Colorimetric determination of citrulline residues in proteins:
To measure the total citrullinated proteins in the lung homogenates, an equal amount of proteins from non-COVID-19 and COVID-19 patients were added to the chromogenic reagent (Diacetyl-monoxime Solution + Antipyrine Solution + Acid-Ferric Solution, at 1:2:3), mixed well, and the tubes covered with aluminum foil. For standard, 0.5 to 10 nmoles of citrulline were prepared as described above. The samples and standards were heated for 25 min in a boiling water bath and cooled to RT on ice. The color developed was read at 464 nm.43Quantification of the citrullinated proteins in the lung homogenates was determined using the citrulline-protein standard curve.