Hypertension or high blood pressure is one of the main contributing factors to developing cardiovascular disease. The condition of blood pressure is determined by various biochemical mechanisms in the body. Angiotensin-converting enzyme (ACE) as a dipeptidyl carboxypeptidase present in the body tissues can increase blood pressure by inducing constriction in the blood vessels. It is mainly in response to the converting action in the rennin-angiotensin system where ACE converts angiotensin I (biologically inactive peptide) to the potent vasoconstrictor angiotensin II which brings about blood vessel constriction and hypertension.
ACE is also responsible for deactivation of bradykinin which is a hypotensive peptide (blood pressure lowering peptide) in the kallikrein-kinin system of the body. Antihypertensive effect of ACE inhibitory peptides is attributed to their ability to react with ACE; therefore ACE will remain unavailable to catalyse the conversion of angiotensin I to angiotensin II.
ACE inhibitory peptides have been identified in a range of food products such as milk, tuna muscles, sardines, chicken muscles, etc. ACE inhibitory peptides can be categorised into different groups based on their actual inhibiting activities. These consist of "true inhibitor type" peptides, "substrate type" peptides and "prodrug type" peptides. Prodrug inhibitor peptides can be converted to the true inhibitor type by means of ACE or by protease enzymes in digestive tract. The hydrolysis of "substrate type" peptides by ACE will generate low-strength inhibiting activity.
In vivo research on spontaneously hypertensive rats demonstrated that "true inhibitor type" and "prodrug inhibitor type" peptides are effective in reducing systolic blood pressure. ACE inhibitory peptides identified in meat generally fall into the category of "true inhibitor type" peptides. The activity of these peptides significantly relies on their bindings to the receptor (the active site or inhibiting site of ACE enzyme) which can result in suppressing the binding interaction of substrate angiotensin I at the active site of enzyme.
The difference between inhibiting power of bioactive peptides (in vitro) and their antihypertensive effect (in vivo) is important to be considered when analysing the isolated bioactive peptides from food proteins.