CONSECUTIVE POLYMERIZATION AND DEPOLYMERIZATION OF KRAFT LIGNIN BY TRAMETES CINGULATA

In extracellular solutions from white-rot fungal cultures, lignin comp onents typically encounter opposing tendencies to be polymerized and d epolymerized. The enzymes most commonly purported to bring about ligni n depolymerization-lignin peroxidase, manganese-dependent peroxidase a nd laccase-can all act as single-electron oxidants. Accordingly they m ay produce from particular lignin monomer residues, either indirectly or directly, phenoxy radicals that will undergo bimolecular coupling, unless they are otherwise preemptively transformed. The present work h as sought to elucidate whether discrimination between polymerization a nd depolymerization is really so precariously established in the proge nitorial step of the biodegradative pathway. Some white-rot fungi expr ess no detectable peroxidase activity of any kind and yet degrade lign ins very effectively. For the sake of simplicity, one of these, Tramet es cingulata, was selected to explore whether there might be some conn ection between lignin polymerization and depolymerization in vivo. The (Mn-free) culture medium was a multicomponent homogeneous solution co ntaining 0.55 gl(-1) softwood kraft lignin, which was adopted as the s ubstrate because of its adequate solubility. As it grew vegetatively, T. cingulata polymerized the dissolved lignin components but then, aft er cessation of primary growth, the high molecular weight polymerized substrate was completely degraded. No lignin peroxidase or Mn-dependen t peroxidase activity was detected in the extracellular culture soluti on during either metabolic phase, although (''laccase-like'') oxidase activity towards 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABT S) persisted throughout the entire process. The results suggest that t he overall effect of T. cingulata upon the kraft lignin substrate is g overned by the consecutive release of distinct polymerizing and depoly merizing enzymes. (C) 1998 Published by Elsevier Science Ltd. All righ ts reserved.