THE ENZYMATIC NATURE OF C'1r

Abstract

Human C'1, a macromolecular complex composed of three subunits, is the zymogen for at least two distinct enzymes. Preparations of one subunit, C'1r, functioned as a protease which converted another subunit, C'1s, to C'1 esterase. The conversion of C'1s to C'1 esterase by C'1r was blocked by Liquoid, phenyl methylsulfonyl fluoride, and calcium ions, but not by soybean trypsin inhibitor, hirudin, or heparin. Preparations of C'1r also possessed two additional functions, i.e., the ability to hydrolyze certain synthetic amino acid esters and to participate in immune hemolysis. Evidence was presented which indicates that these three functions are properties of a single entity, C'1r, but not of the same portion of its molecular structure. These observations suggest that C'1r has at least two active sites, one for its reaction with C'1q, an additional subunit of C'1, and one for its reaction with C'1s; together, the three subcomponents, C'1q, C'1r, and C'1s, form a single functional unit, the first component of complement.

THE INHIBITION OF PLASMIN, PLASMA KALLIKREIN, PLASMA PERMEABILITY FACTOR, AND THE C'1r SUBCOMPONENT OF THE FIRST COMPONENT OF COMPLEMENT BY SERUM C'1 ESTERASE INHIBITOR

“…Under these circumstances, the formation of Cq esterase in mixtures of Cqr and Cqs was strikingly inhibited by the presence of Cq esterase inhibitor. Preparations of C~lr also possess esterolytic properties (36). Proof that the esterase in preparations of Cqr is identical with this subcomponent of complement awaits its further purification, but it is noteworthy that the hydrolysis of its most unique substrate, N-acetyl-Larginine methyl ester, was readily inhibited by C'I esterase inhibitor.…”

“…Preparations containing Cqr in its active state convert Cqs to Crl esterase, probably enzymatically, and hydrolyze AAME, among other syn-thetic substrates (36). C'I esterase inhibitor blocked both of these reactions.…”

“…In the test tube, C'1 esterase is generated from the Cqs fragment of the CI1 molecule through the enzymatic action of a second fragment, the C'lr subcomponent. This action of CPlr is apparently enzymatic (36). In the present experiments, the role of C'I esterase inhibitor was studied by taking advantage of the inhibition of Cqr by Liquoid.…”

Serum levels of C1q, C1r and C1s in normal and pathologic sera

“…Activation of the first component of C by immune complexes requires an internal activation step involving Clq and Clr and results in the proteolytic cleavage of precursor Cls (13)(14)(15). Cls can also be activated by the reaction of Cls proesterase with proteolytic enzymes in the absence of an immunologic mechanism (16).…”

Partial purification and autocatalytic activation of the subunit of the first component of human complement, C1r

“…Autocatalytic activation of C1 r was measured with the pH-stat method at pH 7.5, using 40 mM AAME in 4 mM phosphate buffer containing 0.15 M NaCI in a final volume of 2.5 ml since AAME is a well-known substrate for C1T [8]. The reaction mixture was maintained at 37°C and pH 7.5 using 20 mM NaOH.…”