Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/19/2012
Publication Date: 2/11/2013
Publication URL: http://handle.nal.usda.gov/10113/56964
Citation: Sullivan, M.L., Foster, J.L. 2013. Perennial peanut (Arachis glabrata Benth.) contains polyphenol oxidase (PPO) and PPO substrates that can reduce post-harvest proteolysis. Journal of the Science of Food and Agriculture. 93(10):2421-2428. Interpretive Summary: Ruminant animals such as dairy cattle poorly utilize plant protein that is broken down during harvest and storage. This has both environmental and economic consequences. It is estimated that it costs farmers over $100 million annually to supplement rations with the needed true protein because of protein breakdown in alfalfa, a major forage crop. U.S. Dairy Forage Research Center scientists have demonstrated that in red clover, reaction of o-diphenolic compounds with an endogenous polyphenol oxidase (PPO) enzyme prevents protein degradation when the forage is preserved by ensiling. It is expected that this system would lead to better protein utilization by ruminant animals. Unfortunately, many important forages, including alfalfa, lack PPO and PPO o-diphenol substrates. Recent studies of another legume forage, perennial peanut, suggest that protein from haylage made from this crop is more efficiently utilized by ruminants than protein of haylage made from alfalfa. Perennial peanut is becoming more widely used as a forage crop in subtropical climates such as the U.S. southern coastal plains and peninsular Florida. In this study, we examined perennial peanut for the PPO enzyme and PPO o-diphenol substrates as a potential explanation for its desirable protein utilization characteristics. We detected the PPO protein in extracts of perennial peanut leaves, and we also measured high levels of PPO enzyme activity. In addition, we found that extracts of perennial peanut that were depleted of PPO o-diphenolic substrates showed reduced protein breakdown upon addition of a purified PPO substrate. The addition of phenolic compounds isolated from perennial peanut leaves to extracts of genetically modified alfalfa that produce the red clover PPO enzyme resulted in reduced protein breakdown. Thus, research related to adapting and optimizing the PPO system in other forage crops will likely be applicable to perennial peanut. Because there is difficulty growing alfalfa in the humid, warm environment of the southern U.S., perennial peanut, adapted to this climate, is an attractive alternative forage legume that sells for the same price as alfalfa without transport costs from the cooler regions where alfalfa is grown. Emphasizing the benefits of perennial peanut forage, including efficiency of protein utilization, will further enhance its market value.
Technical Abstract: Studies of perennial peanut (Arachis glaburata Benth.) suggest its hay and haylage have higher levels of rumen undegraded protein (RUP) than other legume forages such as alfalfa. Higher RUP can result in more efficient utilization of nitrogen by ruminant animals with positive economic and environmental effects. We sought to determine whether, like red clover, perennial peanut contains polyphenol oxidase (PPO) and PPO substrates that might be responsible for increased RUP. In this study, perennial peanut extracts contained immunologically detectible PPO protein and high levels of PPO activity (>100 nkatal/mg protein). Addition of the PPO substrate, caffeic acid, to perennial peanut extracts that were depleted of endogenous substrates reduced proteolysis by 90%. Addition of phenolics prepared from perennial peanut leaves to extracts of either transgenic PPO-expressing or control (non-expressing) alfalfa showed peanut phenolics could reduce proteolysis 70% in a PPO-dependent manner. Two likely PPO substrates were present in perennial peanut leaves including caftaric acid. We concluded that perennial peanut contains PPO and PPO substrates that, together, are capable of inhibiting post-harvest proteolysis, suggesting a possible mechanism for increased RUP in this forage. Research related to adapting and optimizing the PPO system in other forage crops will likely be applicable to perennial peanut.