Dihexa: An Angiotensin IV-Derived Research Compound

Dihexa is a small peptide that has been changed a great deal from its parent. It comes from angiotensin IV, a fragment of the renin-angiotensin system. It was made in the lab of Joseph Harding and Jay Wright at Washington State University. Their goal was to build angiotensin IV analogs that enzymes could not break down so fast. Interest in dihexa has spread far past the expert papers. But the core evidence is still narrow. It is made up of test-tube studies on receptors and nerve cells, plus a small set of rodent tests. Most of this work came from the group that first made it.

This profile covers where dihexa came from, how it is built, how it is thought to work and how far the early data reach. Incredible Peptides does not supply dihexa. The compounds we do offer are listed in the research peptides catalog. Nothing here concerns use outside the laboratory.

From angiotensin II to angiotensin IV

The renin-angiotensin system makes a chain of peptides. It does this by cutting one peptide down, step by step. Angiotensin I has ten amino acids. It is turned into angiotensin II, which has eight. Angiotensin II acts mainly at the AT1 and AT2 receptors. Enzymes called aminopeptidases then trim the front end, or N-terminus. This gives angiotensin III. When one more unit is cut off, the result is angiotensin IV. It is the six-unit peptide Val-Tyr-Ile-His-Pro-Phe. These are units 3 to 8 of angiotensin II.

In the early 1990s, Harding, Wright and their team found a separate binding site for angiotensin IV. It held the peptide tightly. They named it the AT4 receptor. In 2001, Albiston and co-workers showed that this site was insulin-regulated aminopeptidase (IRAP). IRAP is a zinc enzyme that sits in the cell membrane. This hinted that angiotensin IV and its analogs might work in part by blocking that enzyme.

Still, plain angiotensin IV is a poor tool for experiments. Enzymes that cut peptides break it down fast. It is also a charged six-unit peptide, so it passes through membranes poorly. Those limits led the team to make a series of shorter, capped analogs.

The structure of dihexa

Dihexa is usually written as N-hexanoic-Tyr-Ile-(6)-aminohexanoic amide. It keeps only the Tyr-Ile core of angiotensin IV. The valine at the front end is swapped for a hexanoyl cap. The His-Pro-Phe piece at the back end is swapped for a 6-aminohexanoic acid spacer. That spacer ends in a primary amide. The name comes from the two six-carbon units on each side of the two-unit core.

Property	Angiotensin IV	Dihexa
Composition	Val-Tyr-Ile-His-Pro-Phe	Hexanoyl-Tyr-Ile-6-aminohexanoyl-NH2
Proteinogenic residues	6	2
N-terminus	Free amine	Acylated (hexanoyl)
C-terminus	Free acid	Primary amide
Formula	C40H54N8O8	C27H44N4O5
Average molecular weight	about 774.9 g/mol	about 504.7 g/mol

Both ends are blocked. This removes the charges on the free amine and the free acid. As a result, the molecule is much more fat-loving, or lipophilic, than its parent. The design had two goals. One was to resist the enzymes that trim peptides from either end. The other was to pass through membranes more easily.

Strictly speaking, dihexa is not a normal peptide. It is a peptidomimetic, a peptide-like molecule built around a two-unit core. We discuss this point in our article on peptide structure-activity relationships.

How to test a capped peptidomimetic

The way dihexa is built shapes how it can be tested. It has no free amine at the front end. So it does not react in ninhydrin-type tests. It also cannot be sequenced by Edman degradation. That means its identity rests on mass spectrometry and chromatography.

Its calculated monoisotopic mass is about 504.33 Da. That gives a protonated ion near m/z 505.34. Tyrosine is the only part that absorbs much UV light by itself. So for impurity checks, UV detection at low wavelengths tells you more than 280 nm does. At those low wavelengths, the amide bonds absorb. Dihexa is also water-repelling. It comes off late in reversed-phase HPLC, and it does not dissolve well in water.

Its 3D shape, or stereochemistry, needs close attention. Isoleucine has two stereocenters. During synthesis, unwanted forms can creep in, such as allo-isoleucine or D-tyrosine versions. These have the exact same mass as the target. To tell them apart, you need chromatography methods built for that job.

These are the same identity and purity questions raised for any synthetic research compound. Our page on analytical quality testing shows how HPLC and mass spectrometry work together to answer them.

The HGF/c-Met idea behind dihexa

The best-known theory of how dihexa works moved the focus away from IRAP. It points instead to a different system. That system is hepatocyte growth factor (HGF) and its receptor, c-Met. c-Met is a receptor tyrosine kinase.

The Washington State group had earlier described another angiotensin IV analog, Norleual. They reported that it blocks HGF/c-Met signaling. They also noted that angiotensin IV looks like part of HGF. That part is a hinge region thought to help HGF pair up, or dimerize.

In work published in 2014, Benoist and colleagues reported that dihexa binds HGF tightly. They also reported that it boosts HGF-driven c-Met activation in vitro. Their proposal was that dihexa works by helping HGF pair up.

The same group reported more results in cultured rat hippocampal neurons. Dihexa raised the number of dendritic spines and the formation of synapses. A c-Met blocker stopped this effect. These are cell-culture findings. They support a theory about how dihexa works. But they do not show that the same thing happens at relevant levels in a whole animal. And how the IRAP and c-Met ideas fit together is still not fully clear. If you are weighing data like this, our guide to binding affinity measurements may help.

The c-Met angle also comes with a caution that is well known in cell biology. Faulty HGF/c-Met signaling is widely documented in tumor biology. So any compound said to boost this pathway deserves careful study. Researchers would want to look at its effects on cell growth and cell movement in cell models.

What the dihexa preclinical data do and do not show

The published live-animal work on dihexa is made up of rodent studies, mostly in rats. These studies report on how stable it is in the body and whether it gets into the brain and spinal cord. They also report changes in behavior and synapse measures in test models. These are early findings in animals.

Several limits apply. Most studies come from one research group or the people it works with. Few other labs have repeated the work, and the studies are small. Public data on how the body handles dihexa (pharmacokinetics) and on its toxicity are thin. The long-term effects of boosting this pathway have not been studied in a planned way. None of this supports conclusions beyond the test systems where the data were gathered.

Where dihexa stands as a research compound

Dihexa is best seen as a capped, fat-loving peptidomimetic made from angiotensin IV. It has a proposed mechanism involving HGF/c-Met, but that mechanism is not fully worked out. It also has a past link to IRAP. Its evidence is preclinical, and most of it comes from one line of labs.

For researchers looking at any synthetic peptide or peptidomimetic, the first steps stay the same. Confirm its identity with mass spectrometry. Check its purity with HPLC. Look for unwanted 3D forms. And read the original papers with a critical eye. Our overview of peptide purity testing covers those methods in more detail.

References

1. Wright JW, Harding JW. Brain renin-angiotensin: a new look at an old system. Progress in Neurobiology, 2011.
2. Albiston AL, McDowall SG, Matsacos D, et al. Evidence that the angiotensin IV (AT4) receptor is the enzyme insulin-regulated aminopeptidase. Journal of Biological Chemistry, 2001.
3. Benoist CC, Kawas LH, Zhu M, et al. J. Pharmacol. Exp. Ther., 2014 (angiotensin IV-derived peptides and dependence on the hepatocyte growth factor/c-Met system).