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In Vitro Generation and Stability of the Lactokinin ß-Lactoglobulin Fragment (142–148)

¹ Department of Life Sciences, University of Limerick, Ireland
² INRA-Laboratoire d’Immuno-Allergie Alimentaire, 91191 Gif Sur Yvette, France
³ Department of Clinical Pharmacology and Therapeutics, Ninewells Hospital and Medical School, University of Dundee, Scotland
⁴ Federal Research Centre of Nutrition and Food, Institute of Dairy Chemistry and Technology, Kiel, Germany

Corresponding author: R. J. FitzGerald; e-mail: dick.fitzgerald{at}ul.ie.

The objectives of this study were to investigate the generation of ß-lactoglobulin fragment (142–148) (ß-LG f(142–148) during the hydrolysis of whey proteins, and the in vitro stability of this fragment upon incubation with gastrointestinal and serum proteinases and peptidases. An enzyme immunoassay (EIA) protocol was developed for the quantification of ß-LG f(142–148) in whey protein hydrolysates and in human blood serum. The minimum detection limit was 3 ng/mL. The level of the peptide in whey protein hydrolysates was influenced by the degree of hydrolysis (DH). As expected, highest levels of this peptide were found in hydrolysates generated with trypsin. Sequential incubation of hydrolysates at different DH values with pepsin and Corolase PP, to simulate gastrointestinal digestion, generally resulted in the degradation of ß-LG f(142–148) as determined by EIA. Reversed-phase HPLC and angiotensin-I-converting enzyme (ACE) activity assays demonstrated that synthetic ß-LG f(142–148) was rapidly degraded upon incubation with human serum. Furthermore, ß-LG f(142–148) could not be detected by EIA in the sera of 2 human volunteers following its oral ingestion or in sera from these volunteers subsequently spiked with ß-LG f(142–148). These in vitro results indicate that ß-LG f(142–148) is probably not sufficiently stable to gastrointestinal and serum proteinases and peptidases to act as an hypotensive agent in humans following oral ingestion. The in vitro methodology described herein has general application in evaluating the hypotensive potential of food protein-derived ACE inhibitory peptides.

Key Words: lactokinin • whey protein • angiotensin-I-converting enzyme • hypertension

Abbreviation key: ACE = angiotensin-I-converting enzyme, B/B₀ (%) = bound activity in the presence or absence of competitor, respectively, ß-LG = ß-lactoglobulin, BLGWP = ß-lactoglobulin-enriched whey protein fraction, ß-LG f(142–148) = peptide fragment corresponding to residues 142 to 148 of bovine ß-lactoglobulin, DH = degree of hydrolysis, EIA = enzyme immunoassay, FAPGG = furanacryloyl-L-phenylalanylglycylglycine, IC₅₀ = concentration of inhibitory substance which reduces ACE activity by 50%, SGID = simulated gastrointestinal digestion, TPCK = L-1-tosylamide-2-phenylethyl chloromethyl ketone, WPC75 = whey protein concentrate 75

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