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The structure of human microplasmin, prepared from plasmin in alkaline solution, has been studied. Microplasmin consists of two polypeptide chains connected by disulfide bonds. One polypeptide is the B chain of plasmin consisting of 230 amino acids, and the other peptide is the COOH-terminal portion of the A chain of plasmin consisting of 31 amino acid residues. Microplasmin has a molecular weight of 28,635, calculated from its primary sequence. It is slightly more positively charged than plasminogen and is a more hydrophobic molecule. The proposed scheme for the formation of microplasmin involves autolysis at specific peptide bonds and scrambling of especially sensitive disulfide bonds in alkaline solution. Plasmin, a trypsin-like serine protease, catalyzes the hydrolytic cleavage of peptide bonds at the COOH sides of arginines and lysines in protein and peptide substrates (1). The enzyme undergoes cannibalistic hydrolysis, plasmin molecules serving as both substrate and enzyme. The autolytic cleavage of plasmin at pH 11.0 leads to the formation of microplasmin (2). Studies of the primary structure of microplasmin and the possible mechanism of its formation are discussed in this report. MATERIALS AND METHODSProtein. Microplasmin was prepared from human plasmin variant 2 and purified by affinity chromatography as described (2).Reduction and S-Carboxymethylation. Lyophilized protein (10 mg) was dissolved in 10 ml of 6 M guanidine hydrochloride/0.25 M Tris-HCl/3 mM EDTA/100 ,1l of 2-mercaptoethanol, pH 8.6, and capped under nitrogen, and the reaction was initiated by the addition of 1.0 ml of a freshly prepared solution of iodoacetic acid (270 mg/ml) and 1.0 M NaOH. The S-carboxymethylation was allowed to continue for 30 min in the dark, at which time 100 ,u1 of 2-mercaptoethanol and 12 ml of glacial acetic acid were added. The mixture was applied to a Sephadex G-10 column (1.6 x 90 cm) and eluted with 50% (vol/vol) acetic acid.Peptide Analysis with HPLC. HPLC was carried out on a ,tBondapak phenylalkyl column (0.4 x 30 cm) in a HewlettPackard model 1090 system. The acetonitrile gradient systems used are described in the legend of each chromatogram.Peptide fractions were pooled and lyophilized.

SummaryA new abnormal, variant, plasminogen Chicago III has been isolated from a patient with recurring deep vein thrombosis. Studies on the plasma fibrinolytic system of four family members showed no inheritance pattern. Kinetics of activation parameters of Chicago III plasminogen with different activators showed lowered catalytic rate constants from 159fold with urokinase, to 3fold with light (B) chain ‧ streptokinase complex, to 1.3fold with streptokinase. The Michaelis constants of activation of Chicago III plasminogen were 16fold higher with streptokinase, 6fold higher with light (B) chain ‧ streptokinase complex, and similar with urokinase. Each of the three activators exhibited different interaction characteristics with this variant zymogen, as they did with variant Chicago I and Chicago II plasminogens (previously reported). However, the latter variants were characterized by having normal catalytic rate constants of activation, with higher than normal apparent Michaelis constants of activation. Chicago III plasminogen, as well as Chicago I and II plasminogens, has a homogeneous population of molecules; the interpretation of the kinetic data was possible only in terms of a single population of molecules. The plasmins derived from Chicago I plasminogen, and also Chicago II and Chicago III plasminogens, have 100% active sites with normal amidase parameters. Basically, a single population of normal isoelectric forms were found in the Chicago III plasma, with three minor forms, about 11% of the total.The kinetic parameters have permitted us to classify the four known plasminogen variants into three different classes. The Class A homozygote (Tochigi) has both an active center defect and a charge mutation difference with normal cleavage of the Arg560-Val peptide bond; the Class B homozygote (Chicago I and Chicago II) has a Km of activation defect with impaired activator binding and normal cleavage of the Arg560-Val peptide bond; and, the Class C homozygote (Chicago III) has both a Km of activation defect with impaired activator binding and a kcat of activation defect, and impaired Arg560-Val peptide bond cleavage.

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