The last few decades have seen increased activity and focus in the domains of functional foods and bioactive peptides. The focus is not only being generated by the scientists but also potential consumers who are keen to find out more about these subjects as the knowledge and awareness about bioactive peptides and their benefits increasingly become available to the masses.
Currently, it is widely known that certain food proteins and food-derived peptides have the ability to create certain beneficial physiological effects, and as such, they come with a great potential for influencing human health, and may also help in lowering the risks associated with certain chronic diseases. Because of this, it has become apparent that the commercial interest in the development of these products with potential health benefits has increased considerably.
Though the bulk of the work in this arena has been focused on milk, cheese, and other dairy products as the major sources of bioactive proteins and peptides, it has been noted that the interest in other sources of food-derived peptides is also on the rise. For example, there are various fish species such as bonito, sardines, salmon, and tuna which are currently being investigated as viable sources of bioactive peptides.
Certain plants are also under scrutiny for their potential use as sources of bioactive peptides, with the major focus currently being on wheat, pumpkin, soya, and certain types of mushrooms. This is a clear indication that we may have several sources of bioactive proteins and peptides and from those numerous sources, it is possible that we could also discover additional biological benefits and effects to human health.
How bioactive proteins and peptides interact with the intestines
Milk was designed to meet the dietary, nutritional, and developmental needs of new-borns and as such, it is clear that its role goes beyond the infant’s nutrition. It also acts in the gut as the mucosal surface where it not only helps in the absorption of the nutrients and minerals, but also helps in systemic gut maturation.
There are various intact proteins, especially those obtained from milk that has been proven to show in vivo effects by acting directly on the intestines. As a matter of fact, certain studies indicate that the epithelial cells of the small intestines have a receptor-mediated binding of lactoferrin and exogenous opioid peptides. These peptides are known to target the intestinal brush border membrane of the small intestines.
There are certain major bioactive milk proteins like immunoglobulins and lactoferrin which have been shown to have partial resistance to some of the digestive enzymes, and as such, it becomes easy to tell their direct effects on the small intestines. Though some of these proteins are precursors to certain bioactive peptides, the major interest has always been on their biological functions in their intact form and that is what we will be focusing on.
The fact that lactoferrin is currently being produced on an industrial level, with an annual turnover of over 70 tones is a subtle indication that there is indeed great commercial interest in this product. The main reason for this is simply due to its bioactive properties. It is a naturally occurring iron-binding glycoprotein present in milk. It is mainly considered as an anti-microbial component with the ability to effectively protect the host from a myriad of microbial infections. It has also been shown to be able to regulate inflammatory responses through the prevention of the release of cytokine from the monocytes and it can also regulate the differentiation, proliferation, and activation of certain immune cells.
Since the concentration of some of these dietary peptides is very high in the small intestines, it may be possible that they have a low affinity to cellular receptors, but their physiological importance may still be very high within the gut system. Certain studies have reported that some food-derived peptides may also possess the ability to stimulate the secretion of some proteins within the gut lumen while others may have the ability to inhibit the reabsorption of amino acid back to the gut system.
Numerous studies have also come to the conclusion that some of the bioactive peptides present within the gut system have the ability to regulate the gastric emptying rate, hence, potentially affecting the growth of tissues as well as increasing the absorptive and secretory capacity of the gut.
How bioactive peptides are absorbed
Most of the bioactive peptides discovered to date are known to cause systematic effects, and as such, their absorption must be through the intestines or through receptor and cell signaling for them to be able to elicit their respective actions. Though the intestines are known to quickly and easily absorb dipeptides and tripeptides, there is still not enough information that documents the absorption of higher molecular weight bioactive peptides.
There is some evidence suggesting that the use of radioactively labeled peptides may rapidly infuse into the jejunum from the systematic circulation, but there is still a need for more studies to be conducted in this arena before conclusive conclusions can be made. However, there are several reports that suggest that at least two long peptides obtained from casein can be detected in the plasma of adults after consuming yogurt or milk.
Additionally, a study indicated that it was possible for caseinmacropeptide to be present in the blood of rats following oral administration. Though there is still a need for a significant amount of research to determine the fate of the absorption of certain bioactive peptides, it seems as if some of the food-derived peptides can be absorbed in the small intestines, however the extent of their absorption is determined by the specific nature of the peptides, which usually tend to decrease with an increase of the peptide’s chain length.
Bioactive proteins and peptides as antihypertensive
Hypertension is a serious condition known to come with increased risks for other potentially fatal conditions such as a stroke and an array of cardiovascular diseases. Bioactive proteins and peptides with anti-hypertensive properties are currently among the most extensively studied groups in the food arena. The majority of these peptides have been tested for their activity when it comes to the inhibition of the activities of angiotensin-I converting enzyme – ACE, hence, helping in lowering the production of the potent vasoconstrictor angiotensin II.
Though many peptides have been subjected to in vitro studies, a good number of them have also been subjected to hypertensive animal models as well as human subjects. Tripeptides from chicken muscles and bonito muscles as well as VPP usually obtained from fermented milk have shown to have the ability to lower blood pressure in rats. In one of the studies where hypertensive rats were given Calpis, it was observed that there was a considerable reduction in angiotensin-I converting enzyme activities in the aortal tissue of the rats compared to the control group which was given just saline. In a study involving eight weeks of placebo-controlled study in hypertensive humans, it was noted that the daily consumption of 100ml of Calpis led to significantly low blood pressure after the eighth week. There are also studies to suggest that peptides may lower blood pressure spontaneously in hypertensive rats by instigating the process of vasodilation through the actions of bradykinin and prostacyclin receptors.
Bioactive proteins and peptides as cholesterol-lowering agents
Elevated blood cholesterol levels are another major risk factor for a variety of cardiovascular diseases. Currently, several studies are looking into the various methods that can be used for preventing or lowering plasma cholesterol levels, with a large chunk of those studies focusing on the use of bioactive peptides and proteins.
The consumption of food peptides, especially those obtained from soybeans, has been shown to contribute to the lowering of serum cholesterol levels in humans. There is a considerable amount of evidence obtained from various controlled studies that suggest that replacing animal protein with soya protein can lead to immense benefits on serum lipids, including the lowering down of total cholesterol, triglycerides, and low-density lipoproteins. It was observed that all these effects can occur without the levels of the so-called good cholesterol being affected.
There are several mechanisms believed to be used by bioactive protein sand peptides in lowering cholesterol levels. Some of them include blocking or preventing the reabsorption of bile acids or cholesterol synthesis, the stimulation of LDL receptor transcription, and the inhibition of cholesterol synthesis.
From the studies, there is enough evidence to suggest that several or all of these mechanisms may work depending on the protein or peptide under study. For example, after the digestion of soybean protein, high molecular weight core peptides are usually left behind. These have the capability of preventing the reabsorption of bile acids, hence, include the ability to lower cholesterol. In a study involving mice as the subjects, it was observed that low molecular weight peptides such as peptides from soybean glycinin could reduce serum cholesterol levels, though they increased the production of fecal cholesterol and bile acids.
About Opioid Peptides
There are numerous reports and lots of literature that give a vivid description of the action of opioid peptides in the gastrointestinal tract where it is believed that they may have localized effects, systemically-mediated effects, and effects mediated by gut hormones. Since there is a humongous number of opioid receptors, each of the receptors is usually responsible for a specific physiological task, and this implies that the effects created by the opioid peptides can be diverse, and may include affecting appetite, depressing respiratory, and gastrointestinal motility. Out of all the already known receptors, µ-receptors are already known to affect emotional behavior, suppression of intestinal motility, and pain sensation, while k-receptor is known to cause sedation as well as regulate the satiety signals, which in turn, affects the intake of foods.
There are diverse sources of food-derived peptides with opioid activities, with the major ones being water-buffalo milk, the hemoglobin from bovine blood, zein from maize, gluten from wheat, soy protein, and hordein from barley. It is possible to achieve the generation of opioid peptides through digesting the parent protein with the help of normal digestive enzymes such as pepsin, trypsin, and chymotrypsin. However, it is yet to be shown that all the opioid peptides can be released in the gut following oral administration.
A fair number of bioactive peptides derived from milk proteins have been shown to be opioid agonists, demonstrating the ability to bind to opioid receptors and exhibit morphine-like effects. Some of the peptides found in this category include lactorphine, serophine, casoxins, and lactoferrin.
About Opioid Agonists
Milk-derived opioid agonists come with a diverse range of physiological effects, including prolonging gastrointestinal transit time, modulating amino acid transport, inhibiting diarrhea, and prolonging analgesia. The β-casomorphins were the first mild derived opioid agonists to be identified, and as such, they have been extensively studied opioid agonist peptides.
These peptides have also been shown to be present in cheese and this is mainly attributed to the proteolytic effects of certain bacteria such as Bacillus cereus, which is usually very active during cheese ripening. Studies conducted in rats suggest that β-casomorphins may also have a huge role to play when it comes to the dietary intake of fats since it has the ability to stimulate the intake of high-fat diets while at the same time suppressing the intake of high carbohydrate diets in satiated rats. They also have depressive effects on the central respiratory system where it has been suggested that they can slow the rate of respiratory frequency as well as the tidal volume in rabbits and rats.
The absorption of casomorphins in adult humans is yet to be observed and as a result, it is generally considered that the effects of this peptide may be limited only to the gastrointestinal tract. In babies and infants, however, there is enough evidence to suggest that casomorphins may be relocated from the blood to both the cardiovascular compartment and the brain stem. In pregnant or lactating mothers, β-casomorphins may go through the mammary glands and can also be detected in the plasma. Due to this, it has been suggested that β-casomorphines may be useful in the modulation of oxytocin and prolactin during lactation.
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