Effects of UV treatment on the proline-linked pentose phosphate pathway for phenolics and L-DOPA synthesis in dark germinated Vicia faba

Abstract

Fava beans ( Vicia faba) have a high phenolic content. Antioxidant properties of phenolics are extensively studied for their ability to inhibit lipid oxidation in food and for improved health. This legume has the potential to be an effective dietary source of polyphenolics for health. The rationale behind this study was to observe the stimulation of nutraceutically relevant phenolic metabolites when germinating fava beans are exposed to ultra violet (UV) and the potential role of the proline-linked pentose phosphate pathway (PPP). Studies have shown phenolic phytochemicals can be stimulated with UV light. The objective was to over-express the PPP through the regulation of the key enzyme, glucose-6-phosphate dehydrogenase (G6PDH), for enhanced phenolics and L-DOPA production and investigate the link to proline metabolism. Guaiacol peroxidase (GPX) activity was monitored to understand the extent to which the UV-stimulated phenolics could be converted to polymeric phenols. L-DOPA is the precursor of dopamine (DA), the neuromodulator and used for treatment of Parkinson's disease. Seeds of fava beans were pretreated in water, exposed to 5, 10 and 15 h UV light and germinated for 8 days in dark. Total phenolics, proline levels and enzyme activities were measured spectrophotometrically. L-DOPA was measured by High-performance liquid chromatography (HPLC). A total of 15 h UV treatment stimulated L-DOPA on day 1, which reduced over next 8 days of germination. Antioxidant activity of the total phenolic acids was determined by inhibition of the β-carotene oxidation and there was stimulation of antioxidant activity in response to 10 h UV treatment on day 2. Total phenolic content increased 10-fold after 15 h UV light exposure on day 6 along with the highest activity of G6PDH. All treatments stimulated GPX activity indicating polymerization of free phenolics in the late stage of germination. Proline levels were also stimulated in response to all treatments. The stimulation of proline concurrent with stimulation of G6PDH and total phenolics in response to 15 h UV indicates a role of proline-linked PPP. Results indicate an increase in nutraceutically-relevant phenolic metabolites along with enhanced antioxidant activity in response to UV stress. This novel approach provides a mechanism to understand and enhance biosynthesis of important phenolic compounds in plant and legume systems for use in nutraceutical applications.