The effects of tolaasin, the toxin produced by Pseudomonas tolaasii on tyrosinase activities and the induction of browning in Agaricus bisporus fruiting bodies

Infection of Agaricus bisporus with Pseudomonas tolaasii or treatment with a tolaasin containing preparation resulted in the activation of tyrosinase and the development of a brown discolouration of the fruitbody. In order to investigate whether tolaasin is responsible for these reactions, mushrooms were treated with different concentrations of HPLC-purified tolaasin. Tola asin was found to induce both browning and the activation of tyrosinase, wi th the level of activation being related to the concentration of toxin appl ied. In water-treated controls, active tyrosinase isoforms with pIs of 4.5 and 4.4 were the main isoforms found but tolaasin treatment resulted in the activation of a tyrosinase isoform with a pi of 5.7. The different active isoforms could be distinguished as the tolaasin activated isoforms had a hi gher affinity for catechol as a substrate (K-m 0.82 mM) than the active iso forms in the control (K-m 2.86 mM). In mushrooms inoculated with P. tolaasi i or treated with tolaasin containing extracts no significant differences w ere found in the mechanism of oxidation of tyrosine, gamma-glutaminyl-4-hyd roxybenzene or gamma-glutaminyl-3,4-dihydroxybenzene to form melanin. In to laasin treated samples, a constant total amount of melanin and sum of the t hree phenols was found. (C) 1999 Academic Press.