Imino acids are related to amino acids, which contain both amino (-NH2) and carboxyl (-COOH) functional groups. Amino acids containing a secondary amine group (the only proteinogenic amino acid of this type is proline) are sometimes named imino acids, though this usage is disputed by some.
The term imino acid is also the obsolete term for imido acids, containing the -C(=NH)-OH group, and should not be used for them.
The amino acid oxidase enzymes are able to convert amino acids into imino acids. Also the direct biosynthetical precursor to the amino acid proline is the imino acid (S)-Δ1-pyrroline-5-carboxylate (P5C).