Human pre-alpha-inhibitor is a positive acute-phase protein that is more susceptible than inter-alpha-inhibitor to proteolysis by stimulated neutrophils

Background Pre-alpha-inhibitor (P alpha I) is a human plasma serine-protein ase inhibitor that is structurally related to inter-alpha-inhibitor (I alph a I). It is composed of a heavy chain named H3 covalently linked to bikunin by means of a glycosaminoglycan chain. We developed an ELISA procedure mak ing it possible to measure P alpha I for the first time and we investigated its levels in sera from patients with inflammatory diseases. Materials and methods We generated rabbit anti-H3 immunoglobulins, which we re used on solid phase and biotinylated antibikunin immunoglobulins to dete ct trapped P alpha I. Results We demonstrate that P alpha I is more susceptible than I alpha I to in vitro proteolysis by stimulated neutrophils. However, the degradation p roducts thus released as well as the other members of the I alpha I family present in serum do not affect the ELISA test. In a panel of control sera w e observed P alpha I concentrations of 25.6 +/- 7.8 mg L-1 (mean +/- SD; n = 30). These values increased to 64.2 +/- 16.06 mg L-1 (mean +/- SD; n = 15 ) in patients with inflammatory diseases, concording with the positive acut e-phase protein nature of P alpha I. However, for all these patients, the s erum concentrations of P alpha I and C-reactive protein poorly correlated ( r = 0.476; P = 0.076). Indeed, four patients had a relatively weaker increa se in their P alpha I level than that of C-reactive protein. More often tha n not their plasma elastase content was then elevated. Conclusion During inflammatory diseases plasma P alpha I levels may be depe ndent on increased synthesis in combination with enhanced catabolism, perha ps implicating neutrophil or other proteinases.