Neil C. Davis

Evidence for In Vivo Breakdown of β1C-Globulin in Hypocomplementemic Glomerulonephritis

Summary. Evidence has been obtained for the presence in vivo of alpha2Dglobulin, a breakdown product of serum f10-globulin, in patients with acute and persistent hypocomplementemic glomerulonephritis. The protein has been identified by immunoelectrophoretic analysis, and the amounts present have been determined by direct measurement of specific antigenic determinants present on alpha2D. 831A-Globulin, another breakdown product of Plcglobulin, may also be present in vivo in severely hypocomplementemic patients, but its levels are much lower than those of alpha2D-globulin. Alpha2D-globulin has been identified by immunoelectrophoretic analysis of fresh EDTA plasma from patients with hypocomplementemic nephritis as an arc in the alpha2 region that shows a reaction of identity with the arc representing alpha2D-globulin produced by aged normal serum. 831A-Globulin was not seen in these patterns. Measurement of specific antigenic determinants has been carried out in both fresh EDTA plasma and aged serum. In the fresh plasma, the concentration of D antigen, found on both Plw- and alpha2D-globulins, has been related to that of B antigen, found only on /31c and taken as a measure of the concentration of this protein. In the hypocomplementemic patients, the concentration of D antigen, in comparison to that of B, was greater than in the normal subjects. Similarly, in aged serum, the level of alpha2D was greater than would be expected from the amount of flic that had been broken down in vitro, measured by the concentration of ,81A. Calculations indicated that the in vivo alpha2D level in severely hypocomplementemic patients ranged from 7.5 to 18% of that which would be found in a pool of aged normal serum in which /3ic is completely broken down. The levels tended to be lower in less severely hypocomplementemic patients, and none could be detected in normal plasma. Only small quantities of A and D antigens are detectable in the urine of patients with hypocomplementemic nephritis. The rate of excretion is about equal to that of the normal subject. The study indicates that the low serum levels of fl3c-globulin that may be present over long periods in patients with persistent hypocomplementemic glomerulonephritis can be ascribed, in part, to in vivo breakdown of this pro

Hypocomplementemia of Membranoproliferative Nephritis DEPENDENCE OF THE NEPHRITIC FACTOR REACTION ON PROPERDIN FACTOR B

Membranoproliferative nephritis in children is frequently associated with a hypocomplementemia produced at least in part by C3 breakdown mediated by a circulating anticomplementary factor known as C3 nephritic factor (C3NeF). C3 breakdown by this factor in vitro requires the presence of a pseudoglobulin cofactor and magnesium. The present study provides evidence that properdin factor B (C3 proactivator) is activated in the nephritic factor reaction and is the direct mediator of C3 breakdown by C3NeF. Depletion of factor B from mixtures of normal human serum (NHS) and plasma from a patient with membranoproliferative nephritis (MPP), either by heating or by immune equivalence absorption, blocks C3 breakdown by C3NeF. Addition of purified factor B to these mixtures restores the anticomplementary effect. When purified factor B is added to mixtures of MPP and purified C3, breakdown also occurs. Associated with the C3 breakdown is a change in the electrophoretic mobility of factor B from the beta to the gamma position, a shift which has been associated with cleavage activation of the molecule. Further, serum factor B levels are often low in patients with membranoproliferative nephritis and bear a rough inverse correlation with C3NeF levels. It thus appears that factor B is the previously described heat-labile C3NeF cofactor. Whether the C3NeF reaction proceeds via a pathway comparable to that activated by the cobra venom factor or via that activated by zymosan or inulin cannot be determined from the present data.

Continuing C3 Breakdown after Bilateral Nephrectomy in Patients with Membrano-Proliferative Glomerulonephritis

Serum levels of complement components and of C3 nephritic factor (C3NeF) were measured serially in two patients with membrano-proliferative glomerulonephritis who were subjected to bilateral nephrectomy and maintained by peritoneal dialysis for 2 wk before renal transplantation. In both patients, low levels of C3 and high levels of preformed alpha 2D, a C3 breakdown product, were present before nephrectomy and remained essentially unchanged during the anephric period. With transplantation, C3 levels rose towards normal and alpha 2D disappeared from the serum. The serum of both patients contained detectable amounts of C3NeF, a factor which has been shown to react with a cofactor found in normal serum to form an enzyme, designated C3 lytic nephritic factor (C3LyNeF), which will cleave C3 to form the breakdown products, beta1A and alpha 2D. The level of C3NeF was high in one patient before nephrectomy, increased somewhat during the anephric period, and fell after transplantation. In the other patient, the C3NeF level was initially lower, remained relatively constant during the anephric period, and was not significantly affected by transplantation. In both patients, levels of C4 and C5 were either normal or elevated over the period of the study and bore no relationship to the C3 level. The following conclusions can be drawn from the data. The high levels of alpha 2D during the anephric period and the disappearance of this protein as C3 levels approach normal at the time of transplantation indicate that the low C3 levels were largely the result of C3 breakdown rather than diminished synthesis. The presence of C3NeF in detectable amounts in both patients suggest that C3LyNeF, formed by the reaction of C3NeF and cofactor, was responsible for the low C3 levels. Finally, the lack of effect of nephrectomy on C3, alpha 2D, and C3NeF levels indicate that the site of C3 breakdown was extrarenal and that C3NeF and cofactor are at least in large part of extrarenal origin.

THE DESTRUCTION OF GLUCAGON, ADRENOCORTICOTROPIN AND SOMATOTROPIN BY HUMAN BLOOD PLASMA*

Human blood contains an inactive proteolytic enzyme, plasminogen, which may be converted in vitro to its active form, plasmin, either spontaneously or by the addition of various agents (1). A similar activation is believed to occur in vivo after exposure to a variety of noxious circumstances (2-6). Since plasmin catalyzes the hydrolysis of corticotropin A (7), it is possible that circumstances which induce the activation of plasmin will result also in an increase in the rate of destruction of adrenocorticotropic (ACTH) and other polypeptide hormones. The present study was designed to determine the in vitro hydrolytic action of human blood