KINETIC STABILIZATION OF KLUYVEROMYCES-MARXIANUS BETA-GALACTOSIDASE BY HISTIDINE AND OTHER AMINO-ACIDS

Kluyveromyces marxianus beta-galactosidase was purified from a commerc ial preparation ('Lact Aid') and its kinetic stability determined at 4 5-degrees-C. All amino acids (except proline) stabilized the enzyme to some degree, but histidine was the most effective. Histidine (0.1 mM) stabilized the enzyme 9-fold by itself and over 40-fold in the presen ce of 5% lactose, which, by itself, was not a stabilizer. Stability in creased with increasing concentrations of histidine up to 10 mM. Lacto se, galactose and sucrose increased stabilization by histidine, but ma ltose and glucose did not. Histidine had no effect on the K(m) for lac tose and slightly reduced V(max); it did not affect the binding consta nt for Mg2+. Increases in ionic strength reduced the effect of histidi ne. The use of histidine analogues and derivatives showed that the alp ha-amino group and N-1 ring nitrogen were required for stabilization. The alpha-carboxyl group was not required, but may determine the exten t of stabilization.