Lipo-CCK

Interaction of Lipo-Gastrin and Lipo-CCK with Phospholipid Bilayers... 

The transfer of DM-CCK to DMPC SUVs occurs rapidly and quantitatively even below the phase transition temperature of DMPC (104). Differently from what was observed in the case of lipo-gastrin, the endotherm of the system DM-CCK/DMPC exhibits a broad peak at 24.97° C corresponding to the phase transition temperature of the DMPC bilayer with statistically inserted lipo-CCK molecules... 

The biological functions of the lipo-gastrin and lipo-CCK peptides were analyzed on these two receptors of known sequence using rat pancreatic acinar cells and the tumoral rat pancreatic acinar cell line AR42J. The CCK-A receptor has been thoroughly characterized in the rat pancreatic acinar cells (140-142) and in its... 

The generally accepted model of the spatial structure of the receptor, however, raises immediately the question of how a membrane-bound ligand can find its way to the receptor binding site. As an escape of the lipo-peptides into the extracellular water phase is energetically highly unfavored, the binding site (or sites) are reached by lateral penetration at the lipid/water interphase where the spectroscopic measurements in model systems are locating the lipo-CCK and... 

The maleoyl-) -alanine N-hydroxysuccinimide ester was conveniently prepared following the procedure of scheme 3 (101,102), and was then used to convert [Nle15]-gastrin-[2-17] and [Thr,Nlel-CCK-9 into the reactive maleimido-derivatives for the final coupling of the 1,2-diacyl-3-mercaptoglycerols to produce the lipo-peptide derivatives DM-gastrin and DM-CCK shown in fig. 11 (51,104). 

Table 4. Fluorescence properties and accessibility of the tryptophan residues of [Nle15j-gastrin-17 and the lipo-gastrin derivative (DM-gastrin), of [Nle.Thr]-CCK-9 and DM-CCK to iodide quenching a) the relative error on ksv amounts to 3%.

Table. 5. Metal ion binding affinities of gastrin and CCK and their lipo-derivatives as determined by CD (Ca2+ binding) and luminescence (Tb3 binding).

In the aggregated states of the lipo-peptides, i.e. in the corresponding SUVs, the peptide headgroups of the inner layer should not be accessible for Ca2 ions and correspondingly metal ion/peptide molar ratios of 1.5 (or 1) 1 for DM-gastrin and 0.5 1 for DM-CCK were theoretically expected. The experimental ratios were for both lipo-peptides significantly higher as reported in table 5. This can be... 

Recent studies of Ananthanarayanan (122) have shown that various peptide hormones are capable of inducing Ca2+ influxes into phosphatidylcholine vesicles. The observed affinity of gastrin and CCK as well as of their lipo-derivatives for Ca2+ in membrane mimicking environments led us to examine rates of Ca2+ influxes induced by these hormones. 

A lateral penetration of the entire lipo-peptide molecule. i.e. including the lipid tail, is difficult to rationalize as the helix boundle of the receptor represents a tight assembly which precludes diffusion of membrane lipids into its core structure in order to maintain its three-dimensional assembly Consequently, it should therefore preclude also a penetration of the lipo-tail of DM-gastrin and DM-CCK, and the lipo-peptides should approach the receptor with the tail inserted Into the lipid bilayer and then protrude into the binding deft across the extracellular loops. 

Fig. 25. Schematic representation of DM-CCK showing the spacer lbold) and the lipo moiety inserted in the membrane.

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