β-Galactosidase assay
The yeast colonies were suspended in SD/-Met/-Leu/-Trp/-His with different concentrations of methionine and cultured at 30 ℃ with 200 rpm shaking until the OD600 reached 0.8. After centrifuging, the precipitate was dissolved in 1.5 ml z-buffer, centrifuged again, and the resulting cell pellet finally re-suspended in 300 μL z-buffer. Liquid nitrogen was used to break down the yeast cell walls, and the solution was incubated at 37 ℃ with added 700 μL z-buffer and 160 μL o-nitropheyl-β-D-galactopyranoside (OPNG). 400 μL stop buffer (100 mM Na2CO3) was added once the reaction had clearly turned yellow. The average β-galactosidase activity was determined based on the wavelength of the supernatant was at OD420.