The role of specific endosperm proteins in low protein digestibility of sorghum

Maria P Oria, Purdue University

Abstract

The overall goal of this project was to obtain a better understanding of the basis for the low protein digestibility of sorghum grain. In the first study sorghum P721N was harvested at selected days after half-bloom and the relationship between protein digestibility and $\alpha$-, $\beta$-, and $\gamma$-kafirin contents and unextractable disulfide bonded complexes was analyzed. Kafirin synthesis was complete by 40 DAHB. After this, disulfide crosslinking occurred in $\beta$- and $\gamma$-kafirin, concomitantly with a large drop in protein digestibility. Digestibility of cooked flour dropped markedly beginning at 35 DAHB. The second study showed the susceptibility of each kafirin to pepsin digestibility. $\alpha$-Kafirin was the least digestible among kafirins. Cooking the flour reduced the digestibility particularly of $\beta$- and $\gamma$-kafirins. When uncooked and cooked flour was treated with a reducing agent, kafirins were much more digestible. In the third study a highly digestible mutant showed a unique protein body microstructure with invaginations that reached the center. $\gamma$-Kafirin was localized in discrete dark staining regions at the base of the folds, in contrast with the peripheral location of this kafirin in normal sorghum. A last study was conducted to characterize a high molecular weight protein that is believed to be related to low protein digestibility. Its solubility, amino acid composition, lack of crossreactivity with kafirins antisera, pI, and N-terminal sequence classified it as a non-kafirin. It was located in a dark staining peripheral ring of the protein body, where $\gamma$-kafirin is also found. In conclusion, the maturation of the grain leads to disulfide bonded complexes, particularly in the $\beta$- and $\gamma$-kafirins. These complexes would exist in the peripheral regions of protein bodies that are resistant to pepsin digestion especially after cooking. A non-kafirin HMW protein located in the outer regions of the protein body may also be associated with the low protein digestibility, although this needs further studies. This resistant periphery retards the digestion of $\alpha$-kafirin in the interior. However, in the case of the highly digestible mutant genotype the large invaginations and the location of $\gamma$-kafirin in discrete regions rather than at the periphery may facilitate enzymatic breakdown.

Degree

Ph.D.

Advisors

Hamaker, Purdue University.

Subject Area

Food science

COinS