In vitro protein digestibility in grain sorghum: effect of genotype and interaction with starch digestibility

Improvement of nutritive value is one of the main goals of sorghum breeding. It is known that one of the reasons of relatively poor nutritive value of sorghum grain is resistance of its seed storage proteins (kafirins) to protease digestion that also affects digestibility of starch. To study genetic aspects and interdependency of these traits we investigated in vitro protein and starch digestibility of the flour of 12 grain sorghum lines and six F-1 hybrids. Comparison of SDS-PAGE spectra of total grain proteins before and after pepsin digestion revealed that the F-1 hybrids had significantly lower indices of protein digestibility than parental lines, with the exception of the F-1 hybrid M35-1A KB/KVV-45, which retained high level of protein digestibility typical for KVV-45 line. The level of starch digestibility in the F-1 hybrids corresponded to its level in maternal lines suggesting importance of maternal genotype in determination of this trait. It was found that starch digestion by amylolytic enzymes increased the amount of protein in individual kafirin fractions, and reduced the amount of high molecular weight proteins as well it reduced subsequent kafirin digestion by pepsin and caused formation of polypeptide (M-r similar to 45 kDa), perhaps, the kafirin dimer, resistant to pepsin digestion. These data are of importance for fundamental investigation of factors influencing kafirins and starch interactions in sorghum endosperm and their digestibility.