Natural Products of Chemistry
A number of biologically active natural products have been isolated but only a few are useful to us as pest control agents. This is mainly due to the lack of their rational exploration, even though understanding their effects on biological systems and the biochemical changes involved are known to be essential. For example, plant resistance to pests usually involves combinations of compounds, many of them short-lived. Their dynamic analysis and functional understanding are needed. Our group has been pursuing this type of study.
Based on our observation during a simple lettuce seedling assay, we found that the lettuce exuded a polyphenol oxidase (PPO) from the roots of the seedlings into the rhizosphere. This can be visualized when an exogenous substrate such as catechin is present in the test solution, the root caps and solution surrounding the roots are stained yellow to orange. Thus, the enzyme exuded from the root of lettuce seedlings oxidizes catechin although the corresponding o-quinone has not yet been characterized. In addition to lettuce, a number of crops such as tomato, alfalfa, corn and wheat also exude PPOs that are membrane-bound copper containing glycoproteins. The role of the PPO exuded from lettuce seedling, as well as other plants, may be a line of passive defense against soil microorganisms and insects. This led us to investigate the role of PPO, using a series of simple phenolic compounds as a model in order to gain new insights into their actions on a molecular basis. The enzyme has been isolated from the lettuce exudate and its structural characterization is underway. This study has been extended to a similar oxidase present in insects known as tyrosinase because it is one of the key enzymes in the insect molting process. We have reported that ecdysteroid-22-O-acyltransferase in the tobacco budworm, Heliothis virescens plays an important role as a line of defense against exogenous ecdysteroids. Accumulation of this kind of knowledge may provide clues to design more appropriate pesticides.
Another aspect of our research is identifying the antimicrobial activity of natural products. This ongoing project has begun to encompass the underlying rationale for structure-antimicrobial activity relationships.
Most Recent Publications
I. Kubo, N. Masuoka, K. Nihei and B. Burgheim. Maniçoba, a quercetin-rich Amazonian dish. J. Food Composit. Anal. 19, 579-588 (2006).
N. Masuoka, K. Nihei and I. Kubo. Effects of alkyl gallates on xanthine oxidase. Mol. Nutr. Food Res. 50, 725-731 (2006).
T. J. Ha and I. Kubo. Slow-binding inhibition of soybean lipoxygenase-1 by dodecyl gallate. J. Agric. Food Chem. 55, 446-451 (2007).
Orozco, A.; Ogura, T.; Hirosawa, T.; Garduño, R.; Kubo, I. In hydrolyzed cow’s milk Helicobacter pylori becomes nonculturable and the growth of Salmonella typhi and Escherichia coli is inhibited. J. Food Sci. 72, M305-M309 (2007).
T. Nitoda, M. D. Fan and I. Kubo. Effects of cuminaldehyde on melanoma cells. Phytother. Res. 22, 809-813 (2008).
I. R. Green, F. E. Tocoli, S. H. Lee, K. Nihei and I. Kubo. Design and evaluation of anacardic acid derivatives as anti-cavity agents. J. Eur. Med. Chem. 43, 1315-1320 (2008).