Analysis of Lupinus albus storage proteins by two-dimensional electrophoresis and mass spectrometry

A laboratory-prepared total protein extract (TPE) and a lupin protein isolate (LPI-E) produced in a pilot plant were submitted to a detailed two-dimensional (2DE) proteomic investigation. Recent findings have indicated that in an established rodent model of hyperlipidemia, moderate daily intakes of LPI-Es lead to a reduction of total and low-density lipoprotein cholesterol levels, and the knowledge of the actual composition of the protein sample used in that study is at the basis of further structure/action investigations. The experimental results indicate that the semi-industrial procedure used for the production of LPI-E damages only marginally the proteins. It does, however, cleave some disulfide bridges and induce mild proteolysis, as confirmed by the higher number of resolved protein spots in the low Mr and acidic pI region of the 2DE map. Out of 72 spots submitted to mass spectrometry and compared with available protein databases, 42 correspond to fragments of beta-conglutin, the 7S globulin of lupin, spanning between positions 37 and 495 of the protein sequence. Using the bioinformatic tool BlastP, these peptides were compared to the alpha'-subunit of beta-conglycinin, the 7S globulin of soybean, this being the most active hypocholesterolemic component of soybean protein, as shown by in vitro and in vivo experiments. At least 18 peptides derived from beta-conglutin, having a percentage identity higher than 50% and a similarity percentage higher than 70% vs the alpha'-subunit of beta-conglycinin, are likely candidates to be the biologically active components of lupin protein.