Relationships of Polyphenol Content with Antioxidant Activity and Key Enzyme Activities Related to Polyphenol Metabolism in Peeled Kernels of Juglans sigillata Dode cv. ‘Qianhe 7’

Abstract

Peeled iron walnut kernels (Juglans sigillata Dode cv. ‘Qianhe 7’) were studied for the changes in the contents of total phenols, total flavonoids and monomeric phenols and the activities of key enzymes associated with phenolic metabolism during the development process and their relationship with antioxidant activity in vitro. The results showed that the contents of total phenols and total flavonoids and antioxidant activity in iron walnut kernels increased during the development process and reached the maximum level at maturation. The contents of total phenols and total flavonoids were 0.05 mg/g and 0.03 mg/g, respectively in the mature kernels, the ferric ion reducing antioxidant power (FRAP) was 0.019 mmol/g, and the free radical scavenging capacities against 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radical were 43.57 and 16.86 mg/mL, respectively. Eight monomeric phenolics were identified in the kernels, and their contents varied greatly during the development process and decreased in general at the late stage of maturity; among these phenolics, the average content of syringic acid was the highest, followed by catechin, and the level of ferulic acid was the lowest. The contents of total phenols, gallic acid, caffeic acid, ferulic acid and quercetin were positively correlated with the FRAP value, while the contents of catechin, eugenic acid and rutin were positively correlated with the DPPH radical and ABTS cation radical scavenging capacities. The metabolic enzyme activities including chorismate mutase (CM), chorismate synthase (CS) and phenylalanine ammonia lyase (PAL) reached the highest levels at the kernel filling stage, while cinnamate 4-hydroxylase (C4H) activity showed an N-shaped trend and reached the maximum value at the late stage of maturity. The activities of 4-coumarate coenzyme A ligase (4CL), chalcone synthase (CHS) and chalcone isomerase (CHI) decreased rapidly and then increased slightly at the maturity stage. Total phenol, gallic acid, caffeic acid, ferulic acid and quercetin were positively correlated with CM, PAL and C4H, but negatively correlated with CS, 4CL, CHS and CHI. Syringic acid and rutin were positively correlated with CS. In conclusion, the polyphenols and antioxidant activity of peeled ‘Qianhe 7’ iron walnut kernels were higher during the ripening stage. This study provides a theoretical basis for the development of functional foods based on iron walnut kernels.

Keywords

• juglans sigillata dode; kernels; polyphenols; antioxidant activities; enzymes related to phenolic metabolism