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 inves tigated in uncooked and cooked sorghum flour. Enzyme-linked immunosorb ent assay revealed that when the untreated flour was incubated with pe psin, 31.0% of alpha-kafirin, 15.3% of beta-kafirin, and 13.5% of gamm a-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 prote in bodies were largely reduced in size. Cooking the flour reduced the digestibility of all three kafirins, particularly of the beta and gamm a fractions. Some cooked protein bodies appeared unaffected by digesti on as revealed by TEM. Sodium bisulfite reversed the effect of cooking , although not completely. It appears that on cooking, disulfide linka ge formation is enhanced among beta- and gamma-kafirins, or among thes e and matrix proteins, which delays alpha-kafirin digestion. A reducin g agent enhances the digestion of these cross-linked proteins and faci litates the exposure of alpha-kafirin to the enzyme.