Studies on Lipoamide Dehydrogenase of Bakers' Yeast

IV. Inactivation by Metal Ions in the Presence of NADH₂

抄録

The inhibition of NADH₂-lipoamide and NADH₂-menadione oxidoreductase activities of enzyme preincubated with NADH2 in the absence of EDTA is found to be due to metal ions contaminating the buffer solution. The activities of the inactivated enzyme are restored by reoxidation of the enzyme in air.
From the inhibitory behaviors of various added metal ions on enzyme activity, these ions could be divided into two groups. Group I ions (Mn⁺⁺, Fe⁺⁺ and Co⁺⁺) strong-ly inhibit the activity of the enzyme when the latter is preincubated with them in the presence of NADH₂. But, when the NADH₂ in the preincubation mixture is exhausted by a slow reaction of the enzyme with 0₂, the enzyme activity is restored almost com-pletely to that of the native enzyme. The restoration of the activity of the enzyme after inactivation by Group II metal ions (Zn⁺⁺, Cd⁺⁺ and Cu⁺⁺), however, is very slow and incomplete because of the tight com-bination of these ions with the catalytic dithiol group of the enzyme.
The authors wish to thank Dr. G. Sunagawa, the director of this Laboratory, for his encouragement during the course of this investigation and Dr. M. Nakamura, the superintendent of Sankyo Tanashi Plant, for the supply of a large quantity of the enzyme source.