End products of protein metabolism
The nitrogen released
from amino acid (protein) catabolism is excreted from the body in forms which
vary in different species of animals. The considerable amount of nitrogen
excreted through the urine is urea.. The human also excretes nitrogen of purine
bases as uric acid. Creatine excretion is exclusively from tissue protein
breakdown. Neutral sulphur excretion does not indicate the nature of dietary or
tissue protein breakdown.
Due to dynamic state of
amino nitrogen of amino acid and differential pattern of metabolism of labile
protein, the excretion of different nitrogenous metabolic end products does not
always give real metabolic picture of protein metabolism. Increased creatine
excretion may indicate increased tissue protein break down, i.e. specially
muscle protein catabolism going on. Increased uric acid excretion may be due to
increased purine catabolism. Increased urea excretion may indicate increased
dietary protein catabolism.
End
products of exogenous protein metabolism
End
products of endogenous protein metabolism
Creatinine and neutral
sulphur remain absolutely unchanged. So they are solely of endogenous origin.
In this connection, the compound creatine, but when present in urine, should be
regarded as derived from endogenous protein metabolism, because it is the
precursor of creatinine. Thus these compounds are wholly endogenous. Half of
uric acid is of endogenous origin and the other half is of exogenous origin.
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