Stimulation of phenolics, antioxidant and antimicrobial activities in dark germinated mung bean sprouts in response to peptide and phytochemical elicitors

Abstract

The phenyl propanoid pathway (PPP) was stimulated in mung bean sprouts through the pentose phosphate and shikimate pathways, by natural elicitors such as fish protein hydrolysates (FPH), lactoferrin (LF) and oregano extract (OE). Elicitation significantly improved the phenolic, antioxidant and antimicrobial properties of mung bean sprouts. The optimal elicitor concentrations were 1 ml/l FPH, 250 ppm LF and 1 ml/l OE for the highest phenolic content that was approximately 20, 35 and 18% higher than control, respectively, on day 1 of dark germination. The antioxidant activity estimated by β-carotene assay in mung bean sprouts was highest on day 1 of germination for all treatments and control. In general, higher antioxidant activity was observed in the elicited sprouts compared with control. In the case of 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay the antioxidant activity for all treatments and control was highest on day 2. Among the different elicitor treatments, OE elicited mung bean sprouts showed the highest antioxidant activity of 49% DPPH inhibition on day 2. This increased activity correlates with high guaiacol peroxidase (GPX) activity indicating that the polymerizing phenolics required during lignification with growth have antioxidant function. For all elicitor treatments a higher glucose-6-phosphate dehydrogenase (G6PDH) activity was observed during early germination following the high phenolic content. This is due to the general mobilization of carbohydrates to the growing sprouts in response to elicitation. In general the GPX activity steadily increased with germination for treatments and control. The higher phenolics produced on day 1 was utilized for GPX-mediated polymerization to form polymeric phenolics and lignin required during germination. The late stage polymerization linked to GPX activity preceded stimulation of G6PDH. This indicated that as phenolics were polymerized by GPX in late stages, G6PDH linked precursors such as NADPH 2 and sugar phosphates were being made available. Antimicrobial activity against Helicobacter pylori was observed in the mung bean sprout extract from control, LF and OE treatments from the day 1 stage. Both the LF and OE elicited extracts showed high antimicrobial activity, which correlated to high antioxidant activity on day 1. The higher antimicrobial activity was also observed with the higher stimulation of G6PDH and GPX activity during early stages of germination. This leads to the hypothesis that enhanced mobilization of carbohydrates (as indicated by G6PDH activity on days 2 and 4), enhanced polymerization of simple phenols (as indicated by GPX activity on day 3) contributed to high antioxidant activity producing intermediary metabolites (day 2).