Circulin

Peptide bonds are cleaved in a nonselective, but not in a completely random manner. Based on anchimeric side-chain assistance, steric factors, and bond strains, acid-labile peptide bonds are predicted to include sites containing Asp, Glu, Ser, Thr, Asn, Gin, Gly, and ProJ22l The disulfide topologies of circulin B and cyclopsychotride, backbone-cyclized peptides with three disulfide bonds, were determined by partial hydrolysis for 5 hours.[22 Occasionally, the bond between adjacent half-cystine residues is cleaved due to the nonselective nature of the mechanism of partial acid hydrolysis.[21] By this procedure, in all cases, a complex mixture of peptide fragments is produced which requires careful chromatographic separation by RP-HPLC for subsequent analysis by mass spectrometry (see Section 6.1.6.2.7). 

The proteins miraculin and circulin from tropical fruits modify taste. Acids taste sweet rather than sour after the tongue has been treated with either protein.945 946 Exposure of the tongue to artichokes often... 

Circulin A Fungisporin Gramicidin S Malformin A(, C Mycobacillin Polymixin Bi Tyrocidine A, B Valinomycin... 

The plant was traditionally boiled in water to produce a medicinal tea that was used to accelerate childbirth. Reports of this use stimulated interest in characterising the bioactive component, which was subsequently found to be a 29 amino acid peptide referred to as kalata Bl.105 Cyclotides have also been reported to possess antimicrobial activity,106 and, for example, synthetic versions of the cyclotides circulin B and cyclopsychotride A are active against both Grampositive and Gram-negative bacteria.106 Impaired antimicrobial activity of cyclotides is observed at high salt concentration in the assay buffer and the phenomenon of activity linked to ionic strength has also been described for thionins and plant defensins and is an indicator of activity at membranes. 

In the antibiotic field, Vogler and Studer have reviewed the chemistry of the polymyxin antibiotics. Studer, Lergier and-Vogler l reported the synthesis of Circulin A, and Ohno and coworkers° synthesized tyrocidin A. 

Derua, R., Gustafson, R. R., Pannell, L. R. Analysis of the disulfide linkage pattern in circulin A and B, HIV-inhibitory macrocydic peptides. Biochem. Biophys. Res. Commun. 

Homodet cyclic Albomycin Circulin A Colistin Aj B Polymyxin Bj, B2... 

The circulins—As early as 1949, Peterson and Reineke characterized circulin as its sulphate. Total hydrolysis yielded D-leucine, L-threonine and L-K,y-diaminobutyric acid together with an optically active isomer of pelargonic acid. The existence of two components, found by Peterson and Reineke was later confirmed by the chromatographic separation of crude circulin into two major components, named circulin A and circulin B. In addition there was evidence for at least three other ninhydrin-positive, biologically active entities. In the hydrolysate of circulin A, L-isoleucine was found besides the amino acids previously reported . Quantitative amino acid analysis showed circulin A and B to be composed of L-a,y-diamino-butyric acid, L-threonine, D-leucine, L-isoleucine and ( + )-6-methyloctanoic acid in the molar ratio 6 2 1 1 1. After partial acid hydrolysis, fractionation and structure determination of the resulting peptides, circulin A and circulin B were formulated as cyclodecapeptides . Very recently, however, Japanese workers have revised the structure of circulin A. According to them, circulin A differs from colistin A only by a replacement of L-leucine in the latter by L-isoleucine Figure 1.7). 

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