Abstract
Sweetpotato shochu is a spirit that is mainly produced in the southern Kyushu area in Japan. During the course of our search for biologically active compounds in sweetpotato shochu distillery by-products, we reported that the extracts of a by-product inhibited lettuce seed germination. In the present study, in order to clarify the active compounds, the by-product was subjected to bioassay-guided fractionation using lettuce. A caffeic acid derivative was isolated from the by-product by bioassay-guided fractionation using high-performance liquid chromatography (HPLC). The derivative was identified as ethyl caffeate by 1H-nuclear magnetic resonance (NMR) and electron impact mass spectrometry (EIMS). The inhibitory activity of ethyl caffeate on the growth of selected plants was examined in the laboratory. At a concentration of more than 0.62mM, ethyl caffeate significantly inhibited lettuce radicle elongation. However, seed germination was not affected. It was observed that the inhibitory activity of ethyl caffeate on lettuce radicle elongation was greater than that of caffeic acid. In the case of weeds, ethyl caffeate inhibited the seed germination of smooth pigweed by 29% and 48% at concentrations of 0.62 and 1mM, respectively ; however, southern crabgrass was not affected. In addition, ethyl caffeate inhibited radicle elongation of smooth pigweed by 79% and 85% at concentrations of 0.62mM and 1mM, respectively ; this inhibitory effect was greater than that observed in southern crabgrass. Caffeic, chlorogenic, 3-O-,4-O-dicaffeoylquinic (3,4-diCQA), 3-O-,5-O-dicaffeoylquinic (3,5-diCQA) and 4-O-,5-O-dicaffeoylquinic (4,5-diCQA) acids, together with ethyl caffeate in a filtrate of the by-product, were identified by retention times and on-line ultraviolet spectra in HPLC analysis. A filtrate of moromi before distillation and a filtrate of the by-product contained ethyl caffeate at concentrations of 13.7mg/L and 8.3mg/L, respectively. However, ethyl caffeate was not detected in the extract of the storage roots of Koganesengan, a sweetpotato cultivar from which shochu is prepared.
