Cruciferous Oilseed Proteins: the Protein Bodies of Sinapis alba Seed

Abstract

The solubility properties of the proteins of oil-free meal of white mustard seed (S. alba) in various aqueous extraction media are described. Electrophoresis on cellulose acetate of a salt extract of the seed meal at pH 7.0 shows the presence of two positively charged protein bands: a slow moving intense band (I) and a less intense band with higher mobility (II). On the basis of Sephadex G100 chromatography and sedimentation behaviour, these bands are deemed to be identical with the two major protein classes (12 S and 1.7 S, respectively) present in this and other Brassica-related species, as described by other workers. Centrifugation after filtration of a seed meal homogenate yields a preparation that is completely soluble in salt solution, and can be shown by electron microscopy to consist entirely of protein body fragments. Only the 12 S protein can be detected in significant quantity in this preparation: this protein at least we may assume to be present in the aleurone (protein) grains observed in micrographs of the cotyledon cells. In germinating seeds, disappearance of protein bodies is accompanied by a diminution in total salt-soluble protein and in the amounts of the 12 S and 1.7 S proteins, supporting their identification as storage proteins. The rate of utilization is the same in the light and in the dark. Proteolytic activity was detected in the ungerminated seed. The level of activity was more than sufficient to account for the subsequent observed rate of protein utilization. Proteolytic activity per seed increased by only 40-70% during 4 days germination.