Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2005
Publication Date: 8/10/2005
Citation: Krishnan, H.B., Bennett, J.O., Kim, W., Krishnan, A.H., Mawhinney, T.P. 2005. Nitrogen lowers the sulfur amino acid content of soybean (glycine max [l.] merr.) by regulating the accumulation of bowman-birk protease inhibitor. Journal of Agricultural and Food Chemistry. 53(16):6347-6354. Interpretive Summary: Soybeans provide 70% of the world's vegetable protein. Although the protein is considered to be of excellent quality, it is deficient with respect to swine and poultry nutrition in the two sulfur containing amino acids, cysteine and methionine. Rations for these animals must be supplemented with costly synthetic methionine to obtain optimal growth. Selective breeding of soybeans has culminated in an increase in the total seed protein but whether there has been a concomitant increase in the sulfur amino acids has not been established. In this study, we have demonstrated that there is a negative relationship between the total protein and sulfur amino acid content. Seeds with high protein had low amounts of sulfur amino acids. Bowman-Birk protease inhibitor, a protein that is especially rich in the sulfur amino acids, was found to be the major contributor to the overall sulfur amino acid content of soybean. Increasing the accumulation of this sulfur rich protein would benefit soybean farmers by providing them with a marketable value-added trait. Additionally, the increased soybean protein quality would make unnecessary the purchase of expensive supplemental amino acids for the formulation of swine and poultry rations.
Technical Abstract: Soybeans in general contain 35 to 40% protein. Efforts are underway to further increase this protein content, thus enhancing the nutritive value. Even though higher protein is a desirable characteristic, whether such an increase will be accompanied by enhanced protein quality is not known. Soybean protein quality could be significantly improved by increasing the concentration of the sulfur-containing amino acids, cysteine and methionine. To ascertain if a relationship existed between protein quantity and quality, a comparison of the amino acids of soybeans differing in protein content was made. Soybeans with higher protein content had significantly lower percentage of sulfur amino acids, while those with lower protein exhibited a higher content of total cysteine and methionine. Nitrogen application elevated the protein content but lowered that of the sulfur amino acids. Transmission electron microscopy examination of thin sections of low protein soybean seeds revealed several protein storage vacuoles that were partially filled with storage proteins. Fluorescence two-dimensional difference gel electrophoresis of soybean seed proteins revealed nitrogen application favored the accumulation of beta-subunit of beta-conglycinin, while decreasing the accumulation of Bowman-Birk protease inhibitor (BBI), a protein rich in cysteine. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of 60% isopropanol-extracted proteins showed a drastic reduction in the accumulation of BBI with increasing protein content. Northern blot analysis indicated that nitrogen had a negative influence on the expression of the BBI gene. Our results indicate that the negative correlation between total protein and sulfur amino acid content is mostly mediated by the differential accumulation of BBI.