624 F. GONZALEZ-MUJICA ET AL. Copyright © 2005 John Wiley & Sons, Ltd. Phytother. Res. 19, 624–627 (2005) Copyright © 2005 John Wiley & Sons, Ltd. PHYTOTHERAPY RESEARCH Phytother. Res. 19, 624– 627 (2005) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ptr.1704 Inhibition of Hepatic Neoglucogenesis and Glucose-6-Phosphatase by Quercetin 3-O- α α α- (2″-galloyl)rhamnoside Isolated From Bauhinia megalandra Leaves Freddy Gonzalez-Mujica 1 *, Norma Motta 1 , Omar Estrada 2 , Elsa Perdomo 2 , Jeannette Méndez 2 and Masahisa Hasegawa 1 Sección de Bioquímica Médica, Instituto de Medicina Experimental, Facultad de Medicina, Universidad Central de Venezuela. Caracas, Venezuela 2 Laboratorio de Productos Naturales, Centro de Química Orgánica, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela In intact microsomes, quercetin 3-O- α α α-(2″-galloyl)rhamnoside (QGR) inhibits glucose-6-phosphatase (G-6- Pase) in a concentration-dependent manner. QGR increased the G-6-Pase K m for glucose-6-phosphate with- out change in the V max . The flavonol did not change the kinetic parameters of disrupted microsomal G-6-Pase or intact or disrupted microsomal G-6-Pase pyrophosphatase (PPase) activity. This result allowed the conclu- sion that QGR competitively inhibits the glucose-6-phosphate (G-6-P) transporter (T1) without affecting the catalytic subunit or the phosphate/pyrophosphate transporter (T2) of the G-6-Pase system. QGR strongly inhibits the neoglucogenic capacity of rat liver slices incubated in a Krebs-Ringer bicarbonate buffer, supplemented with lactate and oleate saturated albumin. The QGR G-6-Pase inhibition might explain the decrease in the liver slice neoglucogenic capacity and, in turn, could reduce glucose levels in diabetic patients. Copyright © 2005 John Wiley & Sons, Ltd. Keywords: neoglucogenesis; glucose-6-phosphatase; quercetin 3-O-α-(2″-galloyl)rhamnoside; Bauhinia; diabetes. Received 20 September 2003 Accepted 2 February 2005 *Correspondence to: Dr F. Gonzalez-Mujica, Sección de Bioquímica Médica, Instituto de Medicina Experimental, Facultad de Medicina, Universidad Central de Venezuela, Apartado postal 50587, Sabana Grande, Caracas, Venezuela. E-mail: gonzalef@latinmail.com Contract/grant sponsor: Consejo de Desarrollo Científico y Humanístico de la Universidad Central de Venezuela; Contract/grant number: 09-33- 4788-00; 09-33-4706-00; 03-12-4422-99. called GLUT 7. Due to the fact that G-6-Pase is a membrane bound enzyme, it shows latency that refers to that portion of the total intrinsic enzymatic activity which is not manifest unless the enzyme preparation is disrupted prior to assay (Nordlie, 1979). The inhibition of the G-6-P phosphohydrolase act- ivity of the G-6-Pase might reduce the endogenous glucose production and, in consequence, be useful in the control of the hyperglycaemia present in diabetes (McCormack et al., 2001). A synthetic analogue of chlorogenic acid has been shown to specifically inhibit T1 and its possible therapeutic use in non-insulin- dependent diabetes has been suggested (Herling et al., 1999). Recently, Waltner-Law et al. (2002) showed that the flavonoid epigallocatechin gallate, a constituent of green tea, represses hepatic glucose production. In Venezuela several plants are used as traditional medicines in the empirical treatment of diabetes. One of them is the Bauhinia species that has been reported to produce hypoglycaemia in rabbits (Roman-Ramos et al., 1992) and to inhibit hepatic neoglucogenesis and G-6-Pase activity (Gonzalez-Mujica et al., 1998). The accompanying paper (Estrada et al., 2005) re- ports QGR to be one of the most powerful flavonoids purified from Bauhinia megalandra leaves that inhibits the microsomal G-6-Pase enzyme. This paper reports the effects of QGR on the rat liver microsomal G-6-Pase and PPase kinetic parameters and on the neoglucogenic activity of rat liver slices, using lactate as the substrate. INTRODUCTION G-6-Pase is the enzyme (EC 3.1.3.9) that catalyses the last step of gluconeogenesis and glycogenolysis (Ashmore and Weber, 1959). The enzyme is located in the endoplasmic reticulum (ER) and nuclear envelopes of liver, kidney and pancreatic islet cells. In mammals the ER enzyme has been described as a multicomponent system (Burchell and Waddell, 1991; Waddell and Burchell, 1991). The catalytic subunit faces the lumen and is able to hydrolyse G-6-P, mannose-6-phosphate (M-6-P) and pyrophosphate (PPi), among other sub- strates, and is bound to a stabilizing protein; a trans- porter protein called T1, which is specific for G-6-P, allows it to cross the ER membrane. The products of the enzyme reaction, phosphate (Pi) and glucose, cross the ER membrane mediated by the T2 and T3 trans- porters, respectively. T2 is complex and is not only able to transport Pi but also able to transport PPi. T3 is a member of the glucose transport family and it has been