Hydrosoluble immunostimulants of bacterial and synthetic origins

Immunostimulating hexapeptide from human casein: amino acid sequence, synthesis and biological properties

A hexapeptide obtained from human casein by enzymatic digestion has been purified, sequenced and synthesized; its structure is: Val-Glu-Pro-Ile-Pro-Tyr. In vitro this hexapeptide stimulates the phagocytosis of opsonized sheep red blood cells by murine peritoneal macrophages. Administered intravenously to adult mice, it enhances the resistance to infection with Klebsiella pneumoniae.

1H NMR studies of natural and synthetic immunostimulating peptides in aqueous solution

The peptides LAla-DGlu (L-alanyl-D-glutamic acid), LAla-DGlu[LLA2pm(Gly)]NH2, i.e. N2-(L-alanyl-D-gamma-isoglutaminyl)-N6-glycyl-LL-2,6-diaminopimelic acid, and LAla-DGlu[LLA2pm(Gly)-D-Ala], i.e. N2-(L-alanyl-D-gamma-glutamyl)-N6-glycyl-LL-alpha-2,6-diaminopimelyl-D-alanine, extracted from a Streptomyces strain and their immunostimulating synthetic lauroyl derivatives were studied by 1H NMR spectroscopy at 270 MHz. The chemical shift analysis confirms (a) the sequence of these compounds, (b) the occurrence of a gamma bond between Glu (or GluNH2) and A2pm, (c) the presence of a carboxamide group on the backbone of the tetrapeptide LAla-DGlu[LLA2pm(Gly)]. Temperature and pH studies strongly suggest that the tetrapeptide and pentapeptide exist under several folded conformations characterized by a proximity between the NH3 group of Gly and the Glu (or GluNH2) residue. THe similarity between the chemical shifts of the corresponding protons in LAla-DGlu, lauroly-LAla-DGlu and muramyl-L:Ala-DGluNH2 shows that the coupling of the dipeptide with a lauroyl or a muramyl group does not induce significant changes in the structure of the peptide moiety. In the lipopeptide series the presence of the lauroyl chain leads to amphipathic substances which form, micelles in aqueous solution as shown by the large increase of the proton linewidths. The immunostimulating properties of this kind of peptide manifest themselves at very low concentration and only after their coupling with a sugar or lipid moiety. These features can be interpreted by the formation of mixed complexes between the immunoadjuvants and cell membrane constituents.

Immunostimulating substances from human casein

Delipidated human casein was digested with trypsin and the enzymatic digest was fractionated on Sephadex G-50. The peptidic fractions were assayed for immunomodulating activity in two in vitro models. Fractions corresponding to molecular weights in the range of 2,000 +/- 600 were found to possess stimulating activity in these models.