title: Alpha 2-macroglobulin conformation determines binding affinity for activin A and plasma clearance of activin A/alpha 2-macroglobulin complex.
abstract: Activin A is a member of the transforming growth factor-beta family of growth factors and a potent regulator of cellular activity. A number of binding proteins for activin A have been identified, including alpha 2-macroglobulin (alpha 2M). Alpha 2M has several conformational states that are known to have different growth factor-binding properties. The effect of alpha 2M conformation on activin A binding has not been characterized. The aims of this study were to determine 1) whether activin A binds preferentially to the native (alpha 2M-N) or "activated" (alpha 2M*) conformation of alpha 2M, 2) the affinity of different alpha 2M conformations for activin A, and 3) the fate of activin A complexed with alpha 2M-N or alpha 2M* in vivo. [125I]Activin A associated with alpha 2M in plasma and follicular fluid and with purified alpha 2Ms. In this qualitative assay, more activin A was associated with alpha 2M* than with alpha 2M-N. The affinity of the activin A-alpha 2M interaction was determined. The Kd values for activin A-alpha 2M* and activin A-alpha 2M-N were 190 +/- 30 and 510 +/- 60 nM, respectively. The plasma clearance profiles and tissue distribution of uncomplexed activin A and purified alpha 2M*-activin A complex were determined. Radiolabeled activin A cleared in a biphasic manner, with rapid clearance over the initial 10 min and substantially slower clearance over the subsequent 20 min. During the slow phase of clearance, activin A formed a complex with circulating alpha 2M-N. In contrast, radiolabeled activin A-alpha 2M* complexes were rapidly cleared from plasma with a half-life of approximately 5 min and were specifically targeted to alpha 2M receptors in vivo. These studies reveal that alpha 2M can maintain activin A in the circulation or rapidly target the hormone for plasma clearance depending on the conformational state of the carrier protein in vivo.
authors.affiliation: Department of Biomedical Sciences, University of Guelph, Ontario, Canada.
authors.forename: C A
authors.initials: CA
authors.lastname: Niemuller
authors.forename: K J
authors.initials: KJ
authors.lastname: Randall
authors.forename: D J
authors.initials: DJ
authors.lastname: Webb
authors.forename: S L
authors.initials: SL
authors.lastname: Gonias
authors.forename: J
authors.initials: J
authors.lastname: LaMarre
mesh_terms: D028341:Activins; D000818:Animals; D001665:Binding Sites; D005260:Female; D006133:Growth Substances / Q000378:metabolism*; D007265:Inhibins / Q000378:metabolism*; D008657:Metabolic Clearance Rate; D051379:Mice; D011487:Protein Conformation; D000511:alpha-Macroglobulins / Q000737:chemistry* / Q000378:metabolism
chemical_list: D006133:Growth Substances; D000511:alpha-Macroglobulins; D028341:Activins; D007265:Inhibins