© 2005 The Japanese Biochemical Society
Regular Paper |
Kinetic Analysis about the Effects of Neutral Salts on the Thermal Stability of Yeast Alcohol Dehydrogenase
Protein Research Laboratory, Pharmaceutical Research Division, Mitsubishi Pharma Corporation, 2-25-1 Shodai-Ohtani, Hirakata, Osaka 573-1153
* For correspondence: Tel: +81-72-856-9262, Fax: +81-72-864-2341, E-mail: Ikegaya.Kazuo{at}mk.m-pharma.co.jp
ABSTRACT
The effects of salts on the rate constants of inactivation by heat of yeast alcohol dehydrogenase (YADH) at 60.0°C were measured. Different effects were observed at low and high salt concentrations. At high concentrations, some salts had stabilizing effects, while others were destabilizing. The effects of salts in the high concentration range examined can be described as follows: (decreased thermal stability) NaClO4 < NaI = (C2H5)4NBr < NH4Br < NaBr = KBr = CsBr = (no addition) < (CH3)4NBr < KCl < KF < Na2SO4 (increased thermal stability). The decreasing effect of NaClO4 on YADH controlled the thermal stability of the enzyme absolutely and was not compensated by the addition of Na2SO4, a salt which stabilized the enzyme. However, Na2SO4 compensation did occur in response to the decrease in thermal stability caused by (C2H5)4NBr. The rate constants of inactivation by heat (k
in) of the enzyme were measured at various temperatures. Effective values of the thermodynamic activation parameters of thermal inactivation, activation of free energy (
G
), activation enthalpy (H
), and activation entropy (S
), were determined. The thermal stability of YADH in 0.8 M Na2SO4 increased more than that of pyruvate kinase from Bacillus stearothermophilus, a moderate thermophile. The changes in the values of H
and S
were great and showed a general compensatory tendency, with the exception of in the case of NaClO4. The temperature for the general compensation effect (T
c) was approximately 123°C. With Na2SO4, the thermal stability of YADH at a temperature below T
c was greater than that in the absence of salt due to the higher values of H
and S
, respectively, and thus was an example of low-temperature enzymatic stabilization. With (C2H5)4NBr, the thermal stability of YADH at a temperature below T
c was lower than that in the absence of salt due to the lower values of H
and S
, respectively, and thus was an example of low-temperature enzymatic destabilization. But with NaClO4, the changes in the values of H
and S
were small and the thermal stability of YADH was thus an example of high-temperature enzymatic destabilization.