The fractionation and analysis of lupin seed proteins with a detailed investigation of the albumins

Abstract

Seed proteins from Lupinus angustifolius cv Uniharvest were fractionated by a modified Osborne method into four solubility classes namely, albumin, globulin, prolamin and glutelin fractions. Protein content and amino acid composition of these fractions and the TCA-precipitable proteins from them were determined.

The albumin fraction (water soluble protein fraction) constituted a considerable proportion being about 37% of the total seed protein. The amino acid composition of the albumin fraction was nutritionally better than that of the globulin fraction, the major protein fraction of the seed. The albumin fraction contained a relatively high content of cysteine, methionine and tryptophan which are the first and second limiting amino acids of legume seed protein. This important fraction was therefore investigated in some detail.

Electrophoresis on polyacrylamide gradient gel (3.5 - 35%) showed a complex band pattern (up to 26 bands) for the albumin fraction compared with the other protein fractions. Sixteen proteins were purified from the albumin fraction by various combinations of methods which were ammonium sulphate fractionation, preparative column electrophoresis on polyacrylamide gel, adsorption chromatography on hydroxyapatite, ion-exchange chromatography on DEAE cellulose and micro-preparative polyacrylamide gel electrophoresis. Fourteen of the purified proteins were obtained with at least 90% purity as shown by polyacrylamide gradient gel electrophoresis (3.5 - 35%). All the proteins purified were analysed for amino acid composition and the major proteins were characterized by molecular weight.

Five proteins of low electrophoretic mobility (MW range 142,700 - 53,800) have amino acid compositions with similar features to those of lupin major globulins with a high content of glutamic acid and low content of methionine and cysteine. Three proteins, one of very low mobility A₄₀ -A₄₀ (MW 394,000) and two of high mobility (MWs 24,800 & 11,000) also have amino acid compositions with similar features to those reported for lupin minor globulins. The very low mobility protein, A₄₀ -A₄₀ has a very well balanced amino acid composition with a relatively high content of cysteine and methionine as well as many other essential amino acids. The two high mobility proteins (A₁₀₀-HA₆ and H₂T-E₂) have a very high content of cysteine but no methionine and low content of many other essential amino acids.

The remaining eight proteins of medium mobility have amino acid compositions typical of legume albumins with a relatively high content of lysine, threonine and valine but relatively low content of glutamic and aspartic acids. Three of these medium mobility proteins have a relatively high content of methionine but low content of cysteine.

The study was extended to five other lupin varieties which are L. angustifolius cv Fest, L. luteus, L. albus, L. pilosus and L. cosentinii where seed proteins were fractionated for comparison with cv Uniharvest. The protein profiles varied considerably among different species. The albumin fractions were the major fractions in L. luteus, L. pilosus and L. cosentinii. The albumin fraction of L. albus was also high whereas the albumin fraction of cv Fest was similar to that of cv Uniharvest. There were negative correlations between the albumin fraction and the globulin and glutelin fractions in all the varieties.

Among these lupin varieties, the amino acid compositions of the first water extracts (H₁T) were different whereas the compositions of the first NaCl extracts (N₁T) were quite similar, showing patterns typical of storage protein fractions. The H₁T of all lupin varieties contained a higher content of cysteine, methionine and to a lesser extent tryptophan than the N₁T. The first SDS extracts (S₁T), only a minor fraction, contained a relatively high proportion of most essential amino acids especially methionine compared with the two major extracts. In most cases, TCA-precipitable proteins from these extracts determined the essential amino acid compositions of the extracts except for cysteine in both H₁T and N₁T and a few essential amino acids in the S₁T.

Electrophoretic patterns of the H₁T from five lupin species were compared with that of cv Uniharvest. The patterns were variable across the different species whereas the patterns of cv Fest and cv Uniharvest showed some species specificity.

Two steps of isolation and purification which are ammonium sulphate fractionation of the H₁T and ion-exchange chromatography on DEAE cellulose of the proteins precipitated at 60% ammonium sulphate saturation were carried out for all the varieties. The results of these are compared with the results from cv Uniharvest and discussed in relation to the type and proportion of the major protein fractionated and the usefulness of the techniques.