Some catalytic and molecular properties of threonine deaminase from Bacillus stearothermophilus

Threonine deaminase [EC 4.2.1.16] was highly purified from Bacillus stearothermophilus. The enzyme exhibited maximum activity at 65 degrees and at pH 9.2--9.6. It was inactivated on dilution and on storage at 4 degrees, but was protected by egg albumin. The enzyme was labile at 65 degrees, but became stable in the presence of egg albumin and isoleucine at pH 7.0. The substrate saturation curve for the enzyme reaction at 40 or 65 degrees was hyperbolic, but in the presence of isoleucine, the curve became sigmoidal (n = 2). The enzyme was more sensitive to isoleucine at 40 degrees than at 65 degrees, while valine slightly inhibited the enzyme at both 40 and 65 degrees. Inhibition of the enzyme by isoleucine was antagonized by valine at 40 and 65 degrees. These properties were essentially similar to those of the enzymes from mesophilic and thermophilic bacteria. The enzyme existed in two forms with different molecular sizes, 1.5-5 X 10(6) and 2 X 10(5) daltons, at pH 7.0 and at temperatures below 40 degrees. The larger component disaggregated into the small one at pH 8.5 or above, at temperatures above 50 degrees or in the presence of isoleucine and valine.