MCD Peptide

Previous studies with mast cell degranulating (MCD) peptide have shown that peptide [Ala(12)]MCD 8 was an inhibitor of IgE binding to mast cell receptors. In an attempt to produce increased inhibition, analogs were synthesized that maintained the alanine residue in position 12 in the MCD peptide sequence and were further modified at both termini. Analogs modified at the C-terminus were [Ala(12),desLys(21)]MCD 2 and [Ala(12),D-Lys(21)]MCD 4. N-terminus modifications were [desLys(6)-Arg(7)-His(8),Ala(12)]MCD 1, [Ala(6), Ala(12)]MCD 6, and [Val(6),Ala(12)]MCD 7. To assess the role of the Proline(12), analogs [D-Ala(12)]MCD 3 and [Meleu(12)]MCD 5 were also synthesized. The analogs were tested for binding to the IgE receptor in cultured mast cells. Inhibitory activity of IgE-caused degranulation was measured using a beta-hexosaminidase assay. Circular dichroism (CD) and molecular modeling of selected analogs were used to follow possible structural differences among these analogs. All analogs showed binding affinity to the IgE receptor and inhibition of IgE-induced mast cell degranulation at different levels. Differences in inhibition were most likely because of diverse interactions of the analogs with the receptor as inferred by the CD and modeling studies. Based on the results of the beta-hexosaminidase assay, analog [Val(6), Ala(12)]MCD 7 proved to be an excellent inhibitor of IgE-mediated mast cell degranulation.

Besides apamin, the structurally related MCD peptide (mast cell degranulating peptide; peptide 401) is another centrally acting peptide from bee venom. In contrast to apamin, it is hardly neurotoxic upon intravenous injection in mice. Following intraventricular injection, as little as 0.3 microgram/animal produce convulsions and respiratory arrest in mice. The clinical picture differs from that elicited by apamin, and apamin is about 10 times more potent than MCD peptide when given intraventricularly. Apamin and MCD peptide injected into the spinal cord of rats in nanogram amounts, produce circumscript hyperexcitation lasting more than one day, however with complete recovery following sublethal doses. Local apamin poisoning differs from local tetanus (elicited by the same way) by its faster time course.

Mast cell degranulating (MCD) peptide, a component of bee venom, is a 22 amino acid peptide with two disulfide bridges. In this first structure-activity study of MCD peptide, three analogs were synthesized and tested: two analogs shortened by omitting sequences 6-10 and 8-13, respectively, and one analog lacking the disulfide bridge between cysteine residues 5 and 19. These analogs were synthesized by solid-phase methods and were compared to MCD peptide in two assays for inflammation: histamine release from mast cells and superoxide anion release from neutrophils. All three analogs produced histamine release, although with only about one fifth of the activity of MCD peptide. Superoxide anion-releasing activity, however, did not parallel histamine release. MCD peptide did not release superoxide anion, while the 6-10 and 8-13 deletion analogs were strong and weak stimulants, respectively, of this anion. CD spectra showed that the secondary structures of the three analogs were very similar to that of MCD peptide, so that a change in secondary structure cannot completely explain the changes in releasing activities. Charge differences between the two deletion analogs and MCD peptide may explain some of the differences in activity. This is the first demonstration that the various activities of MCD peptide can be separated, and provides a lead through which the purported antiinflammatory activity of MCD peptide may possibly be explored in the future.