Table 2.

Spectral constants and relative binding efficiencies for PCB 136 atropisomers with rat hepatic microsomal P450 enzymes^a

Spectral parameter	(+)-PCB 136	(-)-PCB 136	(±)-PCB 136
Hepatic microsomes from MC-treated ratsb
ΔAmax (absorbance unit/nmol P450)	0.012	0.010	0.011
Ksapp (μM)	13	6.9	8.8
ΔAmax/Ksapp (absorbance unit/nmol P450/M)	920	1500	1300
Hepatic microsomes from PB-treated ratsc
ΔAmax (absorbance unit/nmol P450)	0.060 ± 0.009	0.053 ± 0.006	0.058 ± 0.007
Ksapp (μM)	2.2 ± 0.47	3.8 ± 0.76	2.6 ± 0.48
ΔAmax/Ksapp (absorbance unit/nmol P450/M)	28000 ± 2800	14000 ± 1500	23000 ± 2100
Hepatic microsomes from DEX-treated ratsc
ΔAmax (absorbance unit/nmol P450)	0.036 ± 0.005	0.024 ± 0.003	0.032 ± 0.004
Ksapp (μM)	12 ± 2.4	16 ± 3.2	12 ± 2.8
ΔAmax/Ksapp (absorbance unit/nmol P450/M)	3100 ± 240	1600 ± 130	2800 ± 360
Hepatic microsomes from CO-treated ratsc
ΔAmax (absorbance unit/nmol P450)	0.046 ± 0.001	0.042 ± 0.001	0.044 ± 0.001
Ksapp (μM)	9.2 ± 1.6	10 ± 1.1	8.8 ± 1.2
ΔAmax/Ksapp (absorbance unit/nmol P450/M)	5300 ± 920	4000 ± 420	5300 ± 900

^aMaximal absorbance change (ΔA_max), apparent spectral dissociation constant (K_s^app) and relative binding efficiency (ΔA_max/K_s^app) values were obtained for hepatic microsomes prepared from MC, PB, DEX and CO-treated male rats as described in Experimental Procedures.

^bSingle measurement.

^cMean ± SE of three determinations.