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Heme Enzyme Patterns in Genetically and Chemically Induced Mouse Liver 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 hepatomas have repeatedly been shown to be deficient in Phase I drug-metabolizing enzymes. Some of these reduced activities are attributable to a diminution of the heme-containing terminal electron acceptor, cytochrome P-450. We recently demonstrated that spontaneous mouse liver tumors exhibit the same deficiency. Therefore, chemically induced and spontaneous liver tumors share common metabolic alterations which are likely to represent intrinsic characteristics of the tumorigenic process and are independent of its etiology. To determine whether the cytochrome P-450 deficit was the result of an altered heme metabolism, we quantitated four hemecontaining proteins in normal mouse liver, spontaneous mouse liver tumors, and those induced by a single injection of diethylnitrosamine: cytochrome P-450; cytochrome b₅; tryptophan 2,3-dioxygenase (EC 1.13.11.11); and catalase (EC 1.11.1.6). The amounts of these components in spontaneous tumors relative to normal liver were 0.35, 0.68, 0.76, and 0.51, respectively. Similar values were obtained with chemically induced tumors. The enzymes -aminolevulinic acid synthase (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 spontaneous tumor relative to liver were 0.49 and 1.51, respectively. Again, similar values were observed for the chemically induced tumors. Alteration of the latter two enzyme activities may be sufficient for the altered hemoprotein patterns seen in mouse liver tumors. Further, this pattern of metabolic alteration is common to both chemically induced and spontaneous tumors. Thus, tumor resistance to cytotoxic agents activated by the monooxygenase system is not necessarily induced by exposure to these agents, nor as a result of selection.

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

² To whom requests for reprints should be addressed.

Received 11/ 1/85. Revised 2/13/86. Accepted 3/ 6/86.

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