Inhibition of cathepsin E by alpha 2-macroglobulin and the resulting structural changes in the inhibitor

Inhibition of Human Pepsin and Gastricsin by α2-Macroglobulin

The inhibitory effects of human alpha2-macroglobulin (alpha2-M), a major plasma proteinase inhibitor, on human pepsin and gastricsin were investigated. The activities of pepsin and gastricsin towards a protein substrate (reduced and carboxymethylated ribonuclease A) were significantly inhibited by alpha2-M at pH 5.5, whereas those towards a peptide substrate (oxidized insulin B-chain) were scarcely inhibited. Under these conditions at pH 5.5, pepsin and gastricsin cleaved alpha2-M mainly at the His694-Ala695 bond and Leu697-Val698 bond, respectively, in the bait regions sequence of alpha2-M. The conformation of alpha2-M was also shown to be markedly altered upon inhibition of these enzymes as examined by native polyacrylamide gel electrophoresis and electron microscopy. These results show the entrapment and concomitant inhibition of those proteinases by alpha2-M.

Disruption of structural and functional integrity of alpha 2-macroglobulin by cathepsin E

alpha 2-Macroglobulin (alpha 2M) is an abundant glycoprotein with the intrinsic capacity for capturing diverse proteins for rapid delivery into cells. After internalization by the receptor- mediated endocytosis, alpha 2M-protein complexes were rapidly degraded in the endolysosome system. Although this is an important pathway for clearance of both alpha 2M and biological targets, little is known about the nature of alpha 2M degradation in the endolysosome system. To investigate the possible involvement of intracellular aspartic proteinases in the disruption of structural and functional integrity of alpha 2M in the endolysosome system, we examined the capacity of alpha 2M for interacting with cathepsin E and cathepsin D under acidic conditions and the nature of its degradation. alpha 2M was efficiently associated with cathepsin E under acidic conditions to form noncovalent complexes and rapidly degraded through the generation of three major proteins with apparent molecular masses of 90, 85 and 30 kDa. Parallel with this reaction, alpha 2M resulted in the rapid loss of its antiproteolytic activity. Analysis of the N-terminal amino-acid sequences of these proteins revealed that alpha 2M was selectively cleaved at the Phe811-Leu812 bond in about 100mer downstream of the bait region. In contrast, little change was observed for alpha 2M treated by cathepsin D under the same conditions. Together, the synthetic SPAFLA peptide corresponding to the Ser808-Ala813 sequence of human alpha 2M, which contains the cathepsin E-cleavage site, was selectively cleaved by cathepsin E, but not cathepsin D. These results suggest the possible involvement of cathepsin E in disruption of the structural and functional integrity of alpha 2M in the endolysosome system.

Identification and partial characterization of alpha 2-macroglobulin from the tuatara (Sphenodon punctatus)

alpha 2-Macroglobulin (alpha 2-M), a large molecular mass proteinase-binding protein, was identified in plasma from tuatara (Sphenodon), a rare reptile endemic to New Zealand. In this genus, alpha 2-M constitutes 11-13% of total plasma protein (approximately 2.2-3.9 mg/mL). Analysis of blood samples collected at approximately monthly intervals from individual tuatara indicated that the plasma level of alpha 2-M remains fairly constant. The subunits of tuatara alpha 2-M have an apparent molecular mass of approximately 160 kDa as determined by SDS-polyacrylamide gel electrophoresis and the intact protein is an oligomer that contains inter-chain disulfide bonds. N-terminal sequence analyses of tuatara alpha 2-M revealed a distinct similarity to alpha-macroglobulins of other vertebrates and that at least two types of alpha 2-M subunits are present in plasma of tuatara.

Human neutrophil and plasma endopeptidase 24.11: quantification and respective roles in atrial natriuretic peptide hydrolysis

Neutral endopeptidase 24.11 activities were quantified on human peripheral blood cell preparations (reflecting the enzyme concentration on the surface of neutrophils) and in the corresponding diluted plasmas by a spectrofluorimetric assay. Despite statistically identical values in both compartments, enzymatic activity towards atrial natriuretic peptide was not comparable. Indeed, incubation of the radiolabelled peptide in whole blood resulted in the thiorphan-sensitive production of the labelled metabolites Phe-Arg-Tyr and the Cys-Phe bond-cleaved peptide. A similar degradation pattern was observed for blood cells but not for plasma, providing evidence for the exclusive involvement of neutrophil endopeptidase in this peptide inactivation. In search for plasma component(s) susceptible to inhibit enzymatic activity, we observed that in the presence of alpha 2-macroglobulin at the physiological concentration of 3.5 mg mL-1, endopeptidase activity decreased from 100% to 51.2 +/- 8.9% (P = 0.002). Our data suggest that this protein could play a role in the endogenous inhibition of plasma endopeptidase activity.