The synthesis of cyclic tetrapeptoid analogues of the antiprotozoal natural product apicidin.

A novel synthetic strategy is described which may be used to prepare analogues of the antimalarial, fungal metabolite apicidin. Compared to the natural product, one analogue shows potent and selective activity in vitro against the parasite Trypanosoma brucei and low mammalian cell toxicity.

GR138676, a novel peptidic tachykinin antagonist which is potent at NK3 receptors.

GR138676, a conformationally constrained analogue of neurokinin B, is a novel, potent NK3 receptor antagonist. GR138676 was a competitive antagonist of neurokinin B-dependent arachidonic acid mobilization from prelabelled Chinese hamster ovary cells stably transfected with a human NK3 receptor gene (pKB 8.3) and of contractions induced by senktide in rat portal vein (pKB 8.2). However, GR138676 was also a competitive antagonist of the increase in intracellular calcium evoked by the selective NK1 agonist, GR73632, in the human astrocytoma U373MG cell-line (pKB 8.3). GR138676 had little activity at NK2 receptors, inhibiting binding of the NK2 antagonist radioligand [3H]-GR100679 to Chinese hamster ovary cells transfected with the human ileum NK2 receptor with a pKi of 6.0. In summary, despite its activity at NK1 receptors, GR138676 will be a useful tool for characterizing NK3 receptors as well as defining the physiological and pathophysiological function of this receptor subtype.

Chloride secretion in response to guanylin in colonic epithelial from normal and transgenic cystic fibrosis mice.

1. Guanylin, a 15 amino acid endogenous gut peptide, increased the short circuit current (SCC) in the epithelium of the mouse colon, but only when applied to the apical and not the basolateral surface. 2. By use of selective blockers of epithelial ion transport and modification of the bathing solution, it was concluded that guanylin increased electrogenic chloride secretion but also had a minor effect on electrogenic sodium absorption. In addition there were small residual currents which remained unresolved. 3. The threshold concentration of guanylin causing a SCC increase was less than 50 nM, but at concentrations 40 times greater no indication of a maximally effective concentration was found. 4. Two guanylin isomers with the same amino acid sequence but with the disulphide bridges joined in an alternate fashion showed no activity. Thus only guanylin with the greatest structural homology to heat stable enterotoxin (STa) showed biological activity. 5. The action of guanylin was virtually eliminated in colonic epithelia from transgenic cystic fibrosis (CF) mice. As these animals lack the chloride channel coded by the CF gene sequence, it is likely that the final effector process in murine colonic epithelia involves the CFTR (cystic fibrosis transmembrane conductance regulator) chloride channel. 6. Opportunistic infections of the gut generating STa lead to diarrhoeal conditions via an action of the toxin on apical guanylin receptors. Thus, as discussed, the CF heterozygote may have a genetic advantage in this circumstance.

Solid-phase synthesis of O-mannosylated peptides: two strategies compared.

A comparison of the O-glycosylation of resin-bound assembled peptides with the incorporation of glycosylated amino acids using established chemistry is presented. Fmoc/tert-butyl-based protecting groups were used for the peptidic moieties in conjunction with acetyl sugar protection. Koenigs-Knorr glycosylations were carried out using protected bromomannose derivatives, the acceptor being threonine or serine, either in solution or within a resin-bound peptide. The characterisation of microgram quantities of glycopeptides by the use of glycosidases in combination with mass spectrometry is also described.

Solid-phase synthesis of a range of O-phosphorylated peptides by post-assembly phosphitylation and oxidation.

A completely general method for the O-phosphorylation of peptides of any given composition using solid-phase methodology is described. Peptides were assembled using Fmoc amino acid active esters, with base used for Fmoc deprotection. Unprotected amino acid side chain hydroxyl groups were phosphitylated and oxidised at the end of the assembly using bis(benzyloxy)(diisopropylamino)phosphine and tert.-butylhydroperoxide respectively. TFA was used for final deprotection of the amino acid side chains and for simultaneous cleavage from the resin. The synthesis of O-phosphopeptides of up to 15 residues in length is described.