Green tea is widely consumed in Asian countries, while black tea is most popular in Western countries. The manufacturing process of green tea differs from that of black tea because freshly picked young leaves of the tea are immediately steamed. This process destroys the enzymes responsible for breaking down the color pigments in the leaves and allows the tea to maintain its green color during the subsequent rolling and drying processes. The amounts of constituent compounds are slightly different from those of black tea. Pharmacological studies using constituent compounds in green tea have been recently reviewed by Kaszkin et al. [10
]. Green tea extracts are more stable than pure epigallocatechin gallate, the major constituents of green tea, because of the presence of other antioxidant constituents in the extract [10
]. In general, herbal medicines are complex mixtures of different compounds that often act in a synergistic fashion and exert their full beneficial effect as total extracts [11
In the present study, we demonstrated that green tea produces an antihyperglycemic effect without affecting insulin secretion in STZ-diabetic mice. We therefore explore the mechanism underlying the green tea effect by investigating the serum protein profiles of db+/db+ mice, a genetic model of type 2 diabetes, using SELDI-TOF-MS. First, we performed a preliminary analysis to determine the peaks (molecular weights) of biomarker proteins that were abnormally contained in the serum of diabetic mice, without identifying individual serum proteins. As a result, we found that the levels of several serum proteins were significantly altered in the diabetic state. Secondly, we investigated which marker proteins are affected by green tea administration. Despite the changes in the levels of several serum proteins after green tea administration, none of the protein peaks specific for diabetes were sensitive to the administration, except for a peak at m/z 4211(4212). The level of this 4211(4212) Da protein was reduced both in the diabetic state and by green tea administration. Thus, no simple reversed effect of green tea on the diabetes-induced modifications of serum protein levels was observed.
The 4211(4212) Da protein has not yet been identified, but only two candidate groups of vertebrate proteins are found by scanning Swiss-Prot database (molecular weight, 4211; molecular weight ranges, 0.1% according to the standard errors of the current MS analysis; pI, 6; pI ranges, 10): pancreatic polypeptide (PP) (primary accession numbers P13083, P37999, P41519, P38000, P11967) and antibacterial peptides [β-defensin C7 precursor (018815), 4 kDa defensin (P56686), cryptidine-5 precursor (P28312), antibacterial peptide BMAP-34 precursor (P56425)]. PP is primarily expressed in the endocrine cells of the pancreas, and the plasma PP concentrations are elevated by food intake [12
]. Peripheral PP administration results in a reduction in food intake and an increase in energy expenditure [12
]. Interestingly, a previous study revealed that PP-containing cell populations in the islets of Langerhans are reduced in db+/db+ mice with mild hyperinsulinemic diabetes [13
], which is consistent with the observed reduction in the intensity of the peak at m/z 4211(4212) in the diabetic state. We speculate that the reduction of BGL by green tea causes a decrease in serum PP concentrations as a means of maintaining energy homeostasis, although more precise studies are required.
The changes in serum protein profiles by green tea also demonstrate the increase in the peak intensities of hemoglobin-related multi-MS signals, suggesting the adverse side effects of green tea, although blood samples from db+/db+ mice tended to exhibit a hemolytic feature compared with those from wild-type mice (data not shown). Interestingly, the hemoglobin-related multi-MS signals shown in Fig. may include hemoglobin α-chains and β-chains [9
], their dimers, and possibly variously glycated hemoglobins. Hemoglobin A1c is used as a marker of diabetes in clinical diagnostic tests. Until now, however, the extent of glycation per hemoglobin that practically occurred is not clear. More detailed analyses of serum protein profiles using SELDI-TOF-MS will provide a more useful clinical index of the diabetic state.
We observed that green tea improved oral glucose tolerance in humans. It is therefore likely that green tea is prophylactic against diabetes and ameliorates diabetic hyperglycemia. Green tea consumption at moderate doses may be associated with a reduced risk of type 2 diabetes in apparently healthy individuals by controlling postprandial hyperglycemia.