D-Amino acids in animal peptides

Secreted peptides from diverse sources have been found to contain a D-amino acid. From the sequence of cloned mRNAs coding for the precursors of such peptides it could be deduced that in all cases tested so far the D-amino acid in the final product is derived from the corresponding L-amino acid present in the primary product of translation. Enzymes catalyzing such an L- to D-isomerization in peptide linkage have been isolated from the venom of a spider and the skin secretions of frogs. Even though these are completely different proteins, the reaction mechanism is the same, namely a de-protonation/re-protonation of the alpha-carbon of an amino acid with concomitant inversion of the chirality. Sequences potentially coding for homologues of the frog enzyme are present in the genome of different vertebrate species.