Physico-chemical Properties of Purified Isoforms of the 12S Seed Globulin from Mustard Seed (Brassica alba)

Abstract

The 12S mustard seed globulin from Brassica alba was isolated from a variety of extraction solutions (i.e., 1.0m NaCl, pH 7.5 (μ = 1.0); 32.6/2.6 mm KH₂PO₄/K₂HPO₄, pH 7.5, containing 0.4m NaCl (μ = 0.5) and distilled water, pH 7.5, μ = 0. Gel filtration chromatography of the crude globulin from the various extracts revealed two heterogeneous forms of the globulin; i.e., a polymerized (490kDa) and non-polymerized (304kDa) form. Further purification of the non-polymerized form by anion-exchange chromatography indicated the presence of two isoforms. Characterization of one of these isoforms (i.e., isoform B) from the various extractions indicated the following: pI of 7.10, thermal denaturation temperature in the range (89.73–90.16°C), identical amino acid composition that was particularly high in glutamic/glutamine residues, and a similar subunit composition. Isoform B globulins from an extraction with a high ionic strength solution had overall comparable lower negatively and higher positively charged surfaces above and below their pIs, respectively; similar secondary and tertiary structures; and similar solubility and spectroturbidity profiles compared to those globulins extracted with distilled water. The isoform B globulins from all three extractions were found to be cryoprecipitable at low temperatures.

Key words:

• extraction
• purification
• globulin
• mustard
• physico-chemical properties