Comparison of different extractive procedures for proteins from the edible seaweeds Ulva rigida and Ulva rotundata

Proteins have been extracted from the edible seaweeds Ulva rigida Agardh and Ulva rotundata Bliding using classical or enzymatic procedures. The protocols using NaOH under reductive conditions or a two-phase system (PEG/K2CO3) produced the best protein yields. The cleavage or the limitation of the linkages between proteins and polysaccharides caused by these experimental conditions probably explains the efficiency of these protocols. In SDS PAGE, the protein fraction obtained after NaOH extraction from U. rotundata is characterised by the presence of three major bands with apparent molecular weights of 45 600, 31 800 and 18 600. The protein fraction from U. rigida presents two specific bands with apparent molecular weights of about 27 000 and 12 000. These fractions are mainly rich in aspartic and glutamic acids, alanine, glycine and contain few hydroxyproline residues (0.91-2.44% total amino acid content). The use of cellulase does not significantly improve the extraction of algal proteins in comparison with the blank procedure (without enzymes). The weak accessibility of the substrates in the intact cell wall could explain these experimental data. The improvement of protein yield after the use of the polysaccharidase mixture (beta-glucanase, hemicellulase, cellulase) partially confirms this hypothesis.