Improving Digestibility of Soy Flour by Reducing Disulfide Bonds with Thioredoxin

Abstract

The Kunitz trypsin inhibitor (KTI) and the Bowman−Birk inhibitor (BBI) of trypsin and chymotrypsin contain disulfide bonds. Glycinin, the major storage protein in soybeans also contains disulfide bonds. Treatment of soy white flour with a NADP−thioredoxin system (NTS) effectively reduced disulfide bonds in soy flour and increased protein digestibility by trypsin and pancreatin as measured by the pH stat method. Treatment of soy flour with NTS increased the digestibility compared to soy white flour by 29.3 and 60.6% for trypsin and pancreatin, respectively. NTS-treated soy flour had similar digestibility by trypsin to autoclaved soy flour and casein, but digestibility by pancreatin was less than autoclaved soy flour and casein. The degree of reduction by NTS was highly correlated to the degree of hydrolysis (DH) by trypsin (R 2 = 0.93) and pancreatin (R 2 = 0.99). The DH of NTS-treated soy flour by trypsin is reflective of both inactivation of trypsin inhibitors and overall protein digestibility while pancreatin hydrolysis is reflective of only overall protein digestibility. The Kunitz trypsin inhibitor (KTI) and the Bowman-Birk inhibitor (BBI) of trypsin and chymotrypsin contain disulfide bonds. Glycinin, the major storage protein in soybeans also contains disulfide bonds. Treatment of soy white flour with a NADP-thioredoxin system (NTS) effectively reduced disulfide bonds in soy flour and increased protein digestibility by trypsin and pancreatin as measured by the pH stat method. Treatment of soy flour with NTS increased the digestibility compared to soy white flour by 29.3 and 60.6% for trypsin and pancreatin, respectively. NTS-treated soy flour had similar digestibility by trypsin to autoclaved soy flour and casein, but digestibility by pancreatin was less than autoclaved soy flour and casein. The degree of reduction by NTS was highly correlated to the degree of hydrolysis (DH) by trypsin (R 2 ) 0.93) and pancreatin (R 2 ) 0.99). The DH of NTS-treated soy flour by trypsin is reflective of both inactivation of trypsin inhibitors and overall protein digestibility while pancreatin hydrolysis is reflective of only overall protein digestibility.

Structural aspects of legume proteins and nutraceutical properties

“…The protein fraction of plant foods with a high cysteine content, the "albumin fraction", according to classical Osborne classification, has been found to be quite resistant to heat denaturation and proteolytictrypsin, chymotrypsin and pepsin-digestion. Stability is conferred by the presence of a high number of disulfide bonds contained in low molecular weight (MW) proteins, such as BBI (Carbonaro, Marletta, & Carnovale, 1992;Clemente et al, 2000;Faris, Wang, & Wang, 2008). Unlike the closely related Kunitz trypsin inhibitor, soybean BBI is a small molecule of 8 kDa and seven disulfide bridges.…”

Increased In Vitro and In Vivo Digestibility of Soy Proteins by Chemical Modification of Disulfide Bonds

“…The sulfhydryl content of soy white flour was determined using a modified procedure initially described by Ellman [12] and Robyt et al [13]. Detailed procedure and sample pretreatment were discussed in our previous publication [14]. The crude protein content of soy white flour and SMBS-treated soy flour for animal feeding was determined by the micro Kjeldahl method with protein conversion factor of 6.25 [15].…”

Role of pulses in nutraceuticals

“…The protein fraction of pulses with a relatively high cysteine content, that is albumin (according to classical Osborne classification), has been found to be resistant to proteolytic digestion by trypsin, as a consequence of high stability conferred by a number of disulfide bonds in low molecular weight proteins (Marletta et al, 1992;Clemente et al, 2000;Faris et al, 2008).…”