Angiotensin-converting enzyme (ACE) is a very important enzyme that maintains a key reaction step in the renin-angiotensin (RA) system, which mediates blood volume and vascular resistance (the degree of constriction of blood vessels). Blood volume and vascular resistance are two of the most important factors that determine blood pressure, so any flaws in this system can lead to detrimental and even fatal situations. The RA system pathway is shown below.

First, the protein angiotensinogen is regularly and naturally produced by the liver and floats around inactive in the bloodstream, until it encounters renin. Renin is released by the kidneys upon neural or chemical stimulation, and it cleaves a portion of angiotensinogen, converting it to angiotensin I. Angiotensin I is then converted by ACE to angiotensin II in the lungs and in blood vessels throughout the body. Angiotensin II has the effect of constricting blood vessels (thus raising blood pressure) and is one of the strongest natural vasoconstrictors known. To control itself, as angiotensin II increases, it causes the amount of renin that is released to subsequently decrease, a process referred to as feedback inhibition.

A medical student measured the rate of angiotensin II formation as a function of angiotensin I concentration (given a constant amount of ACE). He found that as [angiotensin I] increased, the rate eventually reached a maximum plateau. He then repeated the experiment and added Captopril, a compound that binds to the same active site as angiotensin I, and noted the results in the graph below:

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