Increased affinity of dimeric enkephalins is not dependent on receptor density

Dimeric analogs of immunosuppressive decapeptide fragment of ubiquitin

Our previous studies revealed that ubiquitin and its decapeptide fragment with the LEDGRTLSDY sequence, located on the exposed molecule loop, strongly suppressed the immune response. This suggested that the loop may serve as a functional epitope of ubiquitin molecule and that a possible mechanism of biological action of the synthesized peptides is associated with interfering in interactions of ubiquitin with other molecules. Ubiquitin is known to exist in oligomeric forms, which can interact with various oligomeric receptors. We designed and synthesized new dimeric analogs of the ubiquitin fragment, to probe whether dimeric peptides may have higher affinity towards the ubiquitin receptors responsible for immunosuppression, which are believed to form oligomeric structures. Three dimerization strategies, N-terminus to N-terminus, C-terminus to C-terminus, and N-terminus to C-terminus (head-to-tail) via PEG derivatives were used to synthesize the dimeric peptides on solid support. In the course of our research, we developed a new and straightforward procedure of dimerization where α-amino groups of the C-terminal lysine residues of two peptide fragments were linked by PEG spacer directly on solid support. The effect of dimeric analogs on the immunological response was tested in the AFC in vitro experiment. The immunological tests showed that the head-to-tail dimerization caused a more profound increase in the biological activity than other tested dimerization methods. Our results suggest that such orientation of peptide components may correspond to orientation of the hypothetic ubiquitin receptors responsible for the immunomodulatory activity.

Dimerization of the immunosuppressive peptide fragment of HLA-DR molecule enhances its potency

Our previous studies revealed that the nonapeptide fragment of HLA-DR molecule, located in the beta chain 164-172 with the VPRSGEVYT sequence, suppresses the immune responses. The sequence is located on the exposed molecule loop, therefore it may be involved in the interactions with other proteins. We suggested that the loop may serve as a functional epitope on the HLA class II surface for intermolecular binding, and that possible mechanism of biological action of the synthesized peptides is associated with interfering of adhesion of HLA class II molecules to their coreceptors. It has been postulated that oligomerization of the coreceptors is required for stable binding to class II HLA. Based on the crystal dimeric structure of HLA-DR molecules, we designed, and synthesized molecules able to induce the putative coreceptors dimerization. The synthesized series of compounds consisted of two VPRSGEVYT sequences linked through their C-termini by spacers of different length: (VPRSGEVYTGn)2K-NH2 ( n = 4-6). The results demonstrate that the dimerization of the nonapeptide fragment of HLA-DR results in enhanced immunosuppressory properties.

Opioid receptors and their pharmacological profiles

Opioid receptors can be divided into three major classes, which are called mu, delta and kappa-receptors. The molecular basis of the receptors is discussed and a hypothesis of the binding of bivalent ligands to the receptor is presented. Furthermore the mechanism of action, the distribution and the probable function of these classes is overviewed. Increasing evidence is accumulating that the classical binding model cannot explain completely the interaction of opioids with their receptors. In addition to the mu-receptors, high affinity mu 1 binding sites have been demonstrated. Similarly, the delta receptors may be divided in delta 1 and delta 2. The significance of these subclasses is not yet fully understood. The high affinity mu 1-binding sites, however, represent probably an activated receptor complex, e.g. the complex between the receptor and the guanine-nucleotide-binding protein.

A computer controlled device to facilitate studies of the kinetics of ligand-binding: binding of diazepam to bovine brain membranes

A device to facilitate kinetic receptor filtration assays is described. The receptor containing membranes and the labeled ligand are placed in two separate syringes and are rapidly mixed into a collecting syringe using a pneumatic ram. Shortly after the start of mixing, a pneumatically controlled valve switches the collecting syringe containing the receptor-ligand mixture to the filtration unit. Filtration is performed on glass/microfiber filters or equivalent by pushing the plunger of the collecting syringe by a stepper motor. A valve positioner controlling several valves allows the filtered membranes to be washed and dried by pressure in any user programmable sequence. Further filtration of the receptor-ligand mixture can be programmed at selected time points. The entire system is controlled by an IBM-PC. With this system, the association and dissociation of diazepam from crude bovine-brain membranes has been studied at 4 degrees C. The dissociation shows a biphasic pattern with half lifes of 1.3 and more than 23 minutes respectively. Association appears to be into a single compartment.

Interaction of dimers of inactive enkephalin fragments with mu opiate receptors

Dimeric analogues of the inactive enkephalin fragment Tyr-D-Ala-Gly were synthesized by cross-linking with alkanediamine at the C-terminus. Biological evaluation of these dimers (H-Tyr-D-Ala-Gly-NH)2.(-CH2-)n (DTREn), where n = 0-6, revealed that the fragment inactive for mu receptors was activated by its dimerization, with the maximum activation found with DTRE2, and that the dimer was highly mu-selective. So-called "handicapped" dimers, which lack one of the essential groupings required for enkephalin activity, were found to be far less active, indicating that the dimer interacts bivalently with mu receptors. It seems, therefore, that mu opiate receptors contain at least two equivalent binding sites which are extremely close to each other.

Interaction of dimeric and monomeric enkephalins with NG108-15 hybrid cells. A kinetic analysis

The binding of the enkephalin dimer [D-Ala2, Leu5-NH-CH2-]2 (DPE2) is characterized by its high affinity for receptors on NG108-15 hybrid cells, the affinity constant K = 4.7 X 10(9) M-1 is up to 8-fold that of monomers (0.6 to 1.0 X 10(9) M-1), and a maximal binding capacity equal to one half that of the monomers. Kinetic studies showed that DPE2 binds with a 2-fold higher rate, k1 = 6.3 X 10(7) M-1min-1, than monomers (2.4 to 3.8 X 10(7) M-1min-1), and dissociates at a slower rate than monomers. Dissociation of DPE2 was consistently bi- or multiphasic but increased about 12% only after 3 hr of dissociation in the presence of a large excess of unlabeled enkephalin. The dissociation kinetics of monomers varied with enkephalin and experimental conditions used. Consistent with the value for the maximal binding capacity, the kinetic studies are interpreted in support of the hypothesis that DPE2 binds by cross-linking two subunits of one receptor.

A novel side-chain-linked antiparallel cyclic dimer of enkephalin

The dimeric cyclic enkephalin analog, (H-Tyr-D-Lys-Gly-Phe-Glu-NH2)2, was isolated as a second major component from the crude product obtained in a solid-phase synthesis of the corresponding cyclic monomer, H-Tyr-D-Lys-Gly-Phe-Glu-NH2. In comparison with [Leu5]enkephalin the cyclic dimer is about equipotent in assays representative for mu-opioid receptor interactions and 1/10 as potent at the delta-receptor. The fact that the enkephalin dimer shows a receptor selectivity pattern distinct from that of the cyclic monomer and of the corresponding linear analog suggests that cyclodimerization via side-chain linkages might be generally useful as a means to produce shifts in the activity profiles of peptide hormones and neurotransmitters.