Peptidylarginine deiminases (PADs; EC 18.104.22.168) are a family of calcium-dependent enzymes that convert peptidylarginine into peptidylcitrulline. The recent finding that patients with rheumatoid arthritis (RA) produce autoantibodies against citrulline-containing epitopes greatly increased the interest in the PAD enzymes and their activities. It is not yet known whether there is a causative relationship between the generation of antibodies targeting citrullinated epitopes and the development of the disease. Characterisation of the structure and function of PADs may help to understand the production process of citrullinated antigens and possibly also the aetiology of RA.
Several assays are known to monitor PAD activity in biological samples. However, these assays either have a low sensitivity or are laborious. Here, we describe the development of a simple, rapid method for the simultaneous analysis of many PAD-containing samples. This new method is based on the binding of an antibody specifically recognising a citrulline-containing epitope in a defined peptide. We show that this method is very sensitive and can be applied to monitor PAD activity in many types of biological samples, such as bacterial lysates, mammalian cell extracts and tissue extracts.