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A Rapid Protein Digestibility Assay for Identifying Highly Digestible Sorghum Lines¹

¹Paper 16120 of the Purdue University Agricultural Research Programs. ²Department of Food Science and the Whistler Center for Carbohydrate Research, Purdue University, West Lafayette, IN 47907-1160. ³Department of Agronomy, Purdue University. ⁴Corresponding author. Phone (765) 494-5886. Fax: (765) 494-7953. E-mail: hamakerb@foodsci.purdue.edu

Protein digestibility in sorghum (Sorghum bicolor (L.) Moench) lines was determined using two standard procedures (pepsin digestibility and pH-stat) and compared with a newly developed, rapid electrophoresis-based screening assay. The new assay was based on the rate of α-kafirin disappearance after pepsin digestion. α-Kafirin, the major sorghum storage protein, makes up ≈60–70% of the total protein in the grain. In the new assay, samples were first digested with pepsin for 1 hr, and undigested proteins were then analyzed by SDS-PAGE. The intensitizes of the undigested α-kafirin bands were measured. Higher band intensity indicated lower protein digestibility. The new assay was significantly correlated with the standard pepsin digestibility assay (r = −0.96, n = 16) after which it was patterned. The same was true of the pH-stat procedure (r = −0.85, n = 16). This implies that the new assay is comparable to existing procedures and can be used for screening sorghum lines for protein digestibility. Two groups consisting of high-protein digestibility and wild-type sorghum lines were identified when the new assay was tested on 48 sorghum lines derived from crosses of wild-type and mutant high protein digestibility lines, indicating that the new assay was efficient in differentiating between the two groups. Advantages of the new assay over the standard procedures include considerable reduction in analysis time and sample size required for the analysis. For example, analysis time was reduced by 20% and sample size by 10% when the new assay was used as compared with the pH-stat procedure. We estimate that ≈60 sorghum lines can be screened in a day by a single operator using the new assay.