All are linear peptides with free termini, so the values match the calculators on this site directly.
Two systems, one enzyme
Blood pressure and blood flow are controlled in part by two peptide systems that pull in opposite directions. The renin–angiotensin system produces angiotensin II, which constricts blood vessels and promotes salt and water retention. The kallikrein–kinin system produces bradykinin, which dilates vessels and increases their permeability. Both begin with a large precursor protein in the blood, cleaved by a specific protease to release a small active peptide.
The two systems are linked by a single enzyme. Angiotensin-converting enzyme (ACE) both activates angiotensin and inactivates bradykinin, so it raises the level of a vasoconstrictor and lowers the level of a vasodilator at the same time.
The angiotensin cascade
The precursor is angiotensinogen, a large protein made mainly by the liver and circulating in the blood. When blood pressure or sodium falls, the kidney releases the protease renin, which cuts off the first ten residues of angiotensinogen. This decapeptide, angiotensin I (DRVYIHPFHL), has little activity of its own.
ACE, found in high amounts on the surface of blood vessels in the lungs, then removes the C-terminal dipeptide His-Leu, producing the octapeptide angiotensin II (DRVYIHPF), the main active hormone. Angiotensin II acts mainly through the AT1 receptor, a G protein-coupled receptor, to:
- constrict arterioles, raising blood pressure;
- stimulate release of aldosterone from the adrenal gland, promoting sodium and water retention by the kidney;
- stimulate thirst and vasopressin release, discussed in the profile of oxytocin and vasopressin;
- promote growth and remodelling of heart and vessel tissue with sustained exposure.
Further trimming produces a family of related peptides with their own activities:
| Peptide | Sequence | Formed by | Main receptor or effect |
|---|---|---|---|
| Angiotensin I | DRVYIHPFHL | Renin from angiotensinogen | Precursor, little activity |
| Angiotensin II | DRVYIHPF | ACE from Ang I | AT1: vasoconstriction, aldosterone |
| Angiotensin III | RVYIHPF | Aminopeptidase A from Ang II | AT1; aldosterone release |
| Angiotensin IV | VYIHPF | Aminopeptidase N from Ang III | Proposed roles in the brain |
| Angiotensin 1–7 | DRVYIHP | ACE2 from Ang II | Mas receptor: vasodilation, opposing Ang II |
ACE2, a homologue of ACE discovered in 2000, removes only the single C-terminal phenylalanine from angiotensin II, producing angiotensin 1–7, which generally opposes the actions of angiotensin II. ACE2 became widely known in 2020 as the cell-surface receptor used by the SARS-CoV-2 virus to enter cells, a role unrelated to its enzymatic function.
How ACE chooses what to cut
ACE is a zinc-dependent dipeptidyl carboxypeptidase: it removes two residues at a time from the C-terminus of its substrates. It has an important restriction: it does not cut a bond when the first residue of the dipeptide to be removed is proline. Angiotensin II ends in His-Pro-Phe, so removing Pro-Phe would mean cutting in front of a proline, and ACE stops there. This is why angiotensin II accumulates rather than being degraded further by the same enzyme that made it.
Bradykinin, RPPGFSPFR, is a much better substrate. ACE removes Phe-Arg to give an inactive heptapeptide, then Ser-Pro to give an inactive pentapeptide. Because ACE also degrades bradykinin, it was originally described as kininase II before it was recognised as the same enzyme that converts angiotensin I.
Bradykinin and the kinins
Bradykinin was discovered in 1949 by Maurício Rocha e Silva and colleagues in São Paulo, who found that venom of the snake Bothrops jararaca, added to blood plasma, released a substance that slowly contracted isolated gut. They called it bradykinin, from the Greek for "slow movement", in contrast to the fast tachykinins described in the profile of substance P.
Bradykinin is released from high-molecular-weight kininogen by the protease plasma kallikrein. Tissue kallikrein releases a slightly longer form, kallidin (Lys-bradykinin, KRPPGFSPFR), from low-molecular-weight kininogen. Both act mainly through the B2 receptor, constitutively present on many cells; a second receptor, B1, is induced during inflammation and responds to fragments lacking the C-terminal arginine.
Through these receptors, bradykinin dilates blood vessels, largely by stimulating release of nitric oxide and prostacyclin from the endothelium, increases vascular permeability, contracts some smooth muscle, and stimulates pain-sensing nerve endings. It is one of the mediators responsible for the pain, redness and swelling of inflammation. Its plasma half-life is well under a minute, reflecting efficient degradation by ACE, neprilysin and other peptidases.
From snake venom to blood pressure drugs
The connection between the two systems led to one of the best-known examples of drug design based on a natural peptide. In the 1960s Sérgio Ferreira, a student of Rocha e Silva, found that Bothrops jararaca venom contained peptides that strengthened the effects of bradykinin. These bradykinin-potentiating peptides turned out to work by inhibiting ACE, which was then shown to be the same enzyme that converts angiotensin I.
One of these venom peptides, the nonapeptide teprotide, lowered blood pressure in patients but could not be given by mouth. Chemists at the Squibb Institute, led by Miguel Ondetti and David Cushman, used the known C-terminal sequences of the venom peptides and a model of the enzyme's active site, based on the related zinc enzyme carboxypeptidase A, to design a small non-peptide inhibitor. The result, captopril, was approved in 1981 as the first orally active ACE inhibitor. Its proline ring mimics the C-terminal proline of the venom peptides, and its thiol binds the zinc ion.
ACE inhibitors lower angiotensin II and raise bradykinin at the same time. The rise in bradykinin contributes to their blood-pressure-lowering effect, and it is also thought to explain two of their best-known side effects, a dry cough and, rarely, swelling of the face and airways. Drugs that block the AT1 receptor directly, such as losartan, act on the angiotensin side only.
Calculating their properties
These peptides are small, unmodified and linear, so the tools on this site give their properties directly, without corrections for amides or disulfides. Angiotensin II is the worked example in the isoelectric point guide, where its pI of 7.0 is derived both numerically and by hand. The comparison is instructive:
| Peptide | Length | Average MW | Monoisotopic | Calculated pI | Charge, pH 7.4 |
|---|---|---|---|---|---|
| Angiotensin I | 10 | 1296.48 | 1295.6775 | 7.2 | −0.1 |
| Angiotensin II | 8 | 1046.18 | 1045.5345 | 7.0 | −0.2 |
| Angiotensin III | 7 | 931.10 | 930.5075 | 9.0 | +0.8 |
| Angiotensin 1–7 | 7 | 899.01 | 898.4661 | 7.0 | −0.2 |
| Bradykinin | 9 | 1060.21 | 1059.5614 | 12.5 | +1.8 |
| Kallidin | 10 | 1188.39 | 1187.6563 | 12.5 | +2.8 |
Removing Asp1 from angiotensin II to give angiotensin III removes the only acidic side chain, and the pI jumps by two units. Bradykinin, with arginines at both ends and no acidic side chains, is strongly basic.
Angiotensin I, angiotensin II and bradykinin are also among the most widely used standards in analytical chemistry. They are well characterised, stable, commercially available in high purity and span a useful mass range, so they appear in calibration mixtures for MALDI and electrospray mass spectrometry and as test peptides for HPLC columns. The mass spectrometry guide explains why well-defined standards matter for accurate mass measurement.
Frequently asked questions
Why is angiotensin I made at all, if it is inactive?
Separating the release of an inactive precursor from its activation gives two points of control. Renin release by the kidney sets how much angiotensin I is produced; ACE, widely present in vessel walls, converts it rapidly and locally.
What is the difference between bradykinin and kallidin?
Kallidin has one extra lysine at the N-terminus. It is released by tissue kallikrein rather than plasma kallikrein, and aminopeptidases can convert it into bradykinin.
Does ACE2 do the same thing as ACE?
No. ACE removes two C-terminal residues and makes angiotensin II; ACE2 removes one and breaks angiotensin II down into angiotensin 1–7. Despite their similar names and related structures, their effects on the system are largely opposite.
Why are these peptides used as mass spectrometry standards?
They are short, have no modifications or disulfides, ionise well, and are easy to obtain pure. Their exact masses are known and span roughly 900–1300 Da, a convenient range for calibrating instruments.
References
- Rocha e Silva M, Beraldo WT, Rosenfeld G (1949) Bradykinin, a hypotensive and smooth muscle stimulating factor released from plasma globulin by snake venoms and by trypsin. American Journal of Physiology 156:261–273.
- Ferreira SH (1965) A bradykinin-potentiating factor (BPF) present in the venom of Bothrops jararaca. British Journal of Pharmacology 24:163–169.
- Cushman DW, Ondetti MA (1991) History of the design of captopril and related inhibitors of angiotensin converting enzyme. Hypertension 17:589–592.
- Donoghue M, Hsieh F, Baronas E, et al. (2000) A novel angiotensin-converting enzyme-related carboxypeptidase (ACE2) converts angiotensin I to angiotensin 1–9. Circulation Research 87:e1–e9.
- Leeb-Lundberg LMF, Marceau F, Müller-Esterl W, Pettibone DJ, Zuraw BL (2005) International Union of Pharmacology. XLV. Classification of the kinin receptor family. Pharmacological Reviews 57:27–77.