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What did isotope experiments reveal about protein turnover in the liver and gut?

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Diagram showing how epinephrine binding to a beta-adrenergic receptor on a cell membrane activates a G-protein, which then stimulates adenylyl cyclase to convert ATP into cyclic AMP (cAMP), leading to physiological effects such as increased heart rate, blood vessel dilation, and glycogen breakdown. — Biochemistry In biochemistry, researchers study the chemical substances and processes that occur in living organisms and cells. In particular, various processes by which cells gain energy, such as epinephrine-stimulated cAMP synthesis, have been deduced through the study of biochemistry.

Isotope experiments conducted in the 1930s fundamentally changed the scientific understanding of metabolism by revealing the "dynamic state" of body constituents. Prior to these studies, it was widely believed that tissue proteins were stable structures that remained unchanged until a cell died.1

By feeding rats amino acids labeled with isotopic nitrogen, researchers discovered that the isotopes were rapidly incorporated into the proteins of the liver and the gut, even though the total protein content of these tissues remained constant. This observation demonstrated that these proteins exist in a dynamic steady state, where high rates of protein synthesis are continuously balanced by equal rates of degradation.1

This turnover is remarkably rapid; for instance, half of the proteins in an average liver cell are synthesized and degraded every five to six days. These findings, pioneered by biochemists such as Rudolf Schoenheimer, proved that animal cells exert coarse metabolic control through the active regulation of protein synthesis and breakdown.12