The term peptide is typically used for polypeptides in the range up to approximately 20 kDa molecular weight. Peptides are usually the products of proteolytic cleavage events (e.g. cleavage of activation peptides from zymogens). Many pathological processes are associated with changes in protease expression or activation and these changes are captured or represented in altered peptide profiles.
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In contrast to cell-associated proteases or the precursor protein respectively, peptides exhibit better permeability between tissue compartments due to the relative low molecular weight. Therefore, the probability to detect proteolytic fragments of tissue-born proteins in body fluids is significantly higher than to identify the protein precursor.
Peptides are suitable to encode conditions diametrically opposed to each other (Coagulation versus Fibrinolysis), which are not accessible by classical Proteomics methods (e.g. after Tryptic Digestion).
Endogenous peptides can be regarded as versatile biomarkers because:

Peptides are capable to reflect changes in the expression of their precursor proteins

Peptides are mirroring activity of the corresponding processing proteases

Peptides have a relatively low molecular weight, that increases the susceptibility to peripheral detection

These properties indicate that peptides as a molecular class are ideal diagnostic marker candidates.