Thesis (MSc (Biochemistry))--University of Limpopo (Medunsa Campus), 2010. / Background: Clausena anisata (C. anisata) (Willd) Hook [Family: Rutaceae] is one of the South African indigenous medicinal plant whose blood glucose lowering effect has been demonstrated in animal models of diabetes mellitus. However, the mechanism(s) by which extracts of these plants exert their blood glucose lowering effect have not been investigated Objectives of the study: The objectives of the current study were to investigate in vitro inhibitory effects of different C. anisata leaf extracts on the activities of human urinary α-amylase, Bacillus stearothermophilus α-glucosidase and rat hepatic glucose 6-phosphatase and to characterize the enzyme inhibitory effect of the extracts on these enzymes with respect to the mode of inhibition (type of inhibition) and IC50 values (measure of inhibition potency). Another objective of the study was to confirm the enzyme inhibitory effects of C. anisata in vivo by measuring postprandial glucose levels in alloxan induced diabetic rats after oral administration of sucrose. Results: Aqueous and methanolic extracts of C. anisata leaves inhibited human urinary α-amylase with IC50 values of 1947 ± 50 and 2436 ± 62 µg/ml respectively. Inhibitions of α-amylase by these extracts were however, significantly less than that of the reference drug acarbose (84 ± 11 µg/ml) (P < 0.001). The mode of inhibition of these extracts on human urinary α-amylase appears to be a reversible non-competitive one. Acetone and hexane extracts of C. anisata inhibited Bacillus stearothermophilus α-glucosidase with IC50 values of 1020 ± 32 and 2068 ± 59 µg/ml respectively. Inhibition of Bacillus stearothermophilus α-glucosidase by these two extracts was also significantly less than that produced by acarbose (36 ± 11 µg/ml) (P < 0.001). The mode of inhibition of Bacillus stearothermophilus α-glucosidase by the acetone extract was found to be reversible competitive in this case. Aqueous and methanolic extracts of C. anisata leaves also inhibited hepatic glucose 6-phosphatase with IC50 values of 493.6 ± 12 and 1012 ± 58 µg/ml respectively. In this case the effect of the aqueous extract was significantly less than that of the reference inhibitor of glucose 6-phosphatase, sodium vanadate (1651 ± 46 µg/ml) (P < 0.05). The mode of inhibition of glucose 6-phosphatase by C. anisata aqueous leaf extract was found to be irreversible. Furthermore, oral administration of C. anisata acetone leaf extract to normal and alloxan induced diabetic rats, 30 minutes before sucrose loading failed to prevent the rise in postprandial glucose levels in these animals. Conclusions: Although both C. anisata aqueous and hexane leaf extract inhibited human urinary α-amylase and Bacillus stearothermophilus α-glucosidase in vitro, this does not appear to be the case in vivo. C. anisata aqueous leaf extract appeared to be a more potent inhibitor of rat hepatic glucose 6-phosphatase than the known inhibitor of the catalytic subunit of this multi-component enzyme system.