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Heme Enzyme Patterns in Rat Liver Nodules and Tumors¹

Department of Pathology, Section of Experimental Pathology, The University of Texas M. D. Anderson Hospital and Tumor Institute at Houston, Houston, Texas 77030

Chemically induced rat hepatocyte nodules and carcinomas have a reduced capacity to oxidize drugs. The reduction in monooxygenase activity results largely from the partial loss of cytochrome P-450, a hemecontaining terminal electron acceptor. To determine whether the cytochrome P-450 deficit was indicative of an altered heme metabolism, we quantitated four heme-containing proteins in normal rat liver and in rat liver nodules and cancers induced by 2-acetylaminofluorene or diethylnitrosamine: cytochrome P-450; cytochrome b₅; catalase (EC 1.11.1.6); and tryptophan 2,3-dioxygenase (EC 1.13.11.11). The amounts of these components in nodules were 45%, 88%, 50%, and 59% of normal liver, respectively; in 2-acetylaminofluorene-induced cancers, 65%, 74%, 64%, and 65%, respectively; and in diethylnitrosamine-induced cancers, 40%, 69%, 56%, and 52%. -Aminolevulinic acid synthease (EC 2.3.1.37), the rate-limiting enzyme in the heme synthetic pathway, and heme oxygenase (EC 1.14.99.3), a degradative enzyme, were also quantitated. The amounts of these enzymes in nodules were 95% and 138% of normal liver, respectively, whereas in 2-acetylaminofluorene-induced cancers, they were 47% and 233%, and in diethylnitrosamine-induced cancers, they were 50% and 175%. These data indicate that four nonmitochondrial liver hemoproteins were diminished to about the same extent in hepatic nodules and cancers. Nodules and cancers also demonstrated an increased capacity for heme degradation, while cancers also demonstrated a decreased capacity for heme synthesis. Thus, the resistance of nodules and tumors to P-450-activated cytotoxic agents may ultimately result from a disturbance in heme metabolism.

¹ This investigation was supported by funds from the Sid Richardson Foundation.

² To whom requests for reprints should be addressed.

Received 9/ 8/86. Revised 11/ 3/86. Accepted 11/ 7/86.