Incorporation of L-azetidine-2-carboxylic acid into hemoglobin in rabbit reticulocytes

L-Azetidine-2-car?ylic acid (AZA), the lower homologue of proline, was incorporated into hemoglobin in rabbit reticulocytes in vitro. Hydrolysis of [¹⁴C] AZA-hemolobin with 3 N p-toluenesulfonic acid and subsequent amino acid analysis resulted in recovery of 90% of the initial radioactivity in fractions containing added free AZA standard. An additional 6% of the radioactivity eluted with homoserine, a known degradation product of AZA. After tryptic digestion of [¹⁴C] AZA-hemoglobin, less than 9% of radioactivity eluted near added free AZA standard, while 91% of radioactivity was located in tryptic peptides elsewhere on the chromatogram. These data provide the first demonstration of AZA incorporation into a mammalian protein.