IN-VITRO METABOLISM OF AFLATOXIN B-1 BY NORMAL AND TUMOROUS LIVER-TISSUE FROM THAILAND

Liver tissues were obtained hom 20 liver cancer patients from Thailand , an area where the incidence of this tumour is high and where exposur e to aflatoxin occurs. The expression of hepatic cytochrome P450s (P45 0) and glutathione S-transferase (GST) was examined and this expressio n was compared to the in vitro metabolism of aflatoxin B-1 (AFB(1)). T here was a >10-fold inter-individual variation in expression of the va rious P450s including CYP3A4 (57-fold), CYP2B6 (56-fold) and CYP2A6 (1 20-fold). Microsomal metabolism of AFB(1) to AFB(1) 8,9-epoxide (as me asured by AFB(1) tris-diol formation) and aflatoxin Q(1) (AFQ(1)), the major metabolite produced, was significantly correlated with CYP3A3/4 expression (P < 0.001) and, to a lesser extent, with CYP2B6 expressio n (P < 0.01). There was a significantly reduced expression of major P4 50 proteins in microsomes from liver tumours compared to microsomes fr om the paired normal liver when analysed by Western immunoblot analysi s. The production of AFQ(1) and AFB(1) tris-diol was almost uniformly reduced in tumours, but interestingly, the production of AFP(1) was si gnificantly increased. The immunoreactive expression of the major huma n classes of cytosolic GSTs (alpha, mu and pi) was also analyzed in no rmal and tumorous liver tissue. The expression of GSTA (alpha) and GST M (mu) class proteins was markedly decreased and GSTP (pi) increased i n the majority of tumour cytosols compared to normal liver. The cytoso lic GST activity (1-chloro-2,4-dinitrobenzene conjugation) was signifi cantly lower in liver tumours compared to normal liver (193 +/- 149 ve rsus 875 +/- 299 nmol/min/mg, P < 0.0001), as was glutathione peroxida se (GPx) activity (cumene hydroperoxide) (26 +/- 23 versus 70 +/- 26 m nol/min/mg respectively, P < 0.0005). Ten out of 14 individuals (71%) were homozygous mull when genotyped for GSTM1. There was no detectable conjugation of AFB(1) 8,9-epoxide to glutathione by cytosol either fr om tumorous or normal liver. Thus, capacity of human cytosols to conju gate reactive AFB(1) metabolites to GSH resembled AFB(1)-sensitive spe cies such as rat, trout and duck rather than resistant species such as mouse and hamster. These data indicate a strong capacity of multiple forms of human hepatic P450s to metabolize AFB(1) to both the reactive intermediate AFB(1) 8,9-epoxide and the detoxification product AFQ(1) . These results suggest that in view of the lack of significant GST-me diated protection against AFB(1) in human liver, variations in express ion of hepatic P450, due either to genetic polymorphisms or to modulat ion by environmental factors, may be important determinants in the ris k of liver cancer development in AFB(1)-exposed populations.