A catechol-type lignan and neolignans are specifically present in the seed coat of tung trees

Abstract

The tung tree produces large seeds with approximately 40–50% of their composition consisting of non-edible oil. α-Eleostearic acid makes up about 80% of tung oil, while linoleic acid and oleic acid make up 7 and 6%, respectively. The oil is readily oxidized due to the three, conjugated double bonds in α-eleostearic acid; however, it is stable inside the tung seed, which is expected to contain strong antioxidative compounds that protect the oil from oxidation. Previously, we isolated and identified a catechol-type lignan (3,3′-bisdemethylpinoresinol) and neolignans (isoamericanol A and americanol A) from the seeds of tung tree fruits that exhibited strong antioxidative activity, similar to that of Trolox. In this study, we show that the catechol-type lignan and neolignans are specifically present in the seed coat of the tung tree fruit seeds. The lignans were not detected in any other tissue, such as the pericarp, bark, leaves, or wood of the tree. The lignan content was determined in each developmental stage of the fruit, and the amount of the three lignans in the tung seeds was the highest in the green mature stage. This was likely because the amount of tung oil was also at its highest during this stage, and the oil composition was stable. Our results indicate that the catechol-type lignan and neolignans are located within the seed coat of the tung tree fruit seeds, where they protect the oil in the endosperm from autoxidation and polymerization processes.