Cultured human monocytes, lymphocytes, Fischer rat liver (TRL-2) cells, and Buffalo rat liver (BRL) cells catalyzed the conversion of 3H(-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene [3H(-)t-7,8-dihydrodiol BP] to r-7, t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (diol epoxide I) and r-7, t-7-8-dihydroxy-<X_Underline>c</X_Underline>-9,10-oxy-7,8,9,10-tetra-hydrobenzo[a]pyrene (diol epoxide II; r-7 indicates that the substituent at the 7-position is the reference, and t and <X_Underline>c</X_Underline> indicate that the substituents trans and cis, respectively, to the reference substituent). These appear to be the most reactive metabolites of benzo[a]pyrene (BP) and were covalently bound to both exogenous and intact cellular DNA in tissue culture media. The cells induced by benzanthracene (BA) exhibited greater levels of DNA binding than the controls and this binding was linear with increasing cell content in human monocytes, in TRL-2 cells and in Buffalo rat liver cells. The binding to DNA was greater than controls in BA-preinduced lymphocytes, but was not linear. The DNA binding in control cells showed a nonlinear increase with increasing cell concentration in all experiments. The addition of human liver epoxide hydrolase (EC 3.3.2.3) to the incubation medium reduced the amount of reactive metabolites binding to DNA by 12–15 % in control and by 23–41 % in BA-induced monocytes. Thus, with whole cell systems of either human monocytes or lymphocytes, the addition of purified human liver epoxide hydrolase reduced the binding of 3H(–)<X_Underline>t</X_Underline>-7,8-dihydrodiol BP metabolites to DNA. Human monocytes and lymphocytes also catalyzed the covalent binding of 3H(–)<X_Underline>t</X_Underline>-7,8-dihydrodiol BP to intact cellular DNA. The addition of 3H(–)<X_Underline>t</X_Underline>-7,8-dihydrodiol BA to tissue culture media caused the inhibition of covalent DNA binding in BA-preinduced monocyte by 58% and lymphocytes by 25%. Previous work has shown that BA is metabolized and converted to BA-diol epoxides by microsomes. These results indicate that BA-diol epoxides and BP diol epoxides are competing for the same binding sites on DNA. On the other hand, the addition of 10 nmol of 3H(–)t-7,8-dihydrodiol BP to the incubation of control and BA-preinduced cell homogenate and further incubation at 37 °C for 25 min showed that the DNA binding in BA-preinduced cell homogenates was much greater than controls. Homogenates of cells induced by BA exhibited a greater level of DNA binding than controls. The increases, were 110% in monocytes, 119% in lymphocytes, 157% in TRL-2 cells and 140% in BRL cells. The increase of binding to cellular DNA resulted from the induction of the mixed-function oxidase enzyme systems by BA treatment. The addition of human liver epoxide hydrolase to tissue culture media reduced DNA binding by 30% in control and by 45% in BA-induced monocytes. The decrease was 47% in control and 66 % in BA-induced lymphocytes. When cells were harvested, washed, disrupted and incubated at 37 °C for 25 min by addition of 3H(–)t-7,8-dihydrodiol BP, DNA binding was still reduced by 29% in control and 33% in BA-induced lymphocytes. However, when the same amounts of bovine serum albumin were added to tissue culture media, no inhibition of DNA binding was detected. These results suggest that the inhibition of DNA binding was due to the effect of epoxide hydrolase.

Keywords:
Binding, 3H-(-trans-7,8-Dihydroxy-7,8-dihydrobenzo[a]pyrene, DNA, Epoxide hydrolase
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© 1988 S. Karger AG, Basel
1988
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