Rat plasma murinoglobulin: isolation, characterization, and comparison with rat alpha-1- and alpha-2-macroglobulins

Murinoglobulin, a newly identified mouse plasma protein with trypsin-protein esterase activity (Saito, A. & Sinohara, H. (1985) J. Biol. Chem. 260, 775-781), was also found in rat plasma and purified to apparent homogeneity. The serum level of rat murinoglobulin was 14.1 mg/ml, amounting to 1/3 of the total serum globulin fraction. Rat murinoglobulin was a monomeric glycoprotein (Mr = 210,000) containing 12% carbohydrate. Rat plasma contained two isoforms of murinoglobulin, termed I and II, which showed complete immunological identity on double diffusion analysis using rabbit antiserum raised against isoform I or II. These antisera also showed partial cross-reactivity towards mouse murinoglobulin and rat alpha-1-macroglobulin but not towards rat or human alpha-2-macroglobulin. The chemical compositions, peptide mapping patterns and electrophoretic mobilities of the two isoforms resembled each other but clearly differed from those of rat alpha-1- or alpha-2-macroglobulin. Rat murinoglobulin inhibited the proteolytic activity of trypsin towards casein and remazol brilliant blue hide powder. The inhibition as to the latter substrate was greater than that as to the former. When molar ratios of inhibitor to trypsin were low, murinoglobulin and the two alpha-macroglobulins stimulated the amidolytic activity of trypsin towards a synthetic substrate. At higher ratios, however, murinoglobulin, but not the alpha-macroglobulins, inhibited the same activity. The trypsin-protein esterase activity of murinoglobulin and the two alpha-macroglobulins was impaired by a molar excess of soybean trypsin inhibitor. Murinoglobulin and the two alpha-macroglobulins were inactivated by methylamine with a concomitant unmasking of the thiol group. Murinoglobulin was much more sensitive to soybean trypsin inhibitor and methylamine than the two alpha-macroglobulins.