Opioid peptides derived from milk proteins

This paper is devoted to opioid peptides derived from milk proteins. The evolutionary aspect of the biological activity of proteins and peptides, protein precursors, amino acid sequences of opioid peptides, and their biological activity are presented herein. Attention was also paid to the relationship between the conformation of b-casomorphins, their hydrophobicity and their ability to interact with opioid receptors. The biological function of opioid peptides derived from milk proteins was shown from the nutritional and pharmaceutical points of view. In this context, the gastrointestinal, analgesic, tolerance and dependence, respiratory, cardiovascular, immunomodulatory and allergic effects of opioid peptides are discussed. Finally, the perspectives of the application of opioid peptides derived from milk proteins in nutrition and pharmacy are also presented. EVOLUTIONARY ASPECT OF THE BIOLOGICAL ACTIVITY OF PEPTIDES DERIVED FROM FOOD PROTEINS Nutrition is an attribute of man’s life determining his phylo- and ontogenetic development. However, before life could evolve on Earth as biologically-active forms of organic molecules, compounds with specific physicochemical properties first had to develop. These were amino acids which led to proteins. Hence, amino acids can be considered as one of the primary nutrients for living organisms which were only later to come into existence. It seems logical that in the primary mixture of organic substances there were no proteins, but only various amino acid polymers which did not show any sense of purpose in their intermolecular structure; moreover, they lacked any functional properties. It was only in the course of evolution that amino acid polymers, phase-liberated from the environment, could interact in a manner that directed their structure to perform biological functions. The course of organic-molecule evolution towards biological activity was determined first by the appearance of the crystalline structure and the optical properties in the amorphic and racemic mixture of the organic compounds, a fact which is also confirmed by the structure of the amino acids themselves. The importance of amino acids in the evolution of biologically-active systems is partly supported by their geological supremacy over carbohydrates. Regarding evolution, it is worth mentioning that in the beginning a predominance developed of aliphatic amino acids over aromatic amino acids containing comparatively high quantities of sulphur. This is interesting, because the biological properties of peptides are mainly affected by their hydrophobic character and the presence of aromatic acids. As aliphatic amino acids predominated in the Precambrian era, it can be presumed that the biological activity of peptides occurred at a later stage. This biochemical evolution of peptides towards protein structures has been confirmed in research results from the areas of molecular biology, palaeobiochemistry and bioelectronics, which allow us to presume that the evolution of protein molecules was aimed at elongating the polypeptide chain, expanding the side chains, as well as developing aromatization, including closing the chain into a ring, and a secondary structure. However, not only was the chemical structure of the protein important in the evolution of life, but also the proteins’ electrical states, which determine their conformation. The phenomenon of protein amphotericity proves their adaptation for oxidoreductive fluctuations in the environment, while on the other hand, the cyclic structure of protein molecules enables them to save the so-called “delocalized electrons”. The cyclization of protein is considered to be another remarkable concept regarding life’s appearance, i.e. the closing of the polypeptide chain into a ring. The formation of such a protein structure offered the possibility of producing metal complexes, the so-called “chelate compounds”. Such a degree of protein molecule configuration could in some cases be sufficient to transfer enzymatic properties, which are the basis of biological activity in living organisms, to a protein. Hence, a question arises of the relationship between the structure of food proteins and their biological functions. It seems barely probable that the food proteins