Effect of L-azetidine 2-carboxylic acid on growth and proline metabolism in Escherichia coli

The effects of L-azetidine 2-carboxylic acid on growth and proline metabolism in a proline-requiring auxotroph of Escherichia coli are described. The homologue inhibited growth of the wild type and it, alone, did not substitute effectively for proline as a growth supplement for the mutant. In medium containing 0.05 mM proline, the addition of increasing amounts of homologue progressively inhibited growth of the wild type but stimulated growth of the mutant at homologue: proline ratios of 10 : 1 and 50 : 1. This suggested that the homologue exerted a “sparing effect” on proline in the mutant.The incorporation of L-[U-¹⁴C]proline and L-[³H]azetidine 2-carboxylic acid into hot trichloroacetic acid-insoluble material in the mutant was measured. Amino acid analysis of the insoluble material from cells incubated with radiolabeled proline alone revealed that proline was partially degraded and metabolized to other amino acids prior to incorporation into protein. The addition of unlabeled homologue to the incubation medium significantly reduced proline catabolism, suggesting that the homologue exerted a sparing effect on proline in this mutant. In medium containing unlabeled proline and radiolabeled L-azetidine 2-carboxylic acid, the homologuewas incorporated both intact and partially degraded prior to incorporation into protein. Alanine was the major L-azetidine 2-carboxylic acid catabolite.