Peptic hydrolysis of ovine beta-lactoglobulin and alpha-lactalbumin - Exceptional susceptibility of native ovine beta-lactoglobulin to pepsinolysis

Ovine beta-lactoglobulin (BLG, a mixture of variants A and B at a ratio of 46/54) and -lactalbumin (ALA) were subjected to pepsin activity. The degree of peptic hydrolysis of native whole BLG reached 63\%, 74\%, 82\% and 87\% after 2, 4, 8 and 20 h hydrolysis, respectively. BLG variant B was degraded completely after 2 It of pepsin digestion while variant A was degraded gradually showing 19\%, 44\%, 61\% and 73\% hydrolysis after 2, 4,alpha 8 and 20 h, respectively. The main factors responsible for the exceptional pepsin susceptibility of ovine BLG are the slightly different tertiary structure of ovine BLG (compared with bovine BLG) as perceived from near circular dichroism spectra at pH 2, and its higher surface hydrophobicity, as demonstrated by a higher binding activity to 1-anilinonaphthalene-8-sulphonate. Reversed phase-high performance liquid chromatograms (RP-HPLC) profiles of the peptic hydrolysates of BLG showed the production of hydrophobic peptides at the early stages of hydrolysis, while more hydrophilic peptides appeared only at a later stage of hydrolysis. Mass spectroscopy analysis allowed the characterisation of 17 and 13 peptides after 2 and 20 h hydrolysis, respectively. Most of the enzyme activity was oriented first towards the N-terminal part of the molecule and later towards the C-terminal part of the protein; little or no activity was observed in the central region of the molecule even after 20 h hydrolysis. Native ovine ALA was almost completely degraded by pepsin, yielding 93\%, 94\%, 95\% and 98\% hydrolysis after 2, 4, 8 and 24 h, respectively. The RP-HPLC profile of the ALA hydrolysate showed 5 major hydrophobic peptides and 7 minor more hydrophilic peptides, which did not change with the time of hydrolysis. (C) 2004 Elsevier Ltd. All rights reserved.