Frozen fresh blood plasma preserves the functionality of native human α 2 -macroglobulin

Human α 2 -macroglobulin (hα 2 M) is a large homotetrameric protein involved in the broad inhibition of endopeptidases. Following cleavage within a bait region, hα 2 M undergoes stepwise transitions from its native, expanded, highly flexible, active conformation to an induced, compact, triggered conformation. As a consequence, the peptidase is entrapped by an irreversible Venus flytrap mechanism. Given the importance of hα 2 M, biochemical studies galore over more than seven decades have attempted to ascertain its role, typically using authentic hα 2 M purified from frozen and non-frozen fresh blood plasma, and even outdated plasma. However, hα 2 M is sensitive once isolated and purified, and becomes heterogeneous during storage and/or freezing, raising concerns about the functional competence of frozen plasma-derived hα 2 M. We therefore used a combination of native and sodium dodecylsulfate polyacrylamide gel electrophoresis, affinity and ion-exchange chromatography, multi-angle laser light scattering after size-exclusion chromatography, free cysteine quantification, and peptidase inhibition assays with endopeptidases of two catalytic classes and three protein substrates, to characterize the biochemical and biophysical properties of hα 2 M purified ad hoc either from fresh plasma or frozen fresh plasma after thawing. We found no differences in the molecular or functional properties of the preparations, indicating that protective components in plasma maintain native hα 2 M in a functionally competent state despite freezing.