Abstract: The objective of this work was to explore the relationship between changes in the activity and fluorescence intensity of neutral lactase induced by ultrahigh pressure treatment. Lactase was treated at different pressure conditions ((0.1, 100, 200, 300, 400, 500 and 600 MPa), at different temperatures (18, 20, 25, 30, 35, 40 and 45 ℃) and for different durations (10, 20, 30, 40, 50 and 60 min)), and its activity was detected by o-nitrophenyl-β-D-galactoside (ONPG) assay. The changes in intrinsic and extrinsic fluorescence intensity of lactase were detected by fluorescence assay, and the correlation between lactase activity and fluorescence intensity was analyzed by Pearson two-tail assay. The results showed that the activity of lactase treated at different pressures and 30 ℃ for 10 min increased with pressure up to 400 MPa and then decreased. The maximum value increased by 13.30% compared with the control group (0.1 MPa). Additionally, the intrinsic and extrinsic fluorescence intensity of lactase were altered, both reaching their maximum values at 400 MPa, and they were significantly correlated with the enzyme activity (P < 0.01). In the case of treatment at 300 MPa and 30 ℃, lactase activity first increased and then decreased with increasing treatment time in the range of 10–60 min. After 20 min, the enzyme activity reached its maximum value, showing a 14.83% increase compared with the control group (0 min). In addition, the intrinsic and extrinsic fluorescence intensity of lactase were changed and reached their maximum values after 20 min, showing a significant (P < 0.01) correlation with the enzyme activity. In the case of 10 min treatment at 300 MPa, the enzyme activity showed an upward and then downward trend with the increase in temperature, and reached its maximum value at 30 ℃, 7.46% higher than that of the control group (18 ℃). At temperatures in the range of 30–45 ℃, the enzyme activity was still higher than that in the control group. Under this condition, the intrinsic and extrinsic fluorescence intensity of lactase were changed, and they were significantly correlated with the enzyme activity (P < 0.01). Therefore, there was a significant correlation between changes in lactase activity and fluorescence intensity after ultrahigh pressure treatment under different conditions (P < 0.01), that is, the change of lactase activity was related to the tertiary structure.

Key words: neutral lactase; ultrahigh pressure treatment; enzyme activity; fluorescence intensity