The over-all 5-methylcytosine (m5C) content of DNA from normal tissues varies considerably in a tissue-specific manner. By high-performance liquid chromatography, we have examined the m5C contents of enzymatic digests of DNA from 103 human tumors including benign, primary malignant and secondary malignant neoplasms. The diversity and large number of these tumor samples allowed us to compare the range of DNA methylation levels from neoplastic tissues to that of normal tissues from humans. Most of the metastatic neoplasms had significantly lower genomic m5C contents than did most of the benign neoplasms or normal tissues. The percentage of primary malignancies with hypomethylated DNA was intermediate between those of metastases and benign neoplasms. These findings might reflect an involvement of extensive demethylation of DNA in tumor progression. Such demethylation could be a source of the continually generated cellular diversity associated with cancer.

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The metabolism and intracellular compartmentalization of adenine nucleotides in a preparation of adult rat heart myocytes showing good morphology, viability, and tolerance to calcium ion has been examined by high performance liquid chromatography. These myocytes contain an average of 23 nmol adenine nucleotide per milligram protein which is about 60% of the adenine nucleotide content of intact rat heart tissue. The loss of adenine nucleotide occurs during the incubation and washing steps that increase the yield of viable cells, rather than during the collagenase perfusion. An analysis of cellular compartments shows that the adenine nucleotide of the cell consists of 17 nmol adenine nucleotide in the cytosol, 5 nmol in the mitochondria, and 1.3 nmol adenosine diphosphate bound to myofibrils per milligram cell protein. Myocytes lose both adenosine triphosphate and adenine nucleotide when incubated anaerobically in the absence of glucose, and the lost adenine nucleotide can be accounted for as increased inosine, adenosine, and inosine monophosphate. Myocytes that contain less than 0.1 nmol of cytosol adenosine triphosphate per milligram cell protein maintain an intact sarcolemma, but are unable to carry out anaerobic glycolysis. Reoxygenation of anaerobic cells results in restoration of energy charge and a net resynthesis of about 2 nmol adenine nucleotide per milligram protein. Adenosine and inosine monophosphate decrease on reaeration of anaerobic cells, whereas inosine levels increase. When iodoacetate is added to block glycolysis, the decline in adenine nucleotide and production of inosine monophosphate are accelerated and there is no resynthesis of adenine nucleotide when anaerobic cells are reoxygenated . Large accumulations of inosine monophosphate are also seen in myocytes treated with an uncoupler of oxidative phosphorylation.

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Plasticity of human tumor populations could account for the reason why many tumorigenic human cell lines lose this feature when grown in culture.