The enzymes involing PCB degradation
T.Senda
Background
Polychlorinated biphenyls (PCBs) are the widely distributed
environmental pollutants, the safe and economical degradation of which is
one of the urgent problem for mankind because of their toxicity and
carcinogenicity. PCBs are strongly resistant to biodegradation due to their
chemical stability. Total amount of PCBs having been released into the
biosphere is ca. 750,000 tons. Parts of the PCBs have been
accumulated and concentrated in the bodies of fishes, birds and so on. The
accumulation of PCBs have been observed even in human bodies. The toxic
symptoms due to PCBs involve headache, pain of joints, hypertension and so
on. The PCBs can be degraded using heat, above 1,200¡C which , however,
gives rise to the production of dioxin notorious for its severe toxity. In
addition, it is impossible to remove the PCBs which have already been
widely spread over the environment using this method. Since 1973, a number
of microorganisms that could degrade PCBs have been isolated and
characterized (Ahmed and Focht, 1973; Furukawa and Matsumura, 1976;
Furukawa and Chakrabarty, 1982; Furukawa, 1982; Bedard et al.,
1986; Furukawa and Miyazaki, 1986; Bedard et al., 1987; Kimbara
et al., 1989; Fukuda, 1993). The major biodegradation pathway of
PCBs in microorganisms has also been established. Thus, four specific
enzymes, biphenyl dioxygenase (BphA), dihydrodiol dehydrogenase (BphB),
2,3-dihydroxybiphenyl dioxygenase (BphC) and 2-hydroxyl-6-oxo-6-phenylhexa-2,4-dienoic acid hydrolase (BphD) are sequentially involved in the
oxidative degradation of PCBs into chlorobenzoates and 2-hydroxypenta-2,4-
dienoate (Furukawa & Miyazaki, 1986). Consequently, the possible
biodegradation of the PCBs have been intensively pursued. To actually utilize
the microorganisms for the degradation of a variety of PCBs, however, the
improvement of the PCB-degradation pathway is required, because the PCB-
degradation pathways sometimes exhibit only narrow substrate specificity.