Why are there proteins in the urine of healthy people?

Proteins are essential nutrients for humans, but why is there a small amount of proteins excreted through urine? Although healthy people have low levels of protein in the urine, they are rich in variety. The proteins in them may be unnecessary wastes of the body or may even have a toxic effect on the body and must be excreted. Compared with small-molecule metabolites, proteins have a complex structure and various functions. Even a small change at the molecular level will affect their subsequent biological functions. A comprehensive comparison of the molecular level modifications of plasma and urine proteome, the difference may provide clues to the presence of protein in the urine. In this study, a total of 9 healthy people’s urine and 9 healthy people’s plasma samples were collected. The samples were analyzed by high-resolution tandem mass spectrometry and unlabeled quantitative proteomics techniques, and the non-limiting modification identification algorithm was used to to compare molecular modifications between two types of samples as a whole. The results showed that the amount of cysteine (Cys) modification to dehydroalanine (Dha) in the urine proteome was higher than that in plasma. The molecular modification [Cys→Dha] destroys the disulfide bond of the original protein, thereby affecting or even changing the structure and biological function of the protein. Therefore, this study revealed the differences in the overall proteome modification of plasma and urine and pointed out that the structure of protein may be irreversibly changed due to the difference in modification of protein molecules, which in turn caused proteins which lost their functions and even toxic proteins from the blood to be excreted into the urine.