Ligninolysis by a purified lignin peroxidase

The lignin peroxidases (LiPs) of white-rot basidiomycetes are generally thought to catalyze the oxidative cleavage of polymeric lignin in vivo. However, direct evidence for such a role has been lacking. In this investigation, ¹⁴C- and ¹³C-labeled synthetic lignins were oxidized with a purified isozyme of Phanerochaete chrysosporium LiP. Gel permeation chromatography of the radiolabeled polymers showed that LiP catalyzed their cleavage to give soluble lower-M_r products. To a lesser extent, the enzyme also polymerized the lignins to give soluble higher-M_r products. This result is attributable to the fact that purified LiP, unlike the intact fungus, provides no mechanism for the removal of lignin fragments that are susceptible to repolymerization. LiP catalysis also gave small quantities of insoluble, perhaps polymerized, lignin, but in lower yield than intact P. chrysosporium does. ¹³C NMR experiments with ¹³C-labeled polymer showed that LiP cleaved it between C_α and C_β of the propyl side chain to give benzylic aldehydes at C_α, in agreement with the cleavage mechanism hypothesized earlier. The data show that LiP catalysis accounts adequately for the initial steps of ligninolysis by P. chrysosporium in vivo.