Elucidating the mechanism of cellular uptake and removal of protein
Thus, although the average liver cell has a life-span of several months, half of its proteins are synthesized and degraded every five to six days.
On the other hand, the proteins of the muscle or the brain, tissues that (unlike the gut or liver) need not adjust to changes in the chemical composition of their milieu, do not turn over as rapidly.
Third, the fate of any nutrient—indeed, often the fate of a particular chemical group or atom in a nutrient—can be followed with relative ease by administering the nutrient labeled with an isotope.
Isotopes are forms of an element that are chemically indistinguishable from each other but differ in physical properties.
It is thus possible to obtain fractions containing predominantly one type of organelle: nuclei (and some unbroken cells); mitochondria, lysosomes, and microbodies; microsomes (i.e., ribosomes and endoplasmic reticulum fragments); and—after prolonged centrifugation at forces in excess of 100,000 times gravity—a clear liquid that represents the soluble fraction of the cytoplasm.
In addition to their utility in the unraveling of metabolic pathways, the use of mutants in the early 1940s led to the postulation of the one gene-one enzyme hypothesis by the Nobel Prize winners Edward L.They cannot reveal, however, the details of the enzymatic steps involved.Clues to the identity of the products involved, and to the possible chemical changes effected by component enzymes, can be provided in any of four ways involving studies with either whole organisms or tissues.Such measurements revealed the cyclic nature of the process; specific amino acids acted as catalysts, stimulating respiration to an extent greater than expected from the quantities added.This was because the added material had been re-formed in the course of the cycle (Homogenates of tissue are useful in studying metabolic processes because permeability barriers that may prevent ready access of external materials to cell components are destroyed.
Search for elucidating the mechanism of cellular uptake and removal of protein:
The high rates of turnover observed in liver and gut tissues indicate that the coarse controls, exerted through the onset and cessation of synthesis of pacemaker enzymes, do occur in animal cells.