Angiotensinogen
Angiotensinogen consists of 453 amino acid residues. It is produced constitutively and released into the circulation mainly by the liver, although other sites are thought to be involved also. It is a member of the serpin family, although it is not known to inhibit other enzymes, unlike most serpins. Plasma angiotensinogen levels are increased by plasma corticosteroid, estrogen, thyroid hormone, and angiotensin II levels.
Angiotensin I
Angiotensin I is formed by the action of renin on angiotensinogen. It appears to have no biological activity and exists solely as a precursor to Angiotensin II.
Angiotensin II
Angiotensin I is converted to Angiotensin II by removal of two terminal residues by the enzyme kininase. Angiotensin II is degraded to angiotensin III by angiotensinases that are located in red blood cells and the vascular beds of most tissues. It has a half-life in humans of 1-2 minutes.
Angiotensin III
Angiotensin III has 40% of the pressor activity of Angiotensin II, but 100% of the aldosterone-producing activity.
Angiotensin IV
Angiotensin IV is a hexapeptide which, like angiotensin III, has some lesser activity.