Dual Enzymatic Approach For Producing Slowly Digested Dextrin And Its Effects On The Glucolipid Metabolism Of Mice

Slowly digestible dextrin(SDD)is a kind of starch derivative,which is slowly digested throughout the entire small intestine and causes attenuated postprandial glycemic response.SDD has been regarded as desirable for the prevention and management of several chronic diseases,such as obesity and type 2 diabetes.As a typical glycosyl transferase for preparing SDD,1,4-α-glucan branching enzyme(GBE,EC 2.4.1.18)is known to catalyze the hydrolysis of α-1,4 glycosidic linkages in starch molecules,followed by the reattachment of the cleaved fragment via a new α-1,6 linkage.In the present study,a two-step modification process successively catalyzed by 1,4-α-glucan branching enzymes(GBEs)from Rhodothermus obamensis STB05(Ro-GBE)and Geobacillus thermoglucosidans STB02(Gt-GBE)was investigated as a strategy to produce SDD.The quality of product was regulated by optimizing modification conditions.In addition,the fine structure was investigated to explore the synergy between Gt-GBE and Ro-GBE.Furthermore,effects of SDD on the glucolipid metabolism of diabetic mice were analyzed.Related research results might offer a novel strategy for the efficient preparation of SDD and provide a theoretical basis for regulating products quality.Firstly,a dual GBE modification process was established on the basis of a two-step procedure.Granular and gelatinized corn starch were successively modified by Ro-GBE and Gt-GBE.This strategy has taken the respective catalytic advantage of the two GBEs,thereby enhancing the reaction efficiency.As compared with the previous Gt-GBE-catalyzed modification,the dual GBE modification process reduced the reaction time by 9.5 h.As compared with single GBE modification with the same duration,the dual GBE modification process significantly improved the slowly digestion property of product.Secondly,the quality of SDD was regulated by changing the substrate concentration,reaction time and Ro-GBE addition.When the substrate concentration increased to 20%(w/w),the rapidly digestible starch(RDS)content of the product reduced to 70.4%,which was 24.4% lower than that of normal corn starch(NCS).Moreover,the higher substrate concentration enhanced the production efficiency of slowly digested dextrin.In addition,the reaction time and the Ro-GBE addition greatly impacted the digestibility and structure of SDD.When the Ro-GBE treatment time prolonged from 0 h to 4 h or the Ro-GBE addition increased from 10 U/g to 30 U/g,branching density of modified produce increased at first,and then became stable.Additionally,the weight-average molecular mass and average chain length decreased,while the relative proportion of external chains and RDS content decreased at first,and then increased.Conversely,the slowly digestible starch(SDS)content increased at first and then decreased.After a 0.5 h Ro-GBE(25 U/g)treatment followed by a 10 h Gt-GBE(25 U/g)treatment,the product showed the lowest in vitro digestibility.The RDS content reduced to 66.6 %,which was 31.2% lower than that seen in NCS,while the SDS and resistant starch contents were 8.8-fold and 7.1-fold greater,respectively.Furthermore,Ro/Gt significantly reduced the postprandial blood glucose fluctuation as detected in ICR mice.The maximum postprandial blood glucose was 45.2%(p <0.0001)lower,as compared with the normal maltodextrin with a similar weight-average molecular mass.Subsequently,the mechanism of synergy between the two GBEs through the analysis and comparison of the particle structure and molecular structure of NCS and its GBE-modified starch.Compared with Gt-GBE,Ro-GBE enhanced modification efficiency on granular starch,and the ratio of A-chains increased(p <0.05).This structural provided more acceptor chains for the following Gt-GBE treatment,which promoted the transglycosylation efficiency of Gt-GBE and retarded digestion of starch.The dual GBE modified product contained compact and highly branched inner chains,more abundant short external chains than those in single Ro-GBE or Gt-GBE modified product.Thirdly,the synergy between Gt-GBE and Ro-GBE was investigated by comparing the granular and molecular structure of native and GBE-modified starch.As compared with Gt-GBE,Ro-GBE more efficiently modified granular NCS and increased the ratio of A-chains of NCS(p <0.05).This process created more acceptor chains for the following Gt-GBE treatment,which promoted the transglycosylation efficiency of Gt-GBE and inhibited its hydrolysis activity.Thus,the dual GBE modified product contained more dense and branched internal chains,more abundant short external chains than those of single Ro-GBE or Gt-GBE modified product.Lastly,the effects of SDD as a dominant dietary carbohydrate on the glycolipid metabolism of type 2 diabetic mice were further investigated.After 10-week treatment,the SDD-containing diet significantly relieved the symptoms of type 2 diabetes.The polydipsia and polyphagia of type 2 diabetic mice were improved and the body weight loss inhibited.The fasting blood glucose and serum insulin content were 50.2% and 46.3% lower,respectively,than those of maltodextrin group.The blood lipid levels were also greatly reduced.Moreover,the pancreatic morphology,hepatic function,renal injury,and insulin resistance of SDD-fed mice were significantly repaired.