Endogenous protein, turnover

The biosynthetic incorporation of amino acids into proteins makes these metabolites valuable endogenous tracers for the characterization of protein turnover. Of the naturally occurring amino acids, administration of a bolus dose of pH]leucine is widely used as a tracer in kinetic investigations of protein synthesis and secretion. 

Poole, B., Ohkuma, S., and Warburton, M. (1978). Some aspects of the intracellular breakdown of exogenous and endogenous proteins. In Segal H.L. and Doyle D.J., eds. Protein Turnover and Lysosome Function. Academic Press, New York, NY USA. pp. 43-58. 

The proteases and other pancreatic enzymes such as pancreatic lipase are eventually degraded (by proteases). Digestion is responsible for the turnover of about 50 g of endogenous protein per day. This comes from the breakdown of the pancreatic enzymes as well as the epithelial cells of the gut, which are replaced every 24 hours. 

Meisler I wanted to ask about the brefeldin experiment you used as a control. Does that prevent turnover or delivery of the endogenous proteins as well If so, does that say anything about the turnover of channels ... 

There are numerous examples of new proteins replacing old in response to an endogenous or environmental cue (20) and many models have been produced for predicting mechanisms for protein turnover. The one protein-one protease concept is not considered likely to be of widespread occurrence in cells (21) but is thought to be the major mechanism controlling Dl turnover. Hilditch (22) have proposed two separate mechanisms for Dl turnover in senescing Festuca leaves, one being rapid and... 

The average minimum requirement of dietary protein for N equilibrium in the adult need only contain 6 gm of essential amino acids. However, the diet of 90 gm of protein usually consumed provides about 45 gm of essential amino acids. To this are added the essential amino acids entering the gut as endogenous proteins. In consequence, about 75 gm of essential amino acids are absorbed daily. This amount can be contrasted with the total amounts of essential and non-essential amino acids in the plasma, namely 0.2 and 0.5 gm. It is thus to be expected that free amino acids will be rapidly transported out of the blood into the tissues. It will also be noted that, although the tissues contain some 70 gm of free amino acids, only 10 gm are as essential amino acids. Nevertheless, the daily turnover of 300 gm of body protein requires 150 gm of essential amino acids. Most of this has to come from recycling of essential amino acid released by the tissues, since the diet customarily provides about 45 gm essential amino acids. Nitrogen equilibrium can still be achieved when the essential amino acid content of the diet is as low as 6 gm. This implies very efficient recycling of amino acids within the body. 

Exograions (from food or snpplements) and endogenous (from body protein turnover) AAs form the AA pool that serves the body s needs for the synthesis of proteins such as ... 

A stimulus which alters the steady-state level of an endogenous cellular component may do so by influencing its rate of synthesis, its rate of break-down, or both. When administered to intact animals, phenobarbital or 3-methylcholanthrene increase (20-50%) the steady-state level of microsomal protein. Similarly, micro-somes from animals pretreated with phenobarbital or 3-methylcholanthrene incorporate radioactive amino acids into protein more rapidly than microsomes from control animals and this effect is blocked by co-administration of actinomycin-D. It was therefore assumed that the increased levels of microsomal protein and enzyme activity after inducers were the result of enhanced synthesis. However, turnover studies have revealed that phenobarbital in particular has a profound effect upon microsomal protein catabolism. Proteins of the endoplasmic reticulum were labelled by injection of radioactive amino acids and the rate at which radioactivity disappeared from the microsomes was compared in control and phenobarbital-treated animals. Assuming a comparable degree of isotope re-utilization in the two groups, this approach provides a relative measure of microsomal-protein turnover. In control animals, radioactivity of total microsomal protein decreases with time with a half-time of about 3 days. In phenobarbital-treated animals, however, there is a marked stabilization of microsomal protein so that almost no radioactivity is lost over a S-day period. The reduced protein catabolism is observed both in total microsomes and in a purified microsomal protein, NADPH cytochrome c reductase. Thus, repeated administration of phenobarbital to animals evokes an increase in... 

Proteases are regulated in vivo by autoinhibition (as zymogens), proteolytic activation, turnover and by endogenous protease inhibitor proteins. The following brief sketch of protease complexity is accompanied by succinct reference to the physiological context and hence the potential pharmacological relevance of plant-derived protease inhibitors to be described later in this review. 

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