Comparative study on the proteases from fish pyloric caeca and the use for production of gelatin hydrolysate with antioxidative activity

Proteases from pyloric caeca extract of three fish species including brownstripe red snapper (Lutjanus vitta), bigeye snapper (Priacanthus tayenus) and threadfin bream (Nemipterus marginatus) were comparatively studied. The extracts from bigeye snapper and threadfin bream exhibited the highest hydrolytic activities toward casein, α-N-benzoyl-dl-arginine-p-nitroanilide and α-N-ρ-tosyl-l-arginine methyl ester at pH 8.0 and 60 °C and pH 8.5 and 55 °C, respectively. The extract of brownstripe red snapper showed the optimal pH and temperature of 8.0 and 60 °C with all substrates used except the optimal temperature was 65 °C when casein was used. All proteases were strongly inhibited by soybean trypsin inhibitor (SBTI) and N-ρ-tosyl-l-lysine chloromethylketone (TLCK) and partially inhibited by N-tosyl-l-phenylalanine chloromethylketone for all substrates tested, suggesting that trypsin-like proteases were the major enzymes. Substrate-gel activity staining of 40–60% ammonium sulfate (AS) fraction revealed that major activity bands were observed with molecular mass of 24, 22 and 20 kDa for brownstripe red snapper, bigeye snapper and threadfin bream, respectively. Those activity bands were partially inhibited by SBTI and TLCK. AS fraction was further used to produce gelatin hydrolysate from the skin of brownstripe red snapper with different degrees of hydrolysis (DH). Hydrolysate with DH of 15% exhibited the highest DPPH and ABTS radical scavenging activities and ferric reducing antioxidant power (p < 0.05). Therefore, the extract from pyloric caeca could be used to produce the gelatin hydrolysates possessing antioxidative activities.