Lipolysis of red clover with differing polyphenol oxidase activities in batch culture.

Polyphenol oxidase (PPO) oxidizes endogenous phenols to quinones, which react with nucleophilic sites of other compounds such as proteins. In red clover (Trifolium pratense L.) , this complexing reaction has been shown to reduce both plant mediated proteolysis and lipolysis. This experiment investigated the role of red clover PPO on lipolysis in the presence and absence of rumen microorganisms. Triplicate macerated shoot samples of two red clover lines, a wild type with a basal level of PPO activity (High PPO) and a mutant with reduced PPO activity (Low PPO), were incubated in anaerobic buffer, with and without strained rumen liquor inoculum (I+ and I-), at 39°C, and sampled at six time (0, 1, 2, 4, 6 and 24 h). At each time point the samples were destructively harvested and lipolysis measured as percentage loss of membrane lipid. Lipolysis data was analysed using a general analysis of variance with repeated measurements (Genstat 8®). The table shows the reducing effect of PPO on lipolysis (High vs Low) but also the elevated level of lipolysis when micro-organisms are present (I+ vs I-). If the PPO effect was solely due to the deactivation of plant lipases this difference should be neutralized through the addition of microbial lipases. The retention of the PPO effect in the I+ treatments suggests that PPO exerts some form of protection on the membrane lipids in a manner similar to the complexing of protein. The lipid in forages is mainly in the form of polar membrane lipids, and polar lipid - phenol complexes could form due to the highly electrophilic nature of the PPO-produced quinones. [ABSTRACT FROM AUTHOR]