Biophysical study of the mode of action of the tetracycline antibiotics. Inhibition of metalloflavoenzyme NADH cytochrome oxidoreductase

A considerable amount of evidence indicates that the tetracyclines may produce their antibiotic action by inhibition of metalloflavoenzymes. In this investigation the inhibitory properties of a series of biologically active and inactive tetracycline analogs on the metalloflavoenzyme, NADH–cytochrome c oxidoreductase, were investigated. Those analogs capable of inhibiting the enzyme appeared to act, at least partially, by chelation of enzyme‐bound metal. The site of chelation on the tetracycline molecule appeared to be the C‐11 to C‐12 chromophore, or the C‐1, C‐2, C‐3 region involving the 2‐carboxamide group of the tetracycline molecule. Both modification of the 2‐carboxamide substituent and epimerization of the 4‐dimethyl‐amino group resulted in loss of inhibitory effect. This parallels known structure‐antibiotic activity relationships for these compounds. Inactive isotetracycline, tetracycline methiodide, and dedimethylaminotetracycline were able to inhibit the enzyme at least as well as the parent tetracyclines. These compounds might lack antibiotic activity due to inability to enter bacterial cells.