Citrullination is a natural biochemical process involving a slight modification of proteins within the body. This modification converts a specific amino acid, arginine, into another amino acid, citrulline, directly within the protein structure. While this process is a normal part of how our bodies function, it also has implications in the development and progression of certain diseases. Understanding this dual nature helps to explain why this seemingly small change in proteins can be quite significant for health.
The Process of Citrullination
Citrullination involves the conversion of the amino acid arginine into citrulline, which occurs within an already formed protein. This transformation is catalyzed by peptidylarginine deiminases (PADs). In humans, there are five known PAD isoforms: PAD1, PAD2, PAD3, PAD4, and PAD6, each with varying tissue expression. These enzymes require calcium ions for their activity.
The modification by PAD enzymes results in a change to the guanidinium group of arginine, converting it into a ureido group and releasing ammonium. Arginine carries a positive charge at a neutral pH, whereas citrulline has no net charge. This loss of positive charge significantly alters the protein’s properties, including its folding, structure, and interactions, and can lead to protein denaturation. The citrullination reaction is irreversible; citrulline cannot be converted back to arginine within the protein.
Normal Roles in the Body
Despite its association with disease, citrullination is a naturally occurring process with purposeful roles in healthy physiological functions. It plays a part in the normal operation of the immune system, including the formation of neutrophil extracellular traps (NETs), which are web-like structures released by neutrophils to trap and neutralize pathogens, aiding in immune defense. Citrullination also contributes to the formation of the skin barrier.
In the epidermis, PAD1 enzyme activity is involved in keratinization, where proteins like keratin and filaggrin are citrullinated. This modification is important for regulating cornification and maintaining the skin’s protective barrier. The process is also involved in the insulation of neurons and the plasticity of the central nervous system, including its influence on gene regulation.
Citrullination and Disease Development
Dysregulation of citrullination or an altered immune response to citrullinated proteins contributes to the development of certain diseases. A prominent example is rheumatoid arthritis (RA), where increased PAD activity and protein citrullination are strongly linked to the disease. In RA, the immune system mistakenly recognizes citrullinated “self” proteins as foreign, leading to the production of autoantibodies called anti-citrullinated protein antibodies (ACPAs), also known as anti-CCP antibodies.
These ACPAs target various citrullinated proteins found in the joints, such as alpha-enolase, fibrinogen, filaggrin, and vimentin. The presence of ACPAs in the blood can occur years before the onset of clinical RA symptoms, suggesting an early role in the autoimmune process. These antibodies can activate immune cells, like macrophages, and promote inflammation and bone erosion within the joints, contributing to the characteristic joint damage seen in RA.
Citrullination is also being investigated in other conditions. For instance, in multiple sclerosis (MS), excessive citrullination of myelin basic protein (MBP) by PAD2 is associated with demyelination, disrupting nerve signal transmission. While more than 45% of MBP can be citrullinated in MS patients, it is typically around 18% in healthy individuals. Elevated citrullination and PAD activity have also been observed in neurodegenerative disorders like Alzheimer’s disease, certain cancers, and inflammatory conditions such as chronic obstructive pulmonary disease (COPD) and inflammatory bowel disease (IBD).
Measuring Citrullination in Healthcare
In healthcare, citrullination is primarily assessed by measuring anti-citrullinated protein antibodies (ACPAs) in blood serum. The anti-cyclic citrullinated peptide (anti-CCP) test commonly detects these antibodies. A positive anti-CCP test is highly specific for rheumatoid arthritis (RA), with specificity ranging from 85% to 99%.
ACPAs are a significant diagnostic and prognostic marker for RA. They can be detected in serum years before joint symptoms, making them valuable for early diagnosis. A positive anti-CCP test indicates a higher likelihood of developing RA and is associated with a more aggressive disease course, with increased risk of joint erosion and deformity. This information assists healthcare providers in patient management and treatment strategies.