Reversible Changes in the Activity of Inorganic Pyrophosphatase of Streptococcus faecalis. The Effect of Compounds Containing SH-Groups.

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Abstract
The activity of inorganic pyrophosphatase (EC 3.6.1.1) from S. faecalis ATCC 8043 decreased rapidly when the cell extracts prepared by lysozyme and osmotic shock were incubated in a buffer solution (pH 8.0) at 25-50.degree. C. An increase in pH (from 8.5 to 10.5) retarded the inactivation considerably. The low activity level was reached in 2.5 h at 37.degree. C (pH 8.0) and this residual activity was stable during prolonged incubation. The enzyme in the low activity form was not rapidly denatured at temperatures < 70.degree. C. The inactivation could be prevented and reversed by cysteine, dithiotreitol and 2-mercaptoethanol, but reductants which did not contain SH-groups were not effective. Reduced glutathione slowed down and oxidized glutathione stimulated the loss of activity. SH inhibitors, such as p-hydroxymercuribenzoate and N-ethylmaleimide, stimulated the inactivation. With pure enzyme, the low, stable activity level obtained with crude extracts during incubation is a property of inorganic pyrophosphatase, and not due to another enzyme with a low capacity to hydrolyze PP1. Inorganic pyrophosphatase in S. faecalis apparently exists in 2 interconvertible forms which differ in activity.