Biosynthesis of a D-amino acid in peptide linkage by an enzyme from frog skin secretions

d-amino acids are present in some peptides from amphibian skin. These residues are derived from the corresponding l-amino acids present in the respective precursors. From skin secretions of Bombinae, we have isolated an enzyme that catalyzes the isomerization of an l-Ile in position 2 of a model peptide to d-allo-Ile. In the course of this reaction, which proceeds without the addition of a cofactor, radioactivity from tritiated water is incorporated into the second position of the product. The amino acid sequence of this isomerase could be deduced from cloned cDNA and genomic DNA. After expression of this cDNA in oocytes of Xenopus laevis, isomerase activity could be detected. Polypeptides related to the frog skin enzyme are present in several vertebrate species, including humans.