Studies on ligninolytic activities of ruminant microbes

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Lignin-carbohydrate complexes (LCCs) restrict the utilization of plant cell walls by ruminants. However, degradation of LCCs into smaller fragments (i.e.oligomers) has been found to occur in the rumen. The objective of this study was to evaluate the ruminal activities of cleaving benzyl ether bonds, which are normally found in grass LCCs, by using lignin model compounds which emit fluorescence by benzyl ether bond cleavage. Two low molecula lignin models, 4-methylumbelliferone veratryl ether (VAU, dimer), and non-phenolic beta-O-4 lignin model compound incorporating 4-methylumbelliferone by benzyl ether linkage (GGU-ET trimer) were chemically synthesized. A coniferyl dehydrogenation polymer bearing fluorescent beta-O-4 benzyl ether model (F-DHP), which contained both phenolic and non-phenolic lignin substructure units, was also synthesized. They were incubated anaerobically with ruminal microbes with and without bactericidal (penicillin 2000IU/ml, streptomycin 0.2mg/ml and chlorampheni … More col 30mug/ml) and fungicidal (cycloheximide 0.5mg/ml) antibiotics, and their degradations were determined by the fluorescence yield. VAU (12muM) were degraded completely within 8hr by ruminal microbes in the presence of cycloheximide, but no significant degradation occurred in the presence of bactericidal antibiotics. GGU-ET was not degraded significantly even without any antibiotics in 48hr. F-DHP showed a partial degradation (about 20%) without any antibiotics. Both the bactericidal and fungicidal treatments showed significant degradation of F-DHP after 48hr although the incubation with sterilized ruminal fluid by a membrane filter (0.22mum pore) showed no degradation. These results indicate that ruminal microbes can cleave non-phenolic benzyl ether linkages in the dimer (VAU) especially by bacteria, but not in the trimeric beta-O-4 model (GGU-ET), and they can also degrade the synthetic lignin polymer with both phenolic and non-phenolic benzyl ether bonds (F-DHP). These results indicate that benzyl ether bonds involved in LCC macormolecules can be partially decomposed by ruminant microbes under anaerobic conditions. Less