Inhibitory activities of extracts from several kinds of seaweeds and phlorotannins from the brown alga Ecklonia stolonifera on glucose-mediated protein damage and rat lens aldose reductase

Diabetic complications, including retinopathy, neuropathy, nephropathy and cataracts, are the leading causes of death in diabetics. Also, an increasing population of diabetic patients has triggered the increasing publicity for treatments and prevention of diabetic complications throughout the world. There are direct evidences indicating the relevance of advanced glycation endproducts (AGE) and aldose reductase (AR) in diabetic complications, both of which are highly implicated in hyperglycemia and oxidative stress. In chronic hyperglycemia, there are evidences of increasing AGE and overexpression of AGE receptors (RAGE), as well as AR-related polyol pathway flux. Glycation, a non-enzymatic Maillard reaction between reducing sugars and proteins, is related to the pathogenesis of diabetic complications. In the later stages of the Maillard reaction, the products undergo conversion to reactive dicarbonyl intermediates to form AGE, which have properties such as cross-linking fluorescence that include pentosidine and crosslines. Excess glucose is metabolized into sorbitol by the NADP+-AR pathway, followed by NAD+-sorbitol dehydrogenase induced fructose formation, which is highly related to AGE formation. In other pathways, glyceraldehyde-3phosphate is degenerated by transition metal ions and oxygen (autoxidation and glycoxidation). Due to the different steps to form the end-products, the development of AGE inhibitors has approached several targets, such as AR inhibition and antioxidant activity. There is growing interest in the benefits of seaweeds as nutraceuticals, which have been promoted by the present work on the inhibitory effects of seaweeds on diabetic complications. The objectives of this present study were to investigate the inhibitory effects of several seaweeds, as well as Ecklonia stolonifera and isolated phlorotannins, on AGE and RLAR. Furthermore, the study took into account the possible-mechanism involved, such as an oxidative stress-mediated pathway, as well as the development of therapeutic or preventing agents for diabetic complications and related diseases. The assay for the inhibition of AGE formation was as follows: To prepare the AGE reaction solution, 10 mg/mL of bovine serum albumin (Sigma, St. Louis, MO, USA) in 50 mM sodium phosphate buffer (pH 7.4), with 0.02% sodium azide to prevent bacterial growth, was added to 0.2 M fructose and 0.2 M glucose. The reaction mixture (950 mL) was then mixed with various concentrations of the samples (50 mL, f.c. 200, 100, 50 mg/mL) dissolved in 10% dimethyl sulfoxide. After incubating at 37°C for 7 days, the fluorescence intensity of the reaction products was determined on a spectrofluorometric detector (FLx800 microplate fluorescence reader; Bio-Tek Instrument Inc., Winooski, VT, USA) with excitation and emission wavelengths at 350 nm and 450 nm, respectively. The inhibition percentage of AGE formation *Corresponding author: Tel: 82-51-620-6335. Fax: 82-51-620-6330. Email: choijs@pknu.ac. kr Received 18 September 2007. Accepted 15 January 2008. FISHERIES SCIENCE 2008; 74: 1363–1365