LIGNINOLYSIS BY A PURIFIED LIGNIN PEROXIDASE

The lignin peroxidases (LiPs) of white-rot basidiomycetes are generall y thought to catalyze the oxidative cleavage of polymeric lignin in vi vo. However, direct evidence for such a role has been lacking. In this investigation, C-14- and C-13-labeled synthetic lignins were oxidized with a purified isozyme of Phanerochaete chrysosporium LiP. Gel perme ation chromatography of the radiolabeled polymers showed that LiP cata lyzed 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 o f lignin fragments that are susceptible to repolymerization. LiP catal ysis also gave small quantities of insoluble, perhaps polymerized, lig nin, but in lower yield than intact P. chrysosporium does. C-13 NMR ex periments with C-13-labeled polymer showed that LiP cleaved it between C(alpha) and C(beta) of the propyl side chain to give benzylic aldehy des at C(alpha), 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.