Table 3. Free sulfhydryl concentration of Kl-β-Gal and its mutants in non-reduced and reduced conditions determined by the DTNB method.

	Free sulfhydryl groups concentration (μmol/g protein)
	Non-reduced conditions	Reduced conditions
Kl-β-Gal	0.0588 ± 0.0199	0.0593 ± 0.0038
R116C/T270C	0.0897 ± 0,0046	0.1087 ± 0.0014*
R116C/T270C/G818C	0.0862 ± 0.0099	0.1250 ± 0.0054**

The ± sign refers to standard error. *Significance p-value with non-reduce condition <0.05; **significance p-value with non-reduce condition <0.01.