Developmental and crosslinking factors related to low protein digestibility of sorghum grain
Abstract
The overall goal of this research was to elucidate the cause of poor protein digestibility in sorghum grain. Protein digestibility in sorghum decreases throughout seed development. Changes that occur in sorghum proteins during development were studied. Digestibility decrease was more strongly related to decrease in moisture content of the seed than developmental stage. However, artificial environment studies revealed that digestibility decrease during seed development cannot be attributed only to moisture loss. Highly digestible free protein body content decreased during development, while content of aggregated protein bodies, that exhibited decreased digestibility after cooking, increased. Quantitation of $\alpha$-, $\beta$-, and $\gamma$-kafirins revealed that free protein bodies are immature protein bodies. Kafirin quantitation revealed that $\gamma$- and $\beta$-kafirin synthesis is completed prior to $\alpha$-kafirin synthesis. This is a new view of sorghum protein body development and is in agreement with proposed models for maize protein body development. Sequential extraction of aggregated protein body fractions with various reducing agent concentrations showed that disulfide crosslinking increases throughout seed development. Protein digestibility in sorghum has been shown to vary from year to year and among genotypes. A high molecular weight (HMW) protein was found in our laboratory in sorghum and evidence indicates that it is associated with the kafirins. Quantitation of HMW protein in six samples with year-to-year differences in digestibility and in seven different genotypes revealed no observable relationship between amount of HMW protein and digestibility. Additionally, no relationship between digestibility and protein disulfide isomerase (PDI) content, an enzyme that catalyzes the formation of disulfide bonds, was found from quantitation of PDI in samples with year-to-year differences in digestibility. Although crosslinking of $\gamma$- and $\beta$-kafirins has been shown to reduce protein digestibility, year-to-year differences in digestibility could not be related to differences in $\alpha$-, $\beta$-, or $\gamma$-kafirin amount. Sequential extraction of the samples exhibiting year-to-year differences in digestibility with increasing concentrations of reducing agent did reveal a negative relationship between amount of easily-accessible $\gamma$-kafirin (extractable with 0.001% 2-mercaptoethanol) and digestibility. This indicates that digestibility may be a function of the amount of $\gamma$-kafirin disulfide crosslinks that can be easily accessed by digestive enzymes.
Degree
Ph.D.
Advisors
Nielsen, Purdue University.
Subject Area
Food science
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