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Inactivation of purified   β  -Galactosidase was done with GdnHCl in the absence and presence of varying [galactose] at 50°C and at pH 4.5. Lineweaver-Burk plots of initial velocity data, in the presence and absence of guanidine hydrochloride (GdnHCl) and galactose, were used to determine the relevant  K    m   and  V   max  values, with p-nitrophenyl   β  -D-galactopyranoside (pNPG) as substrate,  S . Plots of ln([ P ]  ∞   − [ P ]  t  ) against time in the presence of GdnHCl yielded the inactivation rate constant,  A . Plots of  A  versus [ S ] at different galactose concentrations were straight lines that became increasingly less steep as the [galactose] increased, showing that  A  was dependent on [ S ]. Slopes and intercepts of the 1/[ P ]  ∞   versus 1/[ S ] yielded  k   +0  and  k'   +0 , the microscopic rate constants for the free enzyme and the enzyme-substrate complex, respectively. Plots of  k   +0  and  k'   +0  versus [galactose] showed that galactose protected the free enzyme as well as the enzyme-substrate complex (only at the lowest and highest [galactose]) against GdnHCl inactivation. In the absence of galactose, GdnHCl exhibited some degree of non-competitive inhibition. In the presence of GdnHCl, galactose exhibited competitive inhibition at the lower [galactose] of 5 mM which changed to non-competitive as the [galactose] increased. The implications of our findings are further discussed.

Kinetic Analysis of Guanidine Hydrochloride Inactivation of β-Galactosidase in the Presence of Galactose