Protein carbamylation is a post-translational modification that can occur in the 
presence of urea. In solution, urea is in equilibrium with ammonium cyanate, and 
carbamylation occurs when cyanate ions react with the amino groups of lysines, 
arginines, protein N-termini, as well as sulfhydryl groups of cysteines. The 
concentration of urea is elevated in the renal inner medulla compared with other 
tissues. Due to the high urea concentration, we hypothesized that carbamylation 
can occur endogenously within the rat inner medulla. Using immunoblotting of rat 
kidney cortical and medullary homogenates with a carbamyl-lysine specific 
antibody, we showed that carbamylation is present in a large number of inner 
medullary proteins. Using protein mass spectrometry (LC-MS/MS) of rat renal 
inner medulla, we identified 456 unique carbamylated sites in 403 proteins, 
including many that play important physiological roles in the renal medulla 
[Data can be accessed at 
https://helixweb.nih.gov/ESBL/Database/Carbamylation/Carbamylation_peptide_sorted.html]. 
We conclude that protein carbamylation occurs endogenously in the kidney, 
modifying many physiologically important proteins.