Methionine reversal of 2-chloro-4-aminobenzoic acid inhibition in Escherichia coli.

During recent years many experiments have demonstrated the usefulness of antimetabolites for studying routes of metabolism. Competitive inhibitors have been used to block specific enzyme systems and to establish enzyme-substrate relationships, and the compounds produced by the action of these enzyme systems have been characterized through their ability to enhance the reversal of the inhibitors by the metabolites in question. When a given product can replace the original metabolite altogether as a reversing agent, it is often assumed to be the only product formed from the metabolite or enzyme system involved (1). However, confirmation of this type of experiment by more direct means, i.e. by showing that in the presence of an inhibitor an organism can use only the exogenous metabolite for growth, has thus far been lacking. An experiment was therefore devised to settle this point. It has been reported that 2-chloro-4-aminobenzoic acid (CAB) inhibition in Escherichia COG can be reversed by p-aminobenzoic acid (PAB), methionine, or pantothenic acid (2-4). The action of PAB against CAB is competitive. Methionine can enhance the reversing power of PAB, or even replace it completely. Shive and Roberts (3) have thus regarded methionine as a product of an enzyme system which is inhibited by CAB. In an experiment designed to study the methionine-pantothenic acid relationship in E. co&i (which will be reported elsewhere), the methionineCAB relationship was also investigated. E. co&i cells were grown in the presence of CAB and an exogenous source of methionine containing Cl4 in the methyl group. The methionine in the cells after growth contained approximately the same specific radioactivity as that which was introduced into the basal medium. It was therefore concluded that synthesis of methionine was completely inhibited by CAB.