Extraction of sulfated polysaccharide from Skipjack tuna (Katsuwonus pelamis) viscera using alcalase enzyme and rainbow trout (Oncorhynchus mykiss) visceral semi-purified alkaline proteases

Fisheries produce a considerable amount of by-products per ton of processed fish, which ranges from 350 to 600 kg. Currently, most of these by-products are used as fish and shrimp meal or as fertilizer, even though they contain significant amounts of quality proteins (enzymes), polysaccharides, and polyunsaturated fats. The present study deals with the extraction of crude alkaline proteases (AP) from rainbow trout (Oncorhynchus mykiss) viscera. The enzyme precipitated at 30–60% saturation displayed the highest yield, purity, and activity compared to other saturation ranges. The maximum activity of AP was obtained at 60 °C and pH 7. AP, ultrasound-AP, Alcalase, and ultrasound-Alcalase were used to isolate sulfated polysaccharides (SPs) from Skipjack tuna (Katsuwonus pelamis) viscera. The extraction yields showed no significant difference between all extracted SPs (p > 0.05). APU-Viscera-SP, AlU-Viscera-SP, and Al-Viscera-SP contained the most sugar (62.22%), sulfate (15.93%), uronic acid (8.84%), and protein (12.69%), respectively. Also, all SPs showed a mixed composition of monosaccharides, including xylose, mannose, rhamnose, GalA (galacturonic acid) and glucuronic acid (GlcA). AP-Viscera-SP, AlU-Viscera-SP, and AlU-Viscera-SP exhibited the highest antioxidant activity in DPPH, ABTS, and metal chelating tests. The highest inhibitor zone against L. monocytogenes, S. aureus, B. cereus, S. enterica, E. coli, and S. typhimurium were in APU-Viscera-SP, and AlU-Viscera-SP, respectively. Overall, the obtained results demonstrated that fish viscera could be a good source of alkaline protease. Besides, this obtained alkaline protease can also be used to extract sulfated polysaccharides with bioactive and functional properties from fish viscera byproducts.