The Enzymology of Methionine Metabolism in Rat Hepatomas

Summary

In this investigation, we studied the effects of diet on the levels of activity of five enzymes that are involved in methionine metabolism in six rat hepatomas, in the livers of animals hosting the tumors, and in control livers. All tumors possess S-adenosylmethionine synthetase, cystathionine synthase, cystathionase, betaine-homocysteine methyltransferase, and methyltetrahydrofolate-homocysteine methyltransferase. The relative content of the enzymes varied from tumor to tumor and was not a predictable function of the growth rate. In contrast to the findings in control and host livers, the levels of four enzymes in the hepatomas rarely were affected by changes in the dietary protein content. Cystathionine synthase was the exception. An increase in this enzyme was observed in hepatomas 9633, 7794A, 7800, and 7787 when the host animals were fed a high-protein ration. However, protein feeding resulted in a decrease in cystathionine synthase in hepatoma 7777. Several examples of significant differences in enzyme activity between host and control liver were observed. These effects of the tumor were unpredictable.