Studies on the Xh antigen in human serum

Pregnancy zone protein, a proteinase binding alpha-macroglobulin. Stopped-flow kinetic studies of its interaction with chymotrypsin

Human pregnancy zone protein (PZP) is a major pregnancy-associated plasma protein, strongly related to alpha 2-macroglobulin (alpha 2M). The proteinase binding reaction of PZP is investigated using chymotrypsin as a model enzyme. The time-course of the interaction is studied by measuring the change in intrinsic protein fluorescence of PZP-chymotrypsin reaction mixtures as a function of time after rapid mixing in a stopped-flow apparatus. Titrations show the changes of fluorescence at equilibrium to correspond with the formation of a chymotrypsin-PZP(tetramer) species. The kinetic results show the formation of the species to take place in an overall second-order process dependent on the concentrations of chymotrypsin and of PZP(dimers), k = 5 x 10(5) M-1 x s-1. Reactions of PZP-thiol groups do not give rise to fluorescence changes. The fluorescence changes most likely reflect the formation of an intermediate with intact thiol esters. Further analysis of the kinetic results suggests that the chymotrypsin-PZP(tetramer) intermediate is formed in two reaction steps: (1) initially native PZP(dimers) are cleaved at bait regions by enzyme molecules, and that is the rate determining reaction of the fluorescence changes; (2) association with another PZP(dimer) or PZP(dimer)-chymotrypsin complex in a very fast reaction that leads to the formation of 1:1 -chymotrypsin-PZP(tetramer) intermediate, probably with intact thiol esters. The interactions studied apparently are established early in the path of the reaction and the fluorescence changes probably reflect noncovalent enzyme-PZP contacts, which are not changed when covalent binding occurs. Further, fluorescence changes are seen only in reactions of PZP with enzymes, not with methylamine.

Partial primary structure of human pregnancy zone protein: extensive sequence homology with human alpha 2-macroglobulin

Human pregnancy zone protein (PZP) is a major pregnancy-associated protein. Its quaternary structure (two covalently bound 180-kDa subunits, which are further non-covalently assembled into a tetramer of 720 kDa) is similar to that of human alpha 2-macroglobulin (alpha 2M). Here we show, from the results of complete or partial sequence determination of a random selection of 38 tryptic peptides covering 685 residues of the subunit of PZP, that PZP and alpha 2M indeed are extensively homologous. In the stretches of PZP sequenced so far, the degree of identically placed residues in the two proteins is 68%, indicating a close evolutionary relationship between PZP and alpha 2M. Although the function of PZP in pregnancy is largely unknown, its close structural relationship to alpha 2M suggests analogous proteinase binding properties and a potential for being taken up in cells by receptor-mediated endocytosis. In this regard our studies indicate a bait region in PZP significantly different from that present in alpha 2M. PZP could be the human equivalent of the acute-phase alpha-macroglobulins (e.g., rat alpha 2M and rabbit alpha 1M) described earlier.

Measurement of pregnancy-associated plasma proteins during human gestation

Studies of four pregnancy-associated plasma proteins (PAPP's) were made on 206 plasma samples obtained from 175 pregnant women between 12 and 44 wk of gestation. The concentration of three PAPP's (A, C, and D) were assayed by quantitative crossed immunoelectrophoresis. They showed a gradual but small rate of increase during the 2nd trimester, which became more rapid in the 3rd trimester. PAPP-A continued to rise steadily during this period, while PAPP-C and PAPP-D (recently identified as human placental lactogen) tended to reach a plateau. Although PAPP-B could not be quantitated because of technical problems, it was detected in over 50% of the samples from the last 2 mo of gestation, but was almost never seen in those obtained during the 12th-28th wk of gestation. Various parameters were analyzed to determine their possible correlation with the PAPP levels during the last month of gestation. The race and age of the mother showed no influence on any of the PAPP's, while parity, sex of fetus, and infant birth weights appeared to affect the plasma concentration of some of the PAPP's. In the two instances studied, mothers of twins showed abnormally high PAPP levels.