, but it increased the activity of the others. In the case of ACE inhibition, in vitro digestion did not affect the activity of pepsin and Alcalase hydrolysates, reduced the activity of Corolase H-pH and Flavourzyme but increased that of Protamex hydrolysate.