MT-1: Research Chemistry and Literature

Origin and identification

The parent hormone is Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2. Truncation work located the pharmacophore in the central tetrapeptide His6-Phe7-Arg8-Trp9, and every analogue discussed here is built around that fragment. The native peptide is short-lived in serum, which limited its usefulness as a laboratory reagent.

The substituted analogue came out of the joint chemistry and anatomy programme run by Hruby, Hadley and co-workers at the University of Arizona. Their method was systematic residue replacement rather than screening. The 1980 report in the Proceedings of the National Academy of Sciences described the Nle4, D-Phe7 peptide as resistant to serum enzymes and far more persistent than the parent in the assays then available. Persistence, not potency alone, is what made the analogue useful as a reagent.

The same laboratory later constrained the backbone directly, closing a lactam bridge between amino acid side chains to hold the turn that a linear chain only samples. MT-1 is the linear member of that lineage and is normally read against the cyclic ones, because the pair separates the contribution of side-chain chemistry from the contribution of conformational restriction. Identity rests on the residue sequence and the mass it implies, not on a single small-molecule identifier.

Preclinical research context

Molecular characterisation of the family began in 1992 with the cloning of a 317-residue receptor from human melanoma complementary DNA, and continued the following year with a 361-residue receptor found in brain, placental and gut tissue rather than in melanoma or adrenal tissue. Subtype identity, not tissue of origin, now organises the pharmacology.

Radioligand binding studies use the iodinated analogue as tracer against receptors expressed transiently in COS cells. Reported dissociation constants run from tens of picomoles per litre at MC1R to a few nanomoles per litre at MC4R and MC5R. Competition curves built with alpha-MSH, beta-MSH, gamma-MSH and ACTH fragments separate the subtypes further, and MC3R stands apart in preferring gamma-MSH where MC1R and MC5R prefer the alpha form.

Functional work reads receptor activation as cyclic AMP accumulation in transfected cell lines, or as adenylyl cyclase activity in cultured melanoma cells. Site-directed mutagenesis on the extracellular face of MC1R showed that replacing Asp184 with alanine abolishes tracer binding while the receptor remains at the cell surface, placing part of the recognition site in the second extracellular loop. Fragment studies across MC1R, MC3R, MC4R and MC5R found no binding epitope for ACTH beyond the alpha-MSH sequence.

Rodent work in this area is largely anatomical and molecular. It maps receptor transcript distribution across tissues and compares pharmacology on cloned rodent receptors against the human orthologues. Affinities are not interchangeable between species, so results are read against the receptor actually expressed in the system under test.

Analytical characterisation

Chromatographic purity is measured by reverse-phase HPLC on a C18 column with a water and acetonitrile gradient modified with trifluoroacetic acid. Detection at 214 to 220 nm responds to the amide backbone. Purity is reported as main peak area against total integrated area, and the shape of the trailing edge usually says more about the synthesis than the headline figure does.

Mass spectrometry confirms that the observed mass agrees with the sequence. Electrospray ionisation gives a multiply charged envelope that deconvolutes to a neutral mass. MALDI time-of-flight gives the singly protonated ion directly. Deletion sequences and oxidised species are visible by mass even when they co-elute with the main peak.

Each lot is characterised by Janoshik Analytical as an independent third party. The Certificate of Analysis states identity, chromatographic purity and net peptide content, with the chromatogram and the spectrum attached rather than summarised. Certificates are lot-specific and should be read against the lot number printed on the vial.

Handling and storage

The lyophilised solid is stable at ambient temperature for the period of transit, and material that arrives dry and intact has not been compromised by shipping at room temperature. On receipt, move the vial to minus 20 degrees Celsius. Protect it from light throughout.

Allow sealed vials to reach room temperature before opening. Cold glass draws moisture from the air, and a lyophilised cake that takes up water will collapse and degrade. Once taken into solution the peptide is far less robust. Hold reconstituted material at 2 to 8 degrees Celsius and treat working solutions as short-lived.

Repeated freezing and thawing is the common cause of loss. Divide reconstituted material into single-use aliquots before the first freeze so that no portion is thawed twice. Record the date of reconstitution against the lot number. Material is for laboratory research use only and is not for human or veterinary use.

References

1. 4-Norleucine, 7-D-phenylalanine-alpha-melanocyte-stimulating hormone: a highly potent alpha-melanotropin with ultralong biological activity
2. Molecular cloning and expression of the human melanocyte stimulating hormone receptor cDNA
3. Molecular cloning of a novel melanocortin receptor
4. Characterisation of melanocortin receptor subtypes by radioligand binding analysis
5. Characterisation of the melanocortin 4 receptor by radioligand binding
6. Identification of ligand binding residues in extracellular loops of the melanocortin 1 receptor
7. The melanocortin 1, 3, 4 or 5 receptors do not have a binding epitope for ACTH beyond the sequence of alpha-MSH

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