Angiotensin-converting enzyme (ACE) is an enzyme that involves the conversion of angiotensin I to angiotensin II. This leads to the blood vessels narrowing or opening up, a process known as vasoconstriction. The functions of the angiotensin-converting enzyme result in a number of medical conditions, for which a certain category of medications have been created.
The ACE is classified as a peptide, which is a shorter form of protein. More specifically, it is an exopeptidase, which catalyzes the ends of peptide or protein bonds to release single amino acids. The angiotensin-converting enzyme is released from cells that comprise the lungs and kidneys.
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Angiotensin I, which originates in the kidneys, lacks any biological activity. This decapeptide—a peptide that consists of ten amino acids—exists as a precursor to angiotensin II, which is its active form. Angiotensin II is an octopeptide, which means that it contains eight amino acids. ACE catalyzes the transformation by removing the two amino acids of angiotensin I.
Angiotensin-converting enzyme is also responsible for the degradation of the peptide bradykinin. It acts as a vasodilator, which is an agent that widens the blood vessels. Therefore, bradykinin has the opposite effect of angiotensin II.
The two functions of the angiotensin-converting enzyme makes the angiotensin-converting enzyme an important part of the renin-angiotensin system (RAS). This is a hormone system responsible for regulating the body's blood pressure and fluid balance, collectively known as extracellular volume. The RAS oversees substances located outside the cells. The "renin" prefix refers to the enzyme that induces the production of angiotensin I, which in turn is transformed into angiotensin II for it to function. Angiotensin II constricts the blood vessels, thus increasing blood flow and causing high blood pressure, or hypertension. Conversely, destruction of bradykinin lessens the ability of blood vessels to widen and restrict blood flow.
The RAS is also known as the renin-angiotensin-aldosterone system (RAAS). This is because angiotensin II causes the release of aldosterone, a hormone that increases the amount of sodium and water entering the blood. This also increases blood pressure since such re-absorption increases the body's extracellular volume.
Besides hypertension, the actions of the angiotensin-converting enzyme contribute to diseases such as heart failure and diabetes. Specialized drugs called ACE inhibitors exist to inhibit the enzyme by decreasing the formation of angiotensin II and degradation of bradykinin. Examples of ACE inhibitors include Benazepril, which is sold under the brand name Lotensin; Capotril, which goes by the Capoten brand name; and Lisinopril, which is branded Prinivil or Zestril.