RESISTANCE OF SORGHUM ALPHA-KAFIRINS, BETA-KAFIRINS, AND GAMMA-KAFIRINS TO PEPSIN DIGESTION

The differences in pepsin digestibility among alpha-, beta-, and gamma-kafirins and the effect of treatment with sodium bisulfite were investigated in uncooked and cooked sorghum flour. Enzyme-linked immunosorbent assay revealed that when the untreated flour was incubated with pepsin, 31.0% of alpha-kafirin, 15.3% of beta-kafirin, and 13.5% of gamma-kafirin remained undigested. Transmission electron microscopy (TEM) showed that protein body digestion was initiated at the surface where beta- and gamma-kafirins are located. When the flour was treated with sodium bisulfite, the kafirins in the residue were much less and protein bodies were largely reduced in size. Cooking the flour reduced the digestibility of all three kafirins, particularly of the beta and gamma fractions. Some cooked protein bodies appeared unaffected by digestion as revealed by TEM. Sodium bisulfite reversed the effect of cooking, although not completely. It appears that on cooking, disulfide linkage formation is enhanced among beta- and gamma-kafirins, or among these and matrix proteins, which delays alpha-kafirin digestion. A reducing agent enhances the digestion of these cross-linked proteins and facilitates the exposure of alpha-kafirin to the enzyme.