Cereal Chem 72:115-121  |  VIEW ARTICLE

Gel Electrophoresis and Amino Acid Analysis of the Nonprotein Nitrogen Fractions of Defatted Soybean and Almond Meals.

W. J. Wolf. Copyright 1995 by the American Association of Cereal Chemists, Inc. 

Defatted soybean and almond meals were extracted with 0-5M trichloroacetic acid (TCA), and the extracts were analyzed for Kjeldahl nitrogen. Minimum extractability of nitrogenous compounds occurred at 0.4- 1.0M TCA and was assumed to represent the nonprotein nitrogen (NPN). Above 3M TCA, the nitrogen solubility increased rapidly as a result of solubilization of the bulk of the storage proteins. Analysis of 0.6M TCA extracts by sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed that the NPN fraction contained polypeptides of 12 kDa for soybean meal and 28, 12, 10, and 7 kDa for almond meal. These polypeptides were retained by 3.5 kDa cutoff dialysis membranes when the TCA extracts were dialyzed against water to remove the TCA and low molecular weight constituents. The polypeptides represent about 1-2% of the meal nitrogen. The NPN fractions were also characterized by amino acid analysis. The predominant free amino acids were arginine, aspartic, and glutamic acids for soybean meal, and aspartic acid, glutamic acid, proline, arginine, and alanine for almond meal. The polypeptides of almond meal NPN contained 65 mol% glycine, suggesting the presence of glycine-rich proteins that are structural elements of plant cell wall proteins.