Resolubilization of Protein from Water-Insoluble Phlorotannin–Protein Complexes upon Acidification

Marine phlorotannins (PhT) from Laminaria digitata might protect feed proteins from ruminal digestion by formation of insoluble non-covalent tannin-protein complexes at rumen pH (6-7). Formation and disintegration of PhT-protein complexes was studied with β-casein (random coil) and bovine serum albumin (BSA, globular) at various pH. PhT had similar binding affinity for β-casein and BSA as pentagalloyl glucose, as studied by fluorescence quenching. The affinity of PhT for both proteins was independent of pH (3.0, 6.0, and 8.0). In the presence of PhT, the pH range for precipitation of tannin-protein complexes widened to 0.5-1.5 pH units around the isoelectric point (pI) of the protein. Complete protein resolubilization from insoluble PhT-protein complexes was achieved at pH 7 and 2 for β-casein and BSA, respectively. It was demonstrated that PhT modulate the solubility of proteins at neutral pH and that resolubilization of PhT-protein complexes at pH deviating from pI is mainly governed by the charge state of the protein.