The oxidation-reduction potentials of lignin peroxidase isozymes H1, H2, H8, and H10 as well as the Mn-dependent peroxidase isozymes H3 and H4 are reported. The potentiometric titrations involving the ferrous and ferric states of the enzyme had Nernst plots indicating single-electron transfer. The Em7 values of lignin peroxidase isozymes H1, H2, H8, and H10 are -142, -135, -137, and -127 mV versus standard hydrogen electrode, respectively. The Em7 values for the Mn-dependent peroxidase isozymes H3 and H4 are -88 and -93 mV versus standard hydrogen electrode, respectively. The midpoint potential of H1, H8, and H4 remained unchanged in the presence of their respective substrates, veratryl alcohol and Mn(II). The midpoint potential between the ferric and ferrous forms of isozymes H1 and H4 exhibited a pH-dependent change between pH 3.5 and pH 6.5. These results indicate that the reductive half-reaction of the enzymes is the following: ferric peroxidase + le- + H+----ferrous peroxidase. Above pH 6.5, the effect of pH on the midpoint potential is diminished and indicates that an ionization with an apparent pKa equal to approximately 6.6-6.7 occurs in the reduced form of the enzymes. A heme-linked ionization group in the ferrous form of the enzymes was confirmed by studying the effect of pH on the absorption spectra of isozymes H1 and H4. These spectrophotometric pH titration experiments confirmed the electrochemical results indicating pKa values of 6.59 and 6.69 for reduced isozymes H1 and H4, respectively. These results indicate the presence of a heme-linked ionization of an amino acid in the reduced form of the lignin peroxidase isozymes similar to that of other plant peroxidases.

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The accidental contamination of dairy cattle feed in Michigan in 1973-74 with polybrominated biphenyls (PBB) led to the contamination of cattle and people consuming their products. This led to an extensive animal and product monitoring and disposal program conducted by the Michigan Department of Agriculture and the Department of Natural Resources. It also led to several studies of the people of Michigan, extensive research on the chemicals, and an unprecedented establishment by the Legislature of a Toxic Substance Control Commission. Only a few relatively minor components of the PBB mixture that contaminated Michigan are metabolized and another group of minor components seem responsible for the toxicity, which, similar to that caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), includes induction of microsomal enzymes, liver hypertrophy, thymic involution, porphyria, anorexia and chloracne. PBB were found to produce the "dioxin-like" toxicity with roughly 0.01% the potency of TCDD. Both non-toxic as well as toxic congeners were found to be tumor promotors. To date it is impossible to unequivocally conclude that any human health effects can be attributed to PBB. The Toxic Substance Control Commission was established as an independent oversight body with responsibility to gather information, investigate, coordinate and make recommendations concerning toxic substances and the handling of toxic substances incidents. The Commission has declared two toxic substances emergencies and made several recommendations for regulating and solving toxic substances problems but its major activities have evolved towards a role as an environmental ombudsman.

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