Precursors of nicotinic acid in Escherichia coli.

It has now been firmly established that mammals and Neurospora are able to convert tryptophan to nicotinic acid via a biosynthetic pathway which contains several well characterized intermediate compounds (2). However, evidence has been accumulating which indicates that in bacteria the biosynthesis of nicotinic acid is accomplished by a different pathway. Volcani and Snell (3) demonstrated that kynurenine and 3-hydroxyanthranilic acid, both of which are intermediates in the “tryptophan pathway,” could neither replace nicotinic acid as a growth factor for a number of nicotinic acid-requiring species of bacteria nor enhance the growth response of these bacteria to suboptimal amounts of nicotinic acid. Stanier and Tsuchida (4) found that a Pseudomonas species adapted to grow with tryptophan as the sole carbon source lacked the enzyme needed for the oxidation of 3-hydroxyanthranilic acid and thus appears to be unable to form nicotinic acid from tryptophan. Yanofsky (5) reported that a series of nicotinic acid auxotrophs of Escherichia coli and Bacillus subtilis could utilize neither tryptophan nor any of the intermediates in the tryptophan pathway as a replacement for nicotinic acid as a growth factor. ,41so, these bacteria appeared to lack the enzyme which splits 3-hydroxykynurenine to 3-hydroxyanthranilic acid and is essential to the operation of the tryptophan pathway for nicotinic acid synthesis. Furthermore, it was demonstrated that when Cl*-tryptophan was supplied to tryptophan auxotrophs of E. coli and B. subtilis the nicotinic acid which was formed by the growing cells was not radioactive (5). These results clearly ruled out tryptophan as a precursor of nicotinic acid in E. coli and B. subtilis. The present investigations were undertaken in an attempt to obtain evidence for the identity of the precursors of nicotinic acid in bacteria. The results to be presented suggest that the probable precursors of this vitamin in E. coli are a 4-carbon dicarboxylic acid and either glycerol or a compound to which glycerol can be easily metabolized. The strongest evidence for this conclusion is provided by the finding that both Cl*-succinic acid and Ci4-glycerol are efficiently incorporated into the nicotinic acid molecule.