SINCE the classical investigations of Warburg, Posener and Negelein (1924) on the glycolytic processes in cancer tissues, the studies dealing with changes of enzymatic processes in relation to cancer have been numerous and are the subject of several reviews, (e.g. Greenstein, 1954; Fishman, 1959). The material studied varies from whole animals to subcellular particles and the significance attributed to the findings differs from causative connection to chance association. Numerous attempts have been made to use the enzymatic changes found in the blood and in the tissues of patients suffering from malignant tumours for diagnostic or prognostic purposes. As the knowledge of the chemistry and morphology of the cellular processes increases, the explanation for the changes of enzymatic processes in tumor cells is modified.The present investigations deal with the effect of the carcinogens dimethyl (DMN) and diethyl (DEN) nitrosamine on the ,3-glucuronidase and lactic dehydrogenase (LDH) activities during the development of tumors in rats. The comparative carcinogenic effect of these substances in the rat has been studied previously (Argus and Hoch-Ligeti, 1961). Changes caused by a carcinogen in the cytoplasmic protein molecules, either directly or consecutive to alteration of template nucleic acids, might manifest themselves in changes of enzyme activities. Methylation of proteins in rat liver slices during incubation with DMN has been described (Magee and Hultin, 1962). With the aid of DMN labelled with radioactive carbon, incorporation of the label into the deoxyribonucleic acid of the liver and probably also of the kidney has been demonstrated. Acid hydrolysis of labelled ribonucleic acid from the liver yielded a 7-methylguanine (Magee and Farber, 1962). The carcinogenic activity of nitrosamine derivatives was thought to be related to protein denaturation (Argus, Leutze and Kane, 1961) or to methylation of sulfhydryl groups in amino acids (Schoental, 1961).The enzyme systems ,8-glucuronidase and LDH were selected for the present study because of their different intracellular distribution. /3-glucuronidase is largely localized in the lysosomes (deDuve, Pressman, Gianetto, Wattiaux and Appelmans, 1955) and, if the intracellular membranes are intact, it is not easily available to substrates. LDH might be localized both in particulate and nonparticulate cellular material. It has been reported to be a mitochondrial enzyme
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