title: Protein turnover during cell growth: a re-examination of the problem of linear incorporation kinetics of radioactively-labelled amino acids into protein and its relationship to growth characteristics.
abstract: Linear kinetics of incorporation of labelled amino acids into protein have been reported in many cell types from bacteria to mammals. Using HeLa S-3 cells as a paradigm in carefully controlled kinetic studies of this growing mammalian cell system, detailed analysis of many of the factors which are involved under steady growth state conditions indicates that linearity is more apparent than real. Quasi-linear kinetics are not primarily due to the time-scales of experiments being too short for adequate distinction between linearity and shallow exponential kinetics, nor to the fact that conventional assay procedures of biological material are too variable (although both have been taken into account), but to the highly complex pattern of protein turnover following synthesis. Synthesis is a zero-order reaction which would inevitably give exponential kinetics if it were not offset by a series of first-order degradation reactions operating at different intensities during the subsequent history of each cohort of protein molecules produced by the cell. These interactions, however, give a net result for incorporation of label into protein which closely approximates linearity, and hides the true complexity of the underlying processes. These findings on labelling kinetics are discussed throughout in relation to the growth characteristics of the cells.
mesh_terms: D002455:Cell Division; D006367:HeLa Cells / Q000166:cytology / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D007930:Leucine / Q000378:metabolism; D009363:Neoplasm Proteins / Q000096:biosynthesis / Q000378:metabolism*; D014316:Tritium
chemical_list: D009363:Neoplasm Proteins; D014316:Tritium; D007930:Leucine