Date of Award

2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Pharmaceutical Sciences

Department

Biomedical and Pharmaceutical Sciences

First Advisor

Navindra P. Seeram

Abstract

This study investigated the phytochemical constituents, primarily gallotannins, present in a proprietary extract, namely MaplifaTM, from leaves of the red maple (Acer rubrum L.) species as well as their biological activities and mechanisms of action. Although the red maple species has been traditionally used as folk medicine by Native American Indians for numerous health benefits, the bioactive chemical constituents of the leaves of the red maple still remain unknown. This study carried out the identification of phytochemicals targeting gallotannins, a class of polyphenols, from red maple leaves by using various chromatographic separation techniques and spectroscopic approaches. Furthermore, compounds from the red maple species were evaluated for bioactivities including anti-oxidant, anti-diabetic (α-glucosidase enzyme inhibition and inhibition of advanced glycation end products, AGEs) and skin lightening effects (anti-tyrosinase and anti-melanogenesis) by using a panel of in-house bioassays. In addition, the mechanisms of the inhibitory effects of red maple phytochemicals on α-glucosidase were characterized by using kinetic assay, fluorescence assay, FTIR and CD (circular dichroism) spectra. Moreover, the mechanisms of inhibition of the gallotannins on the formation of AGEs were elucidated at each individual stage of AGEs formation by applying MALDI-TOF mass spectroscopy, HPLC-FL analysis, G.K. peptide assays, and assessing the conformational changes induced by protein glycation by using CD.

Plant derived natural products have served as an important resource for medicinal compounds over decades. The red maple is an indigenous plant species of eastern North America and was extensively used as an herbal remedy by the indigenous peoples of eastern North American region for many ailments including abdominal pain, diarrhea and eyes diseases.

Recent studies have shown that extracts of red maple plant parts exhibit free radical scavenging capacities and this antioxidant property was associated with its polyphenolic content. Red maple extracts have also been investigated for their antimicrobial and anticancer effects. In addition, our group has recently demonstrated that a red maple bark extract was able to decrease blood glucose levels in mice after sucrose loading indicating the presence of bioactive compounds that could be relevant to diabetes management. Furthermore, our laboratory has recently isolated a number of new gallotannins, named maplexins A-I, from the stem and bark of the red maple species and has shown that these compounds display potent α-glucosidase inhibitory activity in vitro. As the total polyphenolic content level in red maple leaves is higher than in bark, it is logical to investigate the chemical constituents of red maple leaves for gallotannins which could also serve as α-glucosidase inhibitors. Therefore, we propose a phytochemical study to develop a proprietary extract from red maple leaves and to isolate and identify phenolic compounds therein and evaluate these compounds for their in vitro antioxidant and anti-diabetic activities. This study also proposes to elucidate the mechanisms of inhibitory effects of maple gallotannins on the α-glucosidase enzyme by using various biophysical tools. Understanding the mechanisms of inhibitory action of these compounds will be critical for the further development of α-glucosidase inhibitors from maple for diabetes management.

Beside the inhibitory effects on the α-glucosidase enzyme, the red maple gallotannins were also evaluated for their inhibitory effects on the formation of advanced glycation end products (AGEs), another therapeutic target of diabetes. AGEs are a heterogeneous group of irreversible adducts from non-enzymatic glucose-protein condensation reactions. It has been reported that AGEs as oxidative derivatives are increasingly being implicated as a potential risk for diabetes. Numerous studies have demonstrated that phenolics with anti-oxidative capacity from fruits or vegetables could effectively interrupt the condensation reaction of glucose and proteins and further decrease the formation of AGEs. Therefore, we hypothesize that gallotannins from red maple species might act as antioxidants and inhibit the formation of AGEs.

In addition to anti-diabetic activities, the skin lightening effects of red maple gallotannins, as a treatment strategy for hyperpigmentary disorders, were also evaluated in this study. Hyperpigmentary disorders, such as melasma, age spot, freckles and lentigies, are caused by the accumulation of abnormal melanin. In the melanin biosynthetic pathway, tyrosinase is an exclusive enzyme responsible for the formation of melanins. Emerging data suggests that several plant-derived constituents may act as natural tyrosinase inhibitors which are of interest to the food (as anti-browning agents) and cosmetic (as skin lightening agents) industries. Therefore, we also evaluated the anti-tyrosinase and anti-melanogenic properties of gallotannins isolated from red maple leaves.

In Manuscript I: the objective was to develop a proprietary standardized red maple leaf extract, named MaplifaTM, for potential nutraceutical and/or cosmeceutical applications. In addition, the total phenolic content of MaplifaTM, was determined as gallic acid equivalents (GAEs) and the major gallotannins present in MaplifaTM were identified as maplexin B, ginnalin B, ginnalin C, ginnalin A, maplexin F and a pair of isomers, 6-O-digalloyl-2-O-galloyl-1,5-anhydro-D-glucitol and 2-O-digalloyl-6-O-galloyl-1,5-anhydro-D-glucitol. The quantitative HPLC-UV analysis revealed that ginnalin A was the predominant gallotannin in MaplifaTM (56.3% by dried weight). Furthermore, MaplifaTM was evaluated for various bioactivities including anti-oxidant, anti-tyrosinase, anti-α-glucosidase and anti-AGEs. MaplifaTM showed potent anti-oxidant activity in the DPPH assay with an IC50 of 78.2 ppm. Also, MaplifaTM showed inhibitory effects on the tyrosinase and α-glucosidase enzymes (IC50 = 154.5 ppm and 37.8 ppm, respectively). Lastly, MaplifaTM displayed inhibitory activity against the formation of AGEs (IC50 = 8.6 ppm), which was 10-fold more potent than aminoguanidine as the positive control. Findings from this study suggest that MaplifaTM could have potential cosmetic (for e.g. skin-whitening/lightening) and/or nutraceutical (for e.g. anti-diabetic) applications but further in vivo studies would be required to confirm this.

In Manuscript II, as our previous phytochemical studies on red maple (Acer rubrum) species lead to the identification of a series of maple gallotannins as potent α-glucosidase inhibitors, we aimed to provide insights into the ligand-enzyme interactions and the binding mechanisms of the maple gallotannins and the α-glucosidase enzyme. The inhibitory effects of four gallotannins from red maple species, namely ginnalin A, ginnalin B, ginnalin C and maplexin F, as well as a synthetic derived gallotannin, namely maplexin J, were evaluated against α-glucosidase. The inhibitory effects of the gallotannins against the α-glucosidase enzyme were determined as a noncompetitive mode. The interactions between the gallotannins and α-glucosidase were further elucidated by using spectroscopic means. The results revealed that the gallotannins inhibited α-glucosidase by forming stable ligand-enzyme complexes which consequently lead to the quenching of their intrinsic fluorescence statically. In addition, a fluorescent probe 1,1′-bis(4-anilino-5 napththalenesulfonic acid (bis-ANS), was used to explore the binding regions of the gallotannins. Lastly, FTIR and CD data showed that the gallotannins bound to α-glucosidase and induced conformational changes to the enzyme. These findings were further discussed in the context of structure activity relationship (SAR) for the potential of developing structural gallotannin analogs as potent α-glucosidase inhibitors.

In Manuscript III, the objective of this study was to evaluate the inhibitory effects of a series of maple gallotannins, namely ginnalins A-C and maplexins F and J, at individual stage of protein glycation using a combination of analytical methods including fluorescence spectroscopy, HPLC-FL, CD, and MALDI-TOF. Both early and middle stages of protein glycation inhibition by the maple gallotannins was determined by using the BSA-fructose assay. In this assay, maplexin F showed the most potent inhibitory effect against the formation of AGEs with an IC50 value of 15.8 μM. In addition, MALDI-TOF analysis revealed that the maple gallotannins were able to reduce the number of fructose that adducted to BSA protein indicating that they were able to inhibit the production of Amadori products. Moreover, the maple gallotannins were effective in the G.K. peptide-ribose assay suggested that the protein cross-linking formation at the late stage of glycation could be prevented by maple gallotannins. Lastly, the spectroscopy analyses revealed that the maple gallotannins were able to reduce the conformational changes of BSA protein that were induced by glycation.

In MANUSCRIPT IV, we initiated a project to investigate the cosmetic skin lightening/whitening applications of MaplifaTM, a proprietary extract purified from the leaves of the red maple species, in enzyme and cell based assays. MaplifaTM is standardized to ca. 45-50% of ginnalin A along with other gallotannins including ginnalin B and C and maplexins. SAR studies showed that increasing the number of galloyl groups attached to the 1,5-anhydro-D-glucitol moiety resulted in greater inhibitory effects on the tyrosinase enzyme. Consequently we synthesized maplexin J, atetragalloyl-glucitol (contains the maximum number of 4 galloyl groups on the1,5-anhydro-D-glucitol core) and confirmed our SAR observations. Purified ginnalins A-C, as representative gallotannins in MaplifaTM, were assayed for their inhibitory effects on melanin production in murine B16F10 cells. Ginnalin A (contains 2 galloyl groups) clearly reduced the melanin content at 50 μM whereas ginnalin B and C (contain 1 galloyl group of each) showed only minor anti-melanogenic effects. Lastly, the mechanisms of the inhibitory effects of ginnalins A-C on melanogenesis in B16F10 cells were elucidated by using real-time PCR and Western blot experiments. The results indicated that ginnalins were able to down-regulate the expression of MITF, TYR, TRP-1 and TRP-2 gene levels in a time and dose-dependent manner and significantly reduce the protein expression of TRP-2 gene. The findings in our study indicate that phytochemicals in red maple leaves possess anti-melanogenic effects and thus may have potential cosmetic skin-whitening applications.

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