Citrullination

Citrullination or deimination is the conversion of the amino acid arginine in a protein into the amino acid citrulline. Citrulline is not one of the 20 standard amino acids encoded by DNA in the genetic code. Instead, it is the result of a post-translational modification. Citrullination is distinct from the formation of the free amino acid citrulline as part of the urea cycle or as a byproduct of enzymes of the nitric oxide synthase family.

Enzymes called arginine deiminases (ADIs) catalyze the deimination of free arginine, while protein arginine deiminases or peptidylarginine deiminases (PADs) replace the primary ketimine group (=NH) by a ketone group (=O). Arginine is positively charged at a neutral pH, whereas citrulline has no net charge. This increases the hydrophobicity of the protein, which can lead to changes in protein folding, affecting the structure and function.

In eukaryotes, citrullination controls the expression of genes, particularly in the developing embryo. The immune system often attacks citrullinated proteins, leading to autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. Fibrin and fibrinogen may be favored sites for arginine deimination within rheumatoid joints. Test for presence of anti-citrullinated protein (ACP) antibodies are highly specific (88-96%) for rheumatoid arthritis (RA), about as sensitive as rheumatoid factor (70-78%) for diagnosis of RA, and are detectable from even before the onset of clinical disease.

Citrullinated vimentin may be an autoantigen in RA and other autoimmune diseases, and is used to study RA. Moreover, antibodies against mutated citrullinated vimentin (MCV) may be useful for monitoring effects of RA therapy. An ELISA system utilises genetically modified citrullinated vimentin (MCV), a naturally occurring isoform of vimentin to improve the performance of the test.

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