The effect of pH on enzyme leakage from isolated mouse heart

A series of tris-maleate-buffered salt solutions, from pH 6.4 to 8.9 was used to assess the effect of pH on the two hour leakage of creatine kinase (CK) from isolated mouse heart. Leakage was directly related to pH. Hourly enzyme leakage was minimal (less than 0.25%) at pH 6.4, but from pH 6.4 to 8.9, it increased more than 10 fold. The pattern of fractional leakage in the second hour, relative to pH, was approximately sigmoidal in shape, and similar to a monoprotic titration curve. This observation suggests that a single proton binding site may govern myocardial enzyme leakage, under the conditions of this study. Similar observations were made using salt solutions, buffered with tris-HCl, from pH 7.3 to 8.6. The functional group of this putative protective site has a pKa of approximately 7.9, and properties consistent with a nitrogen base of the type found in terminal amino, epsilon-amino, and/or imidazolium groups.