Abstract
A quantitative analytical method for five aporphine alkaloids, nuciferine (1), nornuciferine (2), N-methylasimilobine (3), asimilobine (4), and pronuciferine (5), and five benzylisoquinoline alkaloids, armepavine (6), norarmepavine (7), N-methylcoclaurine (8), coclaurine (9), and norjuziphine (10), identified as the constituents responsible for the melanogenesis inhibitory activity of the extracts of lotus flowers (the flower buds of Nelumbo nucifera), has been developed using liquid chromatography-mass spectrometry. The optimum conditions for separation and detection of these 10 alkaloids were achieved on a πNAP column, a reversed-phase column with naphthylethyl group-bonded silica packing material, with CH3CN–0.2% aqueous acetic acid as the mobile phase and using mass spectrometry equipped with a positive-mode electrospray ionization source. According to the protocol established, distributions of these 10 alkaloids in the petal, receptacle, and stamen parts, which were separated from the whole flower, were examined. As expected, excellent correlations were observed between the total alkaloid content and melanogenesis inhibitory activity. Among the active alkaloids, nornuciferine (2) was found to give a carbamate salt (2′′) via formation of an unstable carbamic acid (2′) by absorption of carbon dioxide from the air.
Highlights
A Nymphaeaceae plant Nelumbo nucifera Gaertn. is extensively cultivated in Eastern Asian countries [1,2,3]
Dried flower buds of N. nucifera were extracted with methanol under reflux to obtain a methanol extract
To clarify the mechanisms of action of these active constituents, we examined the effects of the principal alkaloids (1, 2, 6, 7, and 9) on expression of mRNAs for tyrosinase, tyrosinase-related proteins (TRPs)-1, and TRP-2 in B16 melanoma 4A5 cells
Summary
A Nymphaeaceae plant Nelumbo nucifera Gaertn. (common name “lotus” in English) is extensively cultivated in Eastern Asian countries [1,2,3]. (common name “lotus” in English) is extensively cultivated in Eastern Asian countries [1,2,3]. All parts of this plant, including the leaves, stamens, flowers, seeds, and rhizomes, have been used as traditional medicines or vegetables for thousands of years [2,3,4]. In the course of our studies on the bioactive constituents from the flower buds of N. nucifera, we have isolated several alkaloids, e.g., nuciferine (1), nornuciferine (2), N-methylasimilobine (3), asimilobine (4), pronuciferine (5), and armepavine (6), with melanogenesis inhibitory activities in theophylline-stimulated murine B16 melanoma 4A5 cells [2]. We propose a simple, rapid, and precise analytical method for liquid chromatography-mass spectrometry (LC-MS) simultaneous quantitative determination of five aporphine alkaloids (1–5) and five benzylisoquinoline alkaloids, (6), norarmepavine (7), N-methylcoclaurine (8), coclaurine (9), and norjuziphine (10), using a one-step sample preparation procedure
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