REDUCTION OF THE 2,2'-AZINOBIS(3-ETHYLBENZTHIAZOLINE-6-SULFONALE) CATION-RADICAL BY PHYSIOLOGICAL ORGANIC-ACIDS IN THE ABSENCE AND PRESENCEOF MANGANESE

Laccase is a copper-containing phenoloxidase, involved in lignin degra dation by white rot fungi. The laccase substrate range can be extended to include nonphenolic lignin subunits in the presence of a noncataly tic cooxidant such as 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS), with ABTS being oxidized to the stable cation radical, ABTS(), which accumulates. In this report, we demonstrate that the ABTS(+) can be efficiently reduced back to ABTS by physiologically occurring o rganic acids such as oxalate, glyoxylate, and malonate. The reduction of the radical by oxalate results in the formation of H2O2, indicating the formation of O-2(-) as an intermediate. O-2(-) itself mas shown t o act as an ABTS(+) reductant. ABTS(+) reduction anal H,O, formation a re strongly stimulated by the presence of Mn2+, with accumulation of M n3+ being observed. Additionally, 4-methyl-O-isoeugenol, an unsaturate d lignin monomer model, is capable of directly reducing ABTS(+). These data suggest several mechanisms for the reduction of ABTS(+) which wo uld permit the effective use of ABTS as a laccase cooxidant at catalyt ic concentrations.