PEPTIDE

 FISH PEPTIDE

Fish pellet contains proteins with bioactive peptides encrypted as part of their sequences. Angiotensin, converting enzyme  inhibitory peptides, which may contribute to lower the blood pressure in mammals, were found in preparations from all species. Prolyl endopeptidase  inhibitors, which may contribute to improve memory and learning in mammals and may help prevent or delay the onset  of Alzheimer's disease in humans, were also found in preparations from different fish species, but only in very small amounts in homogenised samples of rakfisk. Antimicrobial compounds were present in most preparations, however, with different concentration-dependent effects on the bacteria investigated: Escherichia coli K12, Bacillus cereus and Listeria monocytogenes.

             There are many examples of biologically active food proteins, with physiological significance beyond the pure nutritional requirements that concern available nitrogen for normal growth and maintenance. Moreover, there are many physiologically active peptides, derived by protease activity from various food protein sources; however, relationships between structural properties and functional activities have not been completely elucidated. Many bioactive peptides have in common structural properties that include a relatively short peptide residue length (e.g. 2-9 amino acids), possessing hydrophobic amino acid residues in addition to proline, lysine or arginine groups. Bioactive peptides are also resistant to the action of digestion peptidases. Antihypertensive peptides, known as Angiotensin I converting enzyme (ACE) inhibitors have been derived from milk, corn and fish protein sources. Peptides with opioid activities are derived from wheat gluten or casein, following digestion with pepsin. Exorphins, or opioid peptides derived from food proteins such as wheat and milk have similar structure to endogenous opioid peptides, with a tyrosine residue located at the amino terminal or bioactive site.

                   Extracts of fish meat contains free amino acids (FAAs) and peptides, which are not incorporated in proteins. The FAAs have been implicated to be responsible for the characteristic taste of seafood. In addition, FAAs play important roles in physiological functions such as osmoregulation and buffer capacity in the tissues of aquatic animals . Different kinds of low-molecular-weight peptides are also present in the extract; however, only a limited number of peptides such as carnosine and anserine  have been identified. Copious amounts of these two histidine-containing dipeptides were distributed in the muscle of marine pelagic fish such as tuna, skipjack, salmon, and eel, but they were not found in mackerel . Their biological roles have been postulated to include the potent intracellular pH-buffering capacity  control of enzyme activity , neurotransmitter function  and inhibition of oxidative reactions.Polypeptide fragments from myofibrillar proteins are susceptible to attack by peptidyl peptidases and aminopeptidases, which will lead to the production of smaller peptides and release of FAAs.

                     Antimicrobial peptides (AMPs) are important mediators of the immune response against bacteria and hepcidin is a 20–25 residues member with known functions in iron regulation and the innate immune response.

                      Antimicrobial peptides (AMPs) are very important mediators of the innate immune response widely distributed from invertebrates to mammals and play an important role against bacterial invasion. In fish, several AMPs, such us pleurocidin, daxin, misgurnin, piscidin, moronecidin, parasin, defensins and hepcidin, have been found in the last decade .  Most of them are small, cationic and hydrophobic molecules that insert into biological membranes and lyse a broad spectrum of pathogens such as bacteria, viruses, fungi and protozoa. Moreover, they may play a role in inflammation and modulation of the immune response.

              The separation and purification of bioactive peptides which will involve development of automated and continuous systems is an important field for Food chemists. Much effort has been given to develop selective column chromatography methods that can replace batch methods of salting out, or using solvent extraction to isolate and purify bioactive peptides. Advances here will enable recovery of bioactive peptides with minimal destruction thus enabling utilization by returning these active peptides to functional food or specific nutraceutical applications.