Proteins - continued metabolism

In cells that constitutively express the apoptotic machinery (now thought to be the majority of cells) but suppress its activity via mechanisms that require continued protein synthesis, metabolic inhibitors enhance apoptosis. The situation, however, is not so clear cut as the above descriptions imply, since multiple mechanisms appear to operate in some cells, depending on the stimulus and the state of the cell (Wyllie et al., 1984 Rukenstein et al., 1991). A similar situation exists for the mechanisms that suppress or protect from apoptosis (Edwards et al., 1991 Ni et al., 1994 Rukenstein et al., 1991). In recent years it has become obvious that a number of initiation... 

It is continuously recycled as part of the biotin-containing protein s metabolism. 

Biosynthesis of Protein. The dynamic equilibrium of body protein was confirmed by animal experiments using A/-labeled amino acids in 1939 (104). The human body is maintained by a continuous equilibrium between the biosynthesis of proteins and their degradative metabolism where the nitrogen lost as urea (about 85% of total excreted nitrogen) and other nitrogen compounds is about 12 g/d under ordinary conditions. The details of protein biosynthesis in living cells have been described (2,6) (see also Proteins). 

Feedback inhibition of amino acid transporters by amino acids synthesized by the cells might be responsible for the well known fact that blocking protein synthesis by cycloheximide in Saccharomyces cerevisiae inhibits the uptake of most amino acids [56]. Indeed, under these conditions, endogenous amino acids continue to accumulate. This situation, which precludes studying amino acid transport in yeast in the presence of inhibitors of protein synthesis, is very different from that observed in bacteria, where amino acid uptake is commonly measured in the presence of chloramphenicol in order to isolate the uptake process from further metabolism of accumulated substances. In yeast, when nitrogen starvation rather than cycloheximide is used to block protein synthesis, this leads to very high uptake activity. This fact supports the feedback inhibition interpretation of the observed cycloheximide effect. 

Connective tissue is composed of apparently very different cells metabolically it is dynamic undergoing continual turnover and so to maintain health means that a balance must be achieved between biosynthesis and degradation. Cells in connective tissue are usually found embedded within a matrix composed of proteins with variable amounts of proteoglycan and genetically determined enzymatic defects in the production of the matrix may result in often serious pathologies. Metabolism in... 

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