The Alpha Macroglobulins of Rat Serum

A three-stage method for isolation of alpha1 macroglobulin and alpha2 macroglobulin from the serum of normal and injured rats is described. The methods successively used, namely gel filtration, ultracentrifugation and chromatography on DEAE-cellulose, were chosen to minimize loss of tryptic esterase-protecting activity. The two proteins differed slightly with respect to the following properties: mol.wt., alpha1 macroglobulin 7.46 X 10(5), alpha2 macroglobulin 7.16 X 10(5); isoelectric focusing, alpha1, macroglobulin pI 4.4, alpha2 macroglobulin pI4.5. Amino acid analyses were identical, except with respect to tyrosine: alpha1 macroglobulin 3.96 +/- 0.24, alpha2 macroglobulin 3.16 +/- 0.32 mol/100 mol of total amino acids. When isolated from the serum of uninjured rats, alpha1 macroglobulin retained the capacity to bind 1.05 mol of trypsin/mol. However, if isolated from serum 2 days after injury only 0.78 mol of trypsin/mol of alpha1 macroglobulin was bound. alpha2 macroglobulin isolated from this latter serum bound on average 0.97 mol of trypsin/mol. When reduced with N-acetylcysteine, both molecules formed subunits of size corresponding to that expected for quarter molecules. When alpha2 macroglobulin was reduced with dithiothreitol, quarter molecules were again produced. alpha1 macroglobulin, however, when thus treated gave a more complex mixture, containing a component having a mol.wt. of less than 6 X 10(4). Antisera raised against the two proteins permitted estimation of the concentration of each protein in the plasmas or sera of normal and injured rats. Plasma from normal male rats contained 3.76 +/- 0.56 mg of alpha1 macroglobulin/ml (n = 33) and 0.016 +/- 0.001 mg of alpha2 macroglobulin/ml (n=33). After injury by injection of turpentine and cortisone, the concentrations in plasma were at 3 days 5.19 +/- 0.81 mg of alpha1 macroglobulin/ml (n = 12) and at 2 days 1.38 +/- 0.35 mg of alpha2 macroglobulin/ml (n = 12). Antisera to the two proteins did not cross-react with one another. The quarter molecules formed by reduction of both proteins showed increased antigenicity.