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Oxidation of polycyclic aromatic hydrocarbons by a tropical isolate of Pycnoporus coccineus and its laccase

Paweena Thongkred^a, Pongtharin Lotrakul^a,*, Sehanat Prasongsuk^a, Tsuyoshi Imai^b, Hunsa Punnapayak^a

 
ABSTRACT:     Basidiocarps of Pycnoporus coccineus and P. sanguineus were collected from Kanchanaburi, Phetchaburi, and Mae Hong Son provinces in Thailand. Liquid culture of P. coccineus isolates provided higher laccase and manganese peroxidase activities than those of P. sanguineus. Lignin peroxidase activity was not detectable in any of the three Pycnoporus isolates. P. coccineus Thongkred 013 BCU, the best laccase producer among these isolates, was selected to optimize laccase production. The highest laccase yield (5.97±0.40 U/ml) was obtained when grown at 28 °C for 8 days in basal medium containing 2% (w/v) glucose and 0.25% (w/v) peptone at an initial pH of 5.0. The optimum conditions for stability and activity of the crude laccase enzyme were at pH 3.5 and room temperature (25±2 °C). FeSO₄ (10 mM) and SDS (1% w/v) were both inhibitory to the laccase activity. When five different types of polycyclic aromatic hydrocarbons (PAHs) were individually added to liquid culture of P. coccineus at 100 ppm, the oxidation of anthracene, pyrene, and fluoranthene detected within 24 h were 59.7, 50.7, and 49.8%, respectively, whereas only 25.3% of benzo[a]pyrene and 32.4% of phenanthrene were oxidized. When P. coccineus crude laccase (1 U/ml) was used, these PAHs were more readily oxidized, with 76.4 and 74.3% of pyrene and anthracene, respectively, being oxidized within 2 h and 84.2% of fluoranthene oxidized within 24 h. Benzo[a]pyrene and phenanthrene oxidation by the enzyme were enhanced when 1 mM 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) was added as a mediator to the reaction mixture, resulting in 66.3 and 50.5% of the initial 100 ppm being oxidized after 24 h.

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* Corresponding author, E-mail: Pongtharin.L@Chula.ac.th

Received 23 Feb 2011, Accepted 30 Aug 2011