Angiotensin-converting enzyme (ACE) is a dipeptidyl caboxy peptidase, located in many tissues and biological fluids, that plays a key role in the regulation of blood pressure and normal cardiovascular functions. The conversion of angiotensin I into angiotensin II is catalyzed by this peptidase. Angiotensin II is a powerful endogenous vasoconstrictor, but it also degrades bradykinin into inactive components, which have a vasodilation effect. Therefore ACE elevates blood pressure and the inhibition of ACE could result in an overall antihypertensive effect. Peptides from dairy origin, have shown great bioactive properties in regard to antimicrobial, anticarcinogenic, cholesterol lowering, antithrombic, immunomodulating, enzyme inhibitory and antihypertensive effects. The search for peptides able to inhibit ACE has been increased due to the potential use of those peptides in drugs, functional foods and nutraceuticals as antihypertensive agents. Other sources have also been identified from which peptides with an ACE inhibitor effect can be extracted or produced, such as corn, soy, gelatin, milk, algae and fish proteins. The inhibitor effect appears to be highly dependent on the molecular size of the peptides, where smaller peptides tend to generate a greater inhibitor effect. In a study by Jeon and others (1999) cod protein hydrolysates were fractionized and a reduction in ACE inhibitor activity was recorded in order from 3 kDa to 5 kDa to 10 kDa and finally 30 kDa. Another study reported that sardine and cod head hydrolysates showed a 30% ACE inhibition while shrimp showed up to 57% ACE inhibition. A study by Theodore and Kristinsson (2008) showed high ACE inhibition (up to 90%) in fish protein hydrolysates prepared from channel catfish protein isolate. They suggested that there was no simple relationship between average peptide size, degree of hydrolysis and ACE inhibition, but their results indicate that the soluble fraction of the hydrolysates, which contains the smallest peptides, is responsible for most of the ACE inhibition. A recent study by Raghavan and Kristinsson (2009) on tilapia protein hydrolysates showed the highest ACE inhibition for low molecular weight peptides (<10 kDa), compared to medium (10-30 kDa) and high (>30 kDa) molecular weight peptides, as well hydrolysates with 25% degree of hydrolysis compared to 7.5%.
Since peptides are widely researched for ACE inhibition, a study by Pripp and Ardo (2007) tried to model a relationship between ACE inhibition and the bitter taste of certain peptides. They found a significant correlation of increased ACE inhibition and bitterness found in certain dipeptides. They attributed the hydrophobicity of those peptides to the relationship between ACE inhibition and bitter taste.
Among many studies it is shown over and over again that the low molecular weight peptides observe much higher ACE inhibitory effects than larger peptides from the same source. Pihlanto and others (2008) published a study in 2007 which determined the ACE inhibitory and radical scavenging effects of potato protein hydrolysates. They observed that after ultrafiltration of the hydrolysate the less than 3 kDa fraction observed the highest ACE inhibitory effect. Miguel and others (2009) studied the ACE inhibitory and antihypertensive effects of bovine casein hydrolysate. They measured the ACE inhibitory effect of the pepsin hydrolyzed bovine casein and a < 3 kDa fraction of the same hydrolysate. The ACE-inhibitory effect was 10 fold higher for the low molecular weight fraction than for the parent hydrolysate. In an antihypertension study with rats, they observed that both the parent and the low MW fraction of the hydrolysate showed effects close to captopril, a synthetic antihypertension drug.
A very interesting study was conducted by Jia and others (2010), who investigated the use of ultrasound for enzymatic preparation of ACE inhibitory peptides from wheat germ protein. They studied the effect of ultrasound treatment before and during hydrolysis and determined that ultrasound pretreatment increased the ACE inhibitory activity up to 40% while the ultrasound treatment during hydrolysis had almost no effect. Further analysis suggested that ultrasonic pretreatment may accelerate the release of hydrophobic amino acids during enzymatic hydrolysis, which may lead to the increased ACE inhibitory effect.
Balti and others (2010) discovered three novel ACE inhibitory enzymes derived by digestive proteases from cuttlefish. The cuttlefish was hydrolyzed by various proteases and tested for ACE inhibitory activity and the most active hydrolysate was further fractionated using gel filtration chromatography. The three novel peptides were identified by ESI mass spectrometry. A study by Rho and others (2009) determined the ACE inhibitory effect of fermented soybean extract and tried to purify and identify the peptide with the ACE inhibitory activity.
Lee and others (2010) discovered a 21 amino acid long peptide, isolated from tuna frame protein hydrolysate, which showed ACE inhibitory activity and antihypertensive effects in spontaneously hypertensive rats. The authors used several proteinases and determined that the pepsin hydrolyzed tuna frame protein observed the highest ACE inhibitory effects. Further fractionation of high (5-10 kDa), medium (1-5 kDa) and low (less than 1 kDa) molecular weight fractions was performed and the 1-5 kDa fractions observed the highest ACE inhibitory activity. From this fraction the novel peptide was isolated by consecutive purification and sequenced by Q-TOF ESI mass spectrometry.
Crynen, S. (2011). Bioactive properties of peptides derived from enzymatic hydrolysis of cod muscle myosin with trypsin, chymotrypsin and elastase. University of Florida.