Consecutive polymerization and depolymerization of kraft lignin by Trametes cingulata

In extracellular solutions from white-rot fungal cultures, lignin components typically encounter opposing tendencies to be polymerized and depolymerized. The enzymes most commonly purported to bring about lignin depolymerization - lignin peroxidase, manganese-dependent peroxidase and laccase - can all act as single-electron oxidants. Accordingly they may 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 has sought to elucidate whether discrimination between polymerization and depolymerization is really so precariously established in the progenitorial step of the biodegradative pathway. Some white-rot fungi express no detectable peroxidase activity of any kind and yet degrade lignins very effectively. For the sake of simplicity, one of these, Trametes cingulata, was selected to explore whether there might be some connection between lignin polymerization and depolymerization in vivo. The (Mn-free) culture medium was a multicomponent homogeneous solution containing 0.55 gl^-1 softwood kraft lignin, which was adopted as the substrate because of its adequate solubility. As it grew vegetatively, T. cingulata polymerized the dissolved lignin components but then, after cessation of primary growth, the high molecular weight polymerized substrate was completely degraded. No lignin peroxidase or Mn- dependent peroxidase activity was detected in the extracellular culture solution during either metabolic phase, although ('laccase-like') oxidase activity towards 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) persisted throughout the entire process. The results suggest that the overall effect of T. cingulata upon the kraft lignin substrate is governed by the consecutive release of distinct polymerizing and depolymerizing enzymes.