## 4 Discussion
The present study revealed that strain B734 was able to generate opioid peptide sequences (YPFP) from WPI. This tetrapeptide (known as morphiceptin, present in the enzymatic digest of bovine casein) is a selective ligand of the \(\upmu\)-opioid receptor. Furthermore, it has been indicated that the biopeptide exhibits considerable opioid potency and is a selective ligand of the \(\upmu\)-opioid receptor (MOR), inducing strong supraspinal antinociception through application directly to the central nervous system [33]. Moreover, the application of morphiceptin analogues in the control of tumor growth and cell proliferation is investigated, which is particularly important for the diagnosis and treatment of various cancers [67]. The present results correspond to findings described by [PERSON] et al. [61], who noted that some BAPs derived from WPI hydrolyzed by a strain of _Lb. heleticus_ (LH-2 CFSM) have multifunctional properties that may influence two or more physiological processes. An example of such a sequence is RELEELNVPGEIVESLSSSEESITR, i.e., a caseinophosphopeptide with mineral-binding, immunomodulatory [40; 41], and anti-caries activities [68; 69]. The mineral-binding bioactivity of this peptide has been confirmed in in vivo studies in an animal model (rats) and in humans [68; 70]. This BAP was identified in the WPI hydrolyses obtained with the use of strain B734 and in samples supplemented with the reference strain (DSMZ 20075). Interestingly, the VQVTSTAV sequence with antihypertensive and antimicrobial activities (present in WPI hydrolyses obtained using the reference strain and in samples with _Lb. helveticus_ B734) was also detected by [PERSON] et al. [61] in unfermented (control) samples of WPI, which indicates the activity of endogenous milk proteases (plasmin, elastase, and cathepsin D, B, and G) and the ability to form some BAPs [71]. Moreover, efficient antimicrobial effects of this peptide (at a concentration of 5 mM) have been confirmed against _Staphylococcus carnosus_ CECT 231, _Serratia marcescens_ CECT 854, and _Escherichia coli_ ATCC 25922, whereas the strongest antibacterial activity was noted toward _Listeria innocua_ CECT 910T [35]. This antibacterial peptide was also detected in water-soluble extracts of the Pecorino Romano and Canestrato Pugliese cheeses [72]. It is worth mentioning that _Lb. helveticus_ is considered the most proteolytic species in the genus _Lactobacillus_[73; 74]. These lactic acid bacteria are involved in the manufacture of long-ripening cheeses, and the proteolytic system of these microorganisms influences the processes of protein hydrolysis contributing to development of characteristics of cheees (including texture, taste, and flavor) and affect generation of BAPs. The present results indicate that certain sequences were released by only one of the tested _Lb. helveticus_ strains (DSMZ 20057 or B734), suggesting that the ability to generate BAPs (in terms of the amount of different sequences) from the selected protein matrix is a strain-dependent feature. This may imply that, depending on the _Lb. helveticus_ strain used, it is possible to obtain hydrolyses of the protein preparation with different BAP profiles and various functional properties. This is supported by results described by [PERSON] et al. [75], who revealed that whey fermented by _Lactobacillus brevis_ exhibited higher ACE inhibitory activity than other samples, which were fermented by other bacteria (_Lb. acidophilus, Lb. plantarum, Lb. casei, Lb. helveticus, Lb. lactis, Lb. paracasei, Lb. reuteri,_ and _Lb. biferentants_). Similar observations were reported by [PERSON] et al. [61], i.e., depending on the microorganisms, WPI hydrolyses differed in the number and amino acid sequences of generated BAPs. Moreover, 19 sequences associated with several categories of bioactivity (ACE inhibitory, opioid, antimicrobial, and antioxidant activities) were detected in hydrolyses obtained by _Lb. helveticus_ LH-2 CFSM, whereas _L.