Potentiation of the anaphylatoxins in vivo using an inhibitor of serum carboxypeptidase N (SCPN). I. Lethality and pathologic effects on pulmonary tissue.

Carboxypeptidase N (EC 3.4.12.7) (SCPN) is a plasma enzyme that efficiently inactivates the anaphylatoxins C3a and C4a and significantly reduces C5a spasmogenic activity by removing the C-terminal arginyl residue from each of these factors. The arginine analog DL-2-mercaptomethyl-3-guanidinoethylthiopropanoic acid (SCPN-INH) is a potent competitive inhibitor of SCPN with a Ki for this carboxypeptidase in serum of 2 x 10(-9) M. Therefore, we have used the SCPN inhibitor to potentiate biologic activity of the anaphylatoxins in vivo. Infusion via the carotid artery of about 40 mg of SCPN-INH into each of 8 adult guinea pigs inactivated the SCPN for at least 3 hours and caused no measurable toxic effects. When cobra venom factor (CVF) is infused into guinea pigs, it activates the alternative pathway of complement, thereby generating the anaphylatoxins C3a and C5a. Ordinarily, infusion of CVF is nonlethal, because the generated anaphylatoxins are rapidly converted to C3a des Arg and C5a des Arg by SCPN. However, CVF (200 micrograms) plus SCPN-INH delivered intravenously in 5 animals induced a lethal reaction in less than 5 minutes. The authors conclude that the lethal effect is due largely to the anaphylatoxins. Histologic sections of the lungs from treated animals show dramatic structural changes consistent with peripheral small airway constriction, bronchial constriction, and vasoconstriction of small muscular arteries. Also, cell aggregates are present in blood vessels. Other histologic changes include severe congestion, pulmonary edema, and an interstitial infiltrate of mononuclear cells. Large doses of chlorpheniramine prevent this lethal reaction. Lethality is apparently attributable to asphyxia and is dependent on the level of CVF administered: eg, 100 micrograms CVF was not lethal in 4 animals given SCPN inhibitor, although signs of respiratory distress were observed. On histologic examination of lungs from guinea pigs given CVF and SCPN-INH, the features are similar to those described when anaphylatoxins are instilled into guinea pig lungs. Intravenous application of purified C3a plus SCPN-INH also proved lethal in 3 of the 6 animals challenged. This is the first evidence that the C3a anaphylatoxin can elicit a lethal response.