Characterization of thermostable alkaline protease from Bacillus halodurans SE5 and its application in degumming coupled with sericin hydrolysate production from yellow cocoon

Bacillus halodurans SE5 was newly isolated and grew well on a medium containing crude sericin extract from cocoon. Thermostable alkaline serine protease (protease_SE5) capable of decomposing sericin extract was purified to homogeneity from culture supernatant with a final yield of 25% and 2.01 x 10(4) U/mg. Among the six natural proteins tested, protease_SE5 showed the highest activity toward sericin. The sericin degumming, bio-bleaching coupled with sericin hydrolysate production from yellow cocoon by protease_SE5 and commercial Alcalase were demonstrated in the presence or absence of dithiothreitol (DTT). The addition of DTT enhanced the efficacy of both proteases. However, without DTT, the protease_SE5 had higher degumming ability and produced sericin hydrolysate 4-times higher than commercial enzyme based on the soluble protein concentration. SDS-PAGE and size exclusion chromatography analysis revealed that the maximal concentration of oli-gopeptides was observed with the hydrolysate prepared by protease_SE5 and showed higher antioxidant activity than those from Alcalase. The appreciable radical scavenging activities of the crude peptide (1.36 +/- 0.07 mM) on ABTS, DPPH, and FRAP assay were 1568 +/- 78, 3.6 +/- 1.6, and 13.6 +/- 0.4 mu mol TE/L, respectively. Protease_SE5 has potential application for one step degumming and preparation of bioactive peptides from yellow cocoon.