Purification and properties of protein Z – a major albumin of barley endosperm

A major albumin of barley grain, called protein Z, has been purified from endosperm flour. Extraction with 0.05 M β-mercaptoethanol and successive use of (NH ₄)₂SO₄-precipitation, anion exchange at pH 7.5, cation exchange at pH 4.5, and anion exchange at pH 8.0 resulted in a highly pure protein as judged from various electrophoretic and immunoelectrophoretic tests. As protein Z is a major protein component of beer, antibodies towards a protein-rich beer fraction could be used to detect the protein during purification. Protein Z consists of at least four antigenically identical molecular forms with isoelectric points in the range 5.55–5.8, but same molecular mass near 40000. Dimer and, probably, tetramer forms were detected by gel filtration in the absence of reducing agents. Monospecific antibodies towards protein Z were prepared. Immunoelectrophoretic properties of the protein were not affected by treatment at pH 1–13 (30 min at 30°C) or up to 100°C (30 min at pH 7). Commonly grown barley varieties contained about 1.5–2.5 mg protein Z/g grain, but a much lower content (∼ 0.2 mg/g grain) was found in a few varieties. Like barley β-amylase, protein Z was present in both salt-extractable "free" (20–30%) and thiol-extractable "latent" (70–80%) forms in the grain. Protein Z contains 2 cysteine and 20 lysine residues per monomer molecule and is relatively rich in leucine and other hydrophobic residues. Protein Z may contribute up to 5% of the total grain lysine in normal varieties and more than 7% in some high-lysine barleys.