Recycling of NAD+ using coimmobilized alcohol dehydrogenase andE. coli

Abstract

The use of immobilized enzymes has opened the possibility of large scale utilization of NAD⁺-linked dehydrogenases, but the applications of this technique were limited by the necessity of providing the large amounts of NAD⁺ required by its stoichiometric consumption in the reaction. After immobilization of alcohol dehydrogenase and intactE. coli by glutaraldehyde in the presence of serum albumin, the respiratory chain was found to be capable of regenerating NAD⁺ from NADH. This NAD⁺ can be recycled at least 100 times, and thus the method is far more effective than any other, and, moreover, does not require NADH oxydase purification. The total NADH oxidase activity recovered was 10–30% of the initial activity. Although, NADH is unable to cross the cytoplasmic membrane, it was able to reach the active site of NADH dehydrogenase after immobilization. The best yield of NADH oxidase activity with immobilized bacteria was obtained without prior treatment of the bacteria to render them more permeable. The denaturation by heat of NADH oxidase in cells that are permeabilized was similar before and after immobilization. In contrast, the heat denaturation of soluble Β-galactosidase required either a higher temperature or a longer exposure after immobilization. The sensitivity of immobilized NADH oxidase to denaturation by methanol was decreased compared to permeabilized cells. As a result, it is clear that the system can function in the presence of methanol, which is necessary as a solvent for certain water insoluble substrates.