What is aging?
In the late 1970s, Earl gradually shifted the focus of his research from the regulation of enzyme activities to that of protein turnover—the process by which damaged or inactivated proteins are removed from cells. How do the cells recognize malfunctioning enzymes? What are the signs of those "troubled" enzymes? In short, what governs the regulation of protein turnover? After studying the activity of normal or "healthy" enzymes for decades, Earl sought to understand how cells can selectively degrade certain proteins which are no longer needed because of changes in their environment. This problem led him to investigate the characteristics of "not-so-healthy" enzymes and their ultimate degradation into smaller pieces, such as amino acids. It was largely an unexplored area of research in biochemistry.
From previous studies in other laboratories, Earl was aware that the amount of enzymes in cells was generally affected by the nutritional state of the organism. Taking this information as a clue, Earl and co-workers set up experiments for the controlled starvation of E. coli , by cutting off the source of nitrogen or carbon during its growth in culture media; and they examined the amounts of fifteen enzymes that were extracted from these starved bacteria. The results were mixed: some enzymes showed an increase in their amounts during starvation; others a decrease; and still others no change. Earl then focused on the enzymes whose level of concentration dropped substantially. Among them was glutamine synthetase, which once again became a central research material in Earl's laboratory.
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