Protein Metabolism

This one I'm bumping for myself, but perhaps some folks might find it interesting.  The red bolding is new.

Original Publication:  4/17/10

Looking for some info on protein, I came across this link:

The page focuses on changes in dietary requirements and/or the impact on the body of malnutrition, disease and stress, but I found this site through the following graphic on protein metabolism:

Something in the numbers doesn't add up here, but I was surprised to see how much protein is "recycled" in our guts.  Perhaps the total protein synthesis is balanced by catabolism/turnover and that is the missing number?  Not sure.  Here is the quoted text referring to this diagram (my apologies for the messed up formatting).  
Protein in the body is not static; protein synthesis and breakdown is constantly taking place. However, the total body protein pool in a healthy adult is constant. Synthesis of protein from endogenous and exogenous amino acids is equal to degradation and external losses. Some proteins have a long lifetime, such as muscle protein and plasma albumin, while others have a high turnover rate. Muscle protein constitutes up to approximately 50% of total body protein, but contributes only approximately 30% of the protein turnover in the body, because in visceral and other organs, protein turnover rates are several times higher than in muscle tissue. 
Protein metabolism is dependent on a vast number of endogenous mediators. These mediators define the balance between anabolic and catabolic processes. Insulin is the major anabolic hormone and also has an important role in amino acid and protein homeostasis. During injury and stress two major alterations in insulin are noted: a catecholamine-mediated suppression of insulin release and an insulin resistance, leading to a release of skeletal muscle amino acid for gluconeogenesis and, at the same time, a decreased utilisation of glucose by insulin-dependent tissues. This mechanism provides glucose to the insulin-dependent tissues that are important for survival and the healing of injury, such as the central nervous system (CNS), immune system and red blood cells. Other hormones, like glucagon and the catecholamines, control or counteract the effects of insulin and are more or less proteolytic; the exact role of catecholamines is still under discussion, however.
In order to define protein requirements, an overview of protein metabolism is necessary. By determining the renewal of proteins susceptible to measurement, such as plasma, muscle and digestive secretion proteins, it has been possible to estimate the daily turnover in proteins. Considerable recycling of endogenous amino acids seems to occur, the quantity amounting to twice the daily intake.   Hence, normal protein metabolism incorporates about 100 g of dietary amino acids and over 200 g of endogenous amino acids daily. Allowance must therefore be made for increased losses of endogenous proteins when assessing patients´ protein intake. 
So fully 2/3rds of our daily protein metabolism comes from proteins in our body -- the amino acid pool in our cells and/or breakdown of tissues.  Another factoid:  Structural proteins have about a 6 month half life, while hormones/peptides/etc. can have half-lives of a matter of minutes.  However hormones etc. constitute a very small amount of protein weight wise.


Lerner said…
"normal protein metabolism incorporates about 100 g of dietary amino acids and over 200 g of endogenous amino acids daily"

now that is interesting as to the amount -- and especially since I have been wondering why a person who is not building muscle or repairing injured organs etc. cannot get by completely on recycling the precious amino acids, as a matter of course. Is it that amino acids can get so broken down that they cannot be repaired? That seems improbable. And why excrete any nitrogen at all? Even if it gets cleaved from catabolizing amino acids why not then use that nitrogen to build new AAs instead of excreting it?

Can it be that protein truly was very plentiful throughout human evolution, so therefore like vitamin C there was no need to be able to create it or to have super-retention for it?
Sanjeev Sharma said…
IMHO in homo sapiens the loss of vitC synthetic ability is more due to uric acid being selected as a better mechanism for some of vitC's actions - reduction capability for example

and less due to vitC being ubiquitous in the food environment.
Myron Schwarzennecker said…
That's interesting because years ago I'd read that humans lost the gene to make uricase, which means that humans are then susceptible to uric acid kidney stones and to gout. Lately Paleos et al have been saying that humans can't make vitamin C because of some mutation.
Sanjeev Sharma said…
indeed selection pressure for high UA or something that demanded UA was so high that UA's downsides / demerits / risks couldn't prevent those gene frequencies from rising
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