The ornithine transaminase (EC 2.6.1.13) concentration in normal liver and in the spectrum of hepatomas of different growth rates was determined by two independent methods, enzymatic assay of activity and immunotitration of enzyme amount.

Kinetic studies showed that the enzyme preparations from liver and hepatoma were indistinguishable on the basis of their pH optima, kinetic properties for substrates, and intracellular distribution. The affinity of the liver enzyme to ornithine at pH 7.4 was Km = 4.0 mm and for 2-oxoglutarate it was Km = 0.4 mm; at pH 8 it was Km = 0.8 mm for ornithine and for 2-oxoglutarate it was Km = 1.8 mm. For the hepatic enzyme, an optimum pH of 8.0 was observed. In the hepatomas the affinity of the enzyme to the substrate and the pH optimum were similar to the values in the liver of control normal rats.

Electrophoretic studies, using the acrylamide gel disc method, indicated that the purified ornithine transaminase from liver, from slowly growing hepatoma 44, and from rapidly growing hepatoma 7777 had the same mobility. Immunotitration showed that the enzymes in liver and hepatomas were immunologically identical.

The concentration of ornithine transaminase in the rapidly growing hepatomas was markedly decreased, indicating a decline in the potential of such neoplasms to channel ornithine into the synthesis of glutamic γ-semialdehyde and l-glutamate.

In the regenerating liver at 24 hr after partial hepatectomy, the enzyme activity was in the same range as that of the sham-operated controls. Since the regenerating liver proliferates at a rate comparable to that of the rapidly growing hepatomas, the markedly decreased activity observed in such tumors appeared to be specific to neoplastic cell growth.

The results of these studies obtained by independent methods for measuring enzyme concentrations support an interpretation that the alteration in the biochemical pattern of tumor cells entails a reprogramming of the pattern of gene expression which is manifested, in part, in the decreased amount of ornithine transaminase in the rapidly growing hepatomas.

The present studies, along with earlier ones in this laboratory, reveal the emergence of a metabolic imbalance in ornithine metabolism which is characterized in the rapidly growing hepatomas by a marked decrease in the activities of ornithine transaminase and ornithine carbamyl transferase in face of an increase in the activity of ornithine decarboxylase. Thus, in the full expression of neoplastic transformation among the three pathways of ornithine utilization, only the one channeling this amino acid into polyamine synthesis is retained and increased.

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This is Paper 3 in a series of publications on “Metabolic Imbalance in l-Ornithine Utilization in Hepatomas of Different Growth Rates.”

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©1974 American Association for Cancer Research.
1974
Cancer Research, Inc.