Wheat (Triticum aestivum L) seeds contain a proteinaceous α-amylase inhibitor. We estimated the amount of the inhibitor that can bind to α-amylase rather than the total amount of the inhibitor, using an ELISA method. During 48 h of imbibition, the amount of the inhibitor did not change, while α-amylase activity increased rapidly. These results suggest that α-amylase synthesized de novo does not bind to the inhibitor. To examine further the interaction between α-amylase and its inhibitor, we localized immunohistochemically both α-amylase and the inhibitor in germinating seeds. In the embryo, we observed α-amylase only in the scutellar epithelial cells at 24 h after the start of imbibition, and later in the scutellar parenchyma cells. The inhibitor, on the other hand, was present between the scutellar parenchyma cells of the embryo at 0 h and 24 h after the start of imbibition, and disappeared later. α-amylase is separated from the inhibitor in the embryo. In the endosperm, we detected α-amylase in the aleurone cells at 24 h after the start of imbibition and later in the starchy endosperm. The inhibitor was present in the aleurone cells and starchy endosperm of the dry seed. During imbibition, the inhibitor in the starchy endosperm disappeared from the area to which α-amylase was secreted. It appears that α-amylase in the starchy endosperm also does not interact with the inhibitor. The aleurone cells are the only place that α-amylase can come into contact with the inhibitor. But, early secretion of α-amylase from the aleurone cells after imbibition started, and rapid disappearance of the inhibitor in the aleurone cells suggest that the α-amylase inhibitor does not work as an effective inhibitor of the endogenous α-amylase synthesized following germination.

The present review assesses the potential of the Phaseolus vulgaris α-amylase inhibitor isoform 1 (α-AI1) starch blockers as a widely used remedy against obesity and diabetes. Consumption of the α-amylase inhibitor causes marginal intraluminal α-amylase activity facilitated by the inhibitor's appropriate structural, physico-chemical and functional properties. As a result there is decreased postprandial plasma hyperglycaemia and insulin levels, increased resistance of starch to digestion and increased activity of colorectal bacteria. The efficacy and safety of the amylase inhibitor extracts, however, depend on the processing and extraction techniques used. The extracts are potential ingredients in foods for increased carbohydrate tolerance in diabetics, decreased energy intake for reducing obesity and for increased resistant starch. Research developments in the distribution and biosynthesis of the α-amylase inhibitor, relevant physico-chemical properties, the molecular starch-blocking mechanism, anti-obesity and anti-diabetes effects, safety of extracts and the need for research into their potential anti-colorectal cancer effect are discussed.

A purified pancreatic α-amylase inhibitor (α-AI) from white beans (Phaseolus vulgaris) was administered orally (100m/g body weight dissolved in 9g NaC/) for 22d to non-diabetic (ND) and type 2 diabetic (neonatal diabetes models n0-STZ and n5-STZ) male Wistar rats. Mean glycaemia (mmo/) declined from day 4 of the α-AI administration in ND rats (5·48 (sem 0·08) v. 4·39 (sem 0·13); P<0·05), n0-STZ diabetic rats (7·94 (sem 0·42) v. 5·56 (sem 0·32); P<0·01) and n5-STZ diabetic rats (17·34 (sem 2·58) v. 11·93 (sem 1·96)), until the end of treatment: ND (5·22 (sem 0·21) v. 3·97 (sem 0·06); P<0·01); n0-STZ (8·10 (sem 0·19) v. 5·21 (sem 0·30); P<0·01); and n5-STZ (16·36 (sem 2·14) v. 7·69 (sem 1·34); P<0·01). There was a decrease in water intake (m/) in the α-AI-treated diabetic rats: n0-STZ (30 (sem 0·10) v. 22 (sem 1·50); P<0·01) and n5-STZ (76 (sem 5·04) v. 57 (sem 4·85); P<0·01). Food intake (/) decreased in all three groups: ND (23 (sem 0·31) v. 20 (sem 0·03); P<0·05); n0-STZ (22 (sem 0·55) v. 16 (sem 0·98); P<0·01); and n5-STZ (31 (sem 0·58) v. 23 (sem 1·20); P<0·01). The enterocyte sucrase and maltase activities (/ proteins) were high (P<0·01) in the untreated diabetic rats, n0-STZ (45 (sem 4) and 152 (sem 10), respectively) and n5-STZ (67 (sem 12) and 151 (sem 10), respectively) with respect to the ND rats (24 (sem 2) and 74 (sem 10), respectively). After α-AI treatment, enzyme activities declined in both diabetic rats, n0-STZ (21 (sem 2) and 85 (sem 11); P<0·01) and n5-STZ (28 (sem 7) and 75 (sem 19); P<0·05), to values close to those in the ND rats. In conclusion, α-AI significantly reduced glycaemia in both the ND and diabetic animals and reduced the intake of food and water, and normalized the elevated disaccharidase levels of the diabetic rats.

An inhibitor of α-amylase was isolated and purified from an extract of white kidney beans (Phaseolus vulgaris). The acute oral administration of the inhibitor (50 mg/kg body weight) to adult Wistar rats together with a starch load (2 g/kg body weight suspended in NaCl (9 g/l)) reduced the increase in glycaemia over the basal value (NaCl, 222 (SEM 49); inhibitor, 145 (SEM 16) mmol/l×180 min; P<0.05) without modifying the insulin response. On administering the inhibitor orally (50 mg/kg body weight dissolved in NaCl (9 g/l)) for 21 d to rats fed on a standard diet, a decline was observed in the glycaemia values on day 0 (NaCl, 5.53 (SEM 0.12); inhibitor, 5.25 (SEM 0.16) mmol/l) relative to those obtained on days 10 (NaCl, 5.00 (SEM 0.14); inhibitor, 4.60 (SEM 0.08) mmol/l; P<0.05) and 21 (NaCl, 5.22 (SEM 0.22); inhibitor, 4.50 (SEM 0.12) mmol/l; P<0.01) of treatment, without modifying the plasma concentration of insulin. There was found to be a significant anorexigenic action of the inhibitor; there was reduced food intake (NaCl, 23.07 (SEM 0.31); inhibitor, 19.50 (SEM 0.49) g/d; P<0.01), a reduced weight gain (NaCl, 52 (SEM 3); inhibitor, −1.33 (SEM 8.9) g/21 d; P<0.01), as well as changes in the activity of some intestinal enzymes such as maltase (NaCl, 87 (SEM 7); inhibitor, 127 (SEM 11) U/g proteins; P<0.05). The present study has shown, for the first time, that the prolonged administration of an α-amylase inhibitor reduces blood glucose levels and body-weight gain in Wistar rats.