Oxygen‐Induced Inactivation of NADH‐Oxidase in Lyophilized Cells of Escherichia Coli

NADH‐oxidase of Escherichia coliwas inactivated when freeze‐dried cells were exposed to oxygen (air). A similar loss in the activity of the enzyme occurred also when a cell‐free extract was lyophilized and then exposed to oxygen. Inactivation of the enzyme was accompanied by liberation of a substance showing flavin‐like fluorescence. Addition of KNO2to the medium before lyophilization suppressed the adverse effect of oxygen both on the viability of the bacteria and on the NADH‐oxidase activity. The possible significance of these findings is discussed.