Surface denaturation of proteins: the thermal inactivation of β-galactosidase (Escherichia coli) on wall–liquid surfaces

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Abstract
Irreversible inactivation of dilute β-galactosidase (Escherichia coli) at relatively low temperatures was found to occur as a result of interactions of β-galactosidase with wall–liquid surfaces. The rate of inactivation was directly proportional to the wall–liquid surface area, but independent of the air–liquid surface area, and the rate was also dependent on the wall composition. A small portion of the β-galactosidase molecules was found to bind strongly to the surfaces of vessels in which the β-galactosidase was stored. Bovine serum albumin eliminated the inactivation and it also eliminated the binding of β-galactosidase to the wall. On the other hand, EDTA eliminated the inactivation, but it did not decrease the amount of β-galactosidase bound. The addition of some transition metals increased the rate of inactivation. Protection of β-galactosidase from surface inactivation by EDTA is not, therefore, a result of decreased binding of the enzyme to the walls of the vessels, but is probably a result of the ability of EDTA to scavenge certain trace metal ions present in solution, which are needed for the inactivation. The content of protein in the solution did not change as a result of the inactivation and, thus, the inactive enzyme does not accumulate at the surface. Since β-galactosidase is often stored for long periods of time and since it is used to decrease the lactose content of milk for lactose intolerant individuals, this study may have practical significance. The presence of metal chelators and extraneous proteins should improve the stability of the enzyme, especially for processes that are carried out at elevated temperatures.Key words: β-galactosidase, surface inactivation, denaturation, protein binding.

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