Jun Kuroki

High affinity of 2,3,4,7,8-pentachlorodibenzofuran to cytochrome P-450 in the hepatic microsomes of rats

The interaction of 2,3,4,7,8-pentachlorodibenzofuran (PenCDF) with cytochrome P-450 isozymes was studied in male Wistar rats using 14C-labeled PenCDF. Three forms of cytochrome P-450 isozymes, P-448 H, P-448 L and P-452, were purified to homogeneity from 14C-PenCDF-treated rat liver microsomes. The purified P-448 H contained 0.847 mole of PenCDF per mole of the hemoprotein, whereas the amounts of PenCDF bound to P-448 L and P-452 were far less than that to P-448 H. These results suggest that cytochrome P-450, particularly P-448 H, functions as the storage site of PenCDF in the rat liver.

Toxicological evaluation of sulfur-containing metabolites of 2,5,2′,5′-tetrachlorobiphenyl in rats

Abstract Sulfur-containing metabolites of 2,5,2′,5′-tetrachlorobiphenyl (TCB), 4-methylthio-TCB (MT-TCB), 4-methylsulfoxyl TCB (MSX-TCB) and 4-methylsulfonyl TCB (MS-TCB) were examined for their acute toxicities, hepatic enzyme inducing activities, accumulation in the liver and lung, and excretion to the feces in rats. TCB and MT-TCB suppressed body weight and recovery of body weight gain was delayed in the MT-TCB-treated rats. MT-TCB and MS-TCB caused an increase in total liver lipid and only MT-TCB brought about an atrophy of the thymus. Treatment with MT-TCB increased cytochrome P-450 content and benzphetamine N-demethylase activity. The same enzymes were also induced by treatment with MSX-TCB. Although TCB administered was excreted mostly as hydroxylated TCB, a part was excreted as unchanged and a very small portion as the sulfur-containing metabolites. MT-TCB, MSX-TCB and MS-TCB were excreted from the MT-TCB- and MSX-TCB-treated rats. The MS-TCB-treated rats excreted only MS-TCB. The same compounds as found in the feces were identified in the liver and lung of the rats treated with those compounds except in the liver of TCB-treated rats. These results indicate that sulfur-containing metabolites, especially MT-TCB, were more important than their parent compound, TCB, from a toxicological point of view.

Unique features of subcellular distribution of 2,3,4,7,8-pentachlorodibenzofuran in rat liver

Abstract Among various polychlorinated dibenzofurans, 2,3,4,7,8-pentachlorodibenzofuran(PenCDF) is recognized to be one of the most important congeners as the causal agents of Yusho, because of its very high toxicity and persistency in the liver. We have already found that this PenCDF has a high affinity to the endoplasmic reticulum of the rat liver, in which PenCDF is bound rather tightly, but with non-covalent binding to P-448 H, an isozyme with a high spin form of cytochrome P-450 induced specifically by treatment with PenCDF. Molar ratio of PenCDF to P-448 H was 1 to 1. We have therefore studied to learn how PenCDF absorbed is transported and incorporated selectively into the liver cells, using 14C-PenCDF. As the results, we found that the 14C-PenCDF added to rat plasma was bound partly to lipoproteins, but mostly to albumin. This binding of PenCDF to albumin also enhanced strongly the cellular uptake of 14C-PenCDF. These findings indicated that albumin is very effective carrier in the blood to facilitate uptake of 14C-PenCDF into the liver cells. Examination of subcellular distribution of 14C-PenCDF incorporated into the liver slices proved that 14C-PenCDF was localized to the endoplasmic reticulum, in which P-448 H was not yet induced. This means that PenCDF has also some affinity to endoplasmic reticulum without P-448 H, although it binds specifically to P-448 H after the induction occurs. These findings strongly suggested that cytochrome P-450, especially P-448 H, might function as the storage site of PenCDF, explaining the high persistency and long-lasting biological effects of PenCDF in rats.