Ornithine δ-Transaminase Activity During the Growth Cycle of Chang's Liver Cells

Abstract

Abstract 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.