Discovery of the Cyclotides

Oldenlandia affinis was used by native women in the Zaire as an aid during childbirth. A tea was made of the leaves and imbibed during labour.

The cyclotides have been recognised as a family of novel circular proteins only in the last few years but the discovery of the first member of this family may be traced back to reports of native medicine applications in the early 1970s.

Kalata B1, was discovered because it is an active ingredient in a herbal medicine used by African women to assist childbirth . While on a Red Cross relief effort in the Congo region in the 1960s a Norwegian doctor, Lorents Gran, noted that during labour African women often ingested a tea made from leaves of the plant Oldenlandia affinis because of its uterotonic effects. The active ingredient was determined to be a peptide that was named kalata B1, after the local name for the native medicine. Subsequent in vivo studies in rats confirmed uterotonic activity of the purified peptide but it was not characterised as a macrocyclic peptide until some 20 year later.

The mid-1990’s was a key period in the discovery of macrocyclic peptides, with several independent groups discovering such peptides while screening for various biological activities and our group determining the three dimensional structure of kalata B1 . In the first fortuitous discovery Schöpke et al., examined Viola arvensis and V. tricolor in a study aimed at the discovery of new saponins. While assaying for the usual hemolytic activity of saponins they discovered a macrocyclic peptide, violapeptide I, with hemolytic activity. At around the same time bio-assay driven screens for anti-HIV and anti-neurotensin activity led to the discovery of the circulins and cyclopsychotride A respectively.

Viola arvensis a cyclotide containing plant. Member of the violaceae family and found in temperate regions of Australia and Europe.

With our report of the three dimensional structure of kalata B1 in 1995 and its sequence homology with the circulins and cyclopsychotride A, we became convinced that macrocyclic peptides might be more common than had earlier been thought and we began searching for other examples. Several other macrocyclic peptides were found in the late 1990s and it became clear that the peptides formed part of a family that we subsequently named the cyclotides.

Several novel cyclotide sequences have been discovered in the last few years , with the known sequences now exceeding 45 and many more currently being characterized in our laboratories. A large proportion of the new cyclotides have been discovered based on their structural properties rather than biological activities. The cyclotides are relatively hydrophobic and can be readily identified from crude plant extracts by their characteristically late elution on RP-HPLC.

The cyclotides described above, all come from plants in the Rubiaceae or Violaceae families but the prevalence of macrocyclic peptides has recently been expanded to include the Cucurbitaceae family. This is based on the discovery of the trypsin inhibitors MCoTI-I and MCoTI-II, 34 residue macrocyclic peptides, from Momordica cochinchinensis . They have no sequence homology to the previously characterized cyclotides, with the exception of the six cysteine residues, but are of a similar size and contain a cystine knot motif (Felizmenio-Quimio, 2001). The MCoTI peptides were originally isolated based on their trypsin inhibitory activity and are homologous to linear cystine knot peptides from the squash family of trypsin inhibitors such as EETI-II and CMTI.

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