The Effect of Acid Denaturation upon the Combining Power of Fibrinogen

In a study of the acidand base-binding capacity of fibrin and fibrinogen, it became evident that the latter is changed by long standing in the presence of a small amount of acid, even when the solution is kept in the refrigerator. The change is shown by the greater tendency of the protein to precipitate. Unless the protein has been previously dialyzed against sodium chloride solution, it forms an insoluble jelly-like precipitate soon after the addition of enough hydrochloric acid to bring the pH to 2.5 or 3.0. This precipitate usually does not form if the protein has been dialyzed. That the dialyzed fibrinogen is also changed by acid is shown by the appearance of a precipitate when the acid is neutralized. This precipitate does not dissolve readily in dilute alkali, whereas native fibrinogen is easily dissolved by a small amount of sodium hydroxide. The fibrinogen is so readily changed by the addition of acid that 0.1 N HCl usually precipitates some of the protein immediately in an insoluble form. A more dilute acid (0.02 N) in equivalent amount may not cause immediate precipitation, but if the fibrinogen solution has not been previously dialyzed, the precipitate is formed after standing several hours in the cold.