Effects of calcium on the stability and activity of guinea pig liver transglutaminase.

The binding of calcium to transglutaminase was studied by a kinetic method and by spectrophotometric titration. By the first method, we have shown that the binding of a single calcium per molecule, with a binding constant of 7500 +/- 1300 M-1 (at 55 degrees C), was responsible for the enhancement of the thermostability. The kinetic constants of the deactivation for the unliganded native form and the liganded native one are 1.47 +/- 0.04 min-1 and 0.32 +/- 0.05 min-1 respectively. The enhancement of thermostability is due to the stabilization of the native form, since the deactivation constants of the liganded and unliganded intermediate forms are equal. The total number of calcium binding sites, determined by titration is 4, and therefore, only 1 of them should be implicated in the thermostability. The 4 apparent association constants have been determined by a non-linear fitting of the data to the Adair equation. We have also shown a positive co-operative behaviour of the transglutaminase when the transferase is monitored versus calcium concentration.