TRANSFORMATIONS OF STRUCTURAL PHENYLPROPANOIDS DURING CELL-WALL DIGESTION

Phenylpropanoids limit the degradation of cell walls of roughages in h erbivores but at the same time undergo transformations in the digestiv e tract. This review outlines the main transformations that occur in t he rumen. All the monomeric aromatics tested are fully degraded under anaerobic conditions which favour electron transfer. Six main strains of bacteria degrade monomeric phenols in the rumen by several mechanis ms. In addition, some fungi and bacteria are able to release, and poss ibly to metabolise the esterified hydroxycinnamic acids found in forag e cell walls. The first step in the degradation of these acids is thei r reduction to non-toxic compounds, which are often growth factors. Ho wever, total degradation of monoaromatics is difficult to achieve in v ivo because of the small population of organisms able to metabolise th em and the limited transit time of the substrates in the rumen. Oligol ignols are also degraded to different extents depending on their size and molecular structure. Lignins are partly solubilised during cell wa ll degradation. They may also undergo other transformations such as de methylation and dehydroxylation. The amount of lignin that seems to be degraded in rumen fluid is low but probably higher than under other a naerobic environments over the same period of time. It is generally ac cepted that the digestibility of forage lignins is low. However, the w ide range of values measured (from minus 0.46 to 0.64) is related eith er to the measurement method or to the transformations that the lignin s may undergo in the digestive tract, or to both. An indigestible frac tion of lignins could serve as a reliable cell wall marker but none of the fractions used to date has proved entirely satisfactory for this purpose. Future research in this field would involve better knowledge about the transformation of phenylpropanoids and the development of mi crobial activity on these compounds. This would improve phenolics degr adation and consequently carbohydrate utilization.