Bioactive milk peptides: a prospectus

Interactions between bovine beta-lactoglobulin and peptides under different physicochemical conditions

The aim of this study was to determine if peptides could interact with beta-lactoglobulin (beta-LG) and what the physicochemical conditions promoting their interaction with the protein are. The binding of negatively charged (beta-LG 125-135 and 130-135), positively charged (beta-LG 69-83 and 146-149), and hydrophobic (alphaS1-CN 23-34 and beta-LG 102-105, both bioactive peptides) peptides to bovine beta-LG was determined using an ultrafiltration method under different physicochemical conditions: pH 3.0, 6.8, and 8.0; buffers of 0.05 and 0.1 M; 4, 25, and 40 degrees C; beta-LG/peptide ratios of 1:5 and 1:10. At pH 3.0, none of the peptides interacted with beta-LG at any temperature, buffer molarity, or beta-LG/peptide ratio probably due to electrostatic repulsions between the highly protonated species. At pH 6.8 and 8.0, charged peptides beta-LG 130-135, 69-83, and 146-149 bound to beta-LG under some physicochemical conditions, possibly by nonspecific binding. However, both hydrophobic peptides probably bind to the inner cavity (beta-barrel) of beta-LG, provoking the release of materials absorbing at 214 nm. Given the known biological activities of the hydrophobic peptides used in this study (opioid and ACE-inhibitory activities), their binding to beta-LG may be relevant to a better understanding of the physiological function of the protein.

Intestinal transport of the lactokinin Ala-Leu-Pro-Met-His-Ile-Arg through a Caco-2 Bbe monolayer

ACE inhibitory peptides are biologically active peptides that play a role in blood pressure regulation. When derived from food proteins during food processing or gastrointestinal digestion, these peptides could function as efficient agents in treating and preventing hypertension. However, in order to exert an antihypertensive effect by inhibition of the ACE enzyme, they have to reach the bloodstream intact. The aim of this research was to assess if the known ACE inhibitory peptide Ala-Leu-Pro-Met-His-Ile-Arg, derived from a tryptic digest of beta-lactoglobulin, could be absorbed through a Caco-2 Bbe cell monolayer in an Ussing chamber and reach the serosal side undegraded. Samples of the mucosal compartment showed high ACE inhibitory activity. No or only little ACE inhibitory activity was detected in the serosal compartment. However, when the serosal sample was concentrated three-fold, a substantial ACE inhibitory activity was registered. Concomitantly, HPLC and MS clearly showed the presence of Ala-Leu-Pro-Met-His-Ile-Arg in the mucosal compartment, whereas in the serosal compartment only MS was able to detect the heptapeptide. In conclusion. under the observed experimental conditions, the ACE inhibitory peptide Ala-Leu-Pro-Met-His-Ile-Arg was transported intact through the Caco-2 Bbe monolayer, but in concentrations too low to exert an ACE inhibitory activity.

Fractionation of beta-lactoglobulin tryptic peptides by ampholyte-free isoelectric focusing

Solutions of tryptic hydrolysate of bovine beta-lactoglobulin were fractionated by liquid-phase IEF in a preparative Rotofor cell at constant power for 2 h without ampholytes in order to identify interactions between peptides. The 20 peptide fractions collected were analyzed by capillary electrophoresis and SDS-PAGE under native, denaturing, and reducing conditions. The hydrolysate was shown to be composed mainly of acidic peptides (pI 2-5, 62%) of molecular mass below 6 kDa, and numerous disulfide bonds were detected. Purified peptides (beta-LG 15-20, 71-75, 76-82, and 84-91) were also focused individually and in mixtures and matched to components of the IEF fractions obtained from the tryptic hydrolysate of beta-LG. The separation of acidic (beta-LG 84-91) and basic (beta-LG 76-82) peptides was achieved by IEF, whereas uncharged peptides (beta-LG 15-20 and 71-75) were poorly separated due to their low electrophoretic mobility. Because no peptide-peptide interaction could be identified by IEF fractionation, it is suggested that electrical fields may decrease electrostatic interactions between charged peptides.

The improvement of milk by cultivation with ciliates

The exceptional nutritional values of milk have long been recognized. However, important drawbacks of milk and dairy products are their relatively high cholesterol contents and their low proportion of polyunsaturated fatty acids. Hypersensitivity to milk proteins can also draw the public away from fluid milk consumption. The nonpathogenic ciliate Tetrahymena thermophila has a unique set of enzymes that converts cholesterol into provitamin D3 and related compounds. It also contains delta 12 fatty acid desaturases and releases various hydrolases, including proteases, lipases, and phospholipases. We investigated the potential use of these peculiar traits to improve milk by short-term incubation with live ciliate suspensions. We found that cholesterol can be reduced to 50% of its initial content in whole milk by a 12-h period of incubation with T. thermophila cells, mostly by conversion to delta 7-desaturated cholesterol derivatives, including provitamin D3. In addition, a considerable increase in gamma-linolenate (18:3, n-6) takes place. The protein profile is markedly changed by proteolysis, with a striking reduction of casein and enhancement in total amino acid content. Despite these changes, no marked alterations in flavor or odor were observed at the end of 12 h of incubation. Thus, Tetrahymena appeared ideally suited to correct for milk draw-backs, making this important source of nutrients more acceptable to the health-conscious public.

Gastric proteinase digestion of caseins in newborn pups of the mouse

Casein micelles of mouse milk consist of alpha-, beta-, gamma-, and kappa-caseins. By digestion with alkaline phosphatase, they were separated as an independent band by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE). The compositions of alpha-, beta-, gamma-, and kappa-caseins were 24.3, 25.1, 9.4, and 41.2% in colostrum, and 36.8, 15.6, 11.9, and 35.7% in mature milk, respectively. Zero-day-old pups were allowed to access either colostrum or mature milk, and the aggregated milk in the stomach was analyzed by SDS-PAGE. Caseins in colostrum were digested more rapidly and efficiently than those in mature milk. Among the seven peptides present in the aggregated caseins, four peptides were colostrum-specific and derived from alpha- and gamma-caseins. It was expected that colostrum-specific and soluble peptides were generated from alpha- and gamma-caseins through gastric proteinase digestion. Amino acid sequence analysis and the pH of the aggregated milk suggested that caseins in the stomach were digested by a chymotrypsin-like proteinase. Caseins in colostrum were different from those in mature milk, with respects to the casein composition as well as the gastric proteinase sensitivity. It is concluded that the lactating mice on the day of parturition supply particular caseins to their young.

The effects of α-lactabumin and whey protein concentrate on dry matter recovery, TCA soluble protein levels, and peptide distribution in the rat gastrointestinal tract

The effects of two dietary proteins on dry matter recovery, trichloroacetic acid (TCA) soluble protein concentration, and peptide distribution in gastrointestinal contents were investigated in rats trained to consume, in a single 2-hour daily meal, diets containing α-lactalbumin (α-LA) or whey protein concentrate (WPC) for two weeks. Compared with the WPC diet, the α-LA diet emptied faster from the stomach. Dry matter recovery was higher in the stomach contents of rats fed the WPC diet than in those given the α-LA diet, but dry matter content in the small intestine was comparable. TCA soluble protein levels in the stomach and the small intestinal contents were also significantly (P &lt 0.001) higher in rats fed the WPC diet. The concentration of peptides having molecular weights (MW) ranging from 12 500–30 000 daltons (Da) was higher in the stomach contents of rats fed the WPC diet. Conversely, the level of peptides ranging from 5000–12 500 Da was higher in the stomach contents of rats fed the α-LA diet. For both diets, the small intestinal contents were characterized by high levels of amino acids and small peptides. These results suggest that the hydrolysis and absorption of α-LA is faster than that of WPC.Key words: In vivo protein digestion, peptides absorption, whey protein concentrate, α-lactalbumin, stomach emptying.