Solubility of Bean Leaf Protein in Ethanol

Abstract

IT has been generally held that leaf proteins are insoluble in 80 per cent ethanol or similar solvents¹. This belief has furnished a useful means of distinguishing two forms of nitrogen, ‘soluble’ (that is, amino-acids and peptides) and ‘insoluble’ (that is, proteins and nucleic acids). We became aware of a possible discrepancy in this logic when an alcohol-soluble ‘peptide’ fraction of bean leaf displayed an amino-acid composition indistinguishable from the bulk protein. Further tests, which included the biuret² and Folin³ assays, trichloroacetic acid precipitability, and stainability with light green, indicated the protein-like nature of this ‘peptide’. The recovery of such presumed protein from fresh bean leaves was about 1 per cent of the total protein. When examining peptide metabolism, this error would be more or less serious depending on the amount of peptide present. The amount usually present in bean leaves was quite low judging by paper chromatographic methods⁴, hence the presence of ethanol-soluble protein was an unfortunate complication.