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During the normal 4-day growth cycle of Chang's liver cells in tissue culture, ornithine δ-transaminase increased and returned to the initial level in a fairly regular and reproducible manner. These changes in activity appeared to be the net result of synthesis and degradation of enzyme. Synthesis of enzyme was prevented or decreased by omitting as essential amino acid from the medium or by the addition of puromycin. Inhibition of synthesis allowed a minimum degradation rate to be determined. Synthesis of enzyme began again when the missing amino acid was restored. If the missing amino acid was glutamine or phenylalanine, the subsequent rise of ornithine δ-transaminase took place as a single continuous stage after transferring the cells to complete medium. In contrast, when arginine was the amino acid omitted from the preceding incubation medium, the subsequent increase of enzymatic activity was biphasic. The addition of actinomycin either initially or after prior incubation of cells in medium lacking an essential amino acid stimulated the first phase rise; the second phase increase then was not observed. Leucine, valine, isoleucine, or ornithine, each of which is inhibitory to ornithine δ-transaminase, increased enzymatic activity when added at a 6 mm concentration and extended the period of normal increase of the enzyme. Other amino acids were without effect. It is proposed that the biosynthesis of ornithine δ-transaminase in Chang's liver cells is repressed by an as yet unidentified metabolic product. In addition, it is suggested that the enzyme is synthesized in response to messenger ribonucleic acid of an appreciable lifetime.

An increase of arginase activity has been produced in Chang's liver cells in suspension cultures by increasing the concentration in the growth medium of lysine, leucine, valine, or ornithine. These amino acids are known to inhibit enzymes in the arginase-initiated reaction sequence from arginine to proline. An increase in arginase also occurred with o-aminobenzaldehyde, a compound which reacts with a metabolite (Δ1-pyrroline 5-carboxylate) of the same reaction chain. A stimulation of arginase has been also shown following substitution of citrulline for arginine in the growth medium. A decrease in arginase was found after addition of proline, a product of the same reaction sequence. Experiments in which protein synthesis was inhibited by puromycin indicate that the rate of accumulation of enzyme protein was involved rather than an activation or inactivation of preformed enzyme. When arginase was stabilized by the addition of manganese to the growth medium, the compounds mentioned above produced the same results as before, although greater in magnitude, indicating an effect on the rate of synthesis rather than on the rate of degradation of the enzyme during general protein turnover. These results suggest that synthesis of arginase in Chang's liver cells during a normal 4-day growth cycle is regulated, in part at least, by product repression.