Solution physicochemical properties of bovine beta 2-microglobulin. Aggregation states

Bovine beta 2-microglobulin (beta 2-m), the light chain of the histocompatibility antigen, was isolated in crystalline form from colostrum. Previous studies from this laboratory on the solution properties of this protein suggest the existence of a time-dependent multiple aggregation phenomenon. To clarify the molecular states of beta 2-m, its solution properties were studied by ultracentrifugation and spectropolarimetry. Sedimentation equilibrium experiments at pH 5.0 (0.08 M NaCl, 0.02 M sodium phosphate) at concentrations less than 0.3 mg/ml give Mr = 11,800. From sedimentation velocity results, we conclude that bovine beta 2-m is a much more symmetrical and compact molecule than either guinea pig or human beta 2-m. At concentrations above 0.4 mg/ml under the same conditions, sedimentation equilibrium experiments show that a monomer to tetramer reversible self-association occurs. Also, the tetramerization increases with decreasing temperature. beta 2-Microglobulin undergoes an irreversible temperature-dependent association to a much larger aggregate over a period of 7 days, as evidenced by sedimentation equilibrium and velocity results. The rate of this aggregation decreases as the pH approaches the isoelectric point (pH 7) from either side. Furthermore, circular dichroism measured at pH 5.0 under time-dependent aggregating conditions showed a marked increase in the percentage of disordered structure, leading to the conclusion that this effect is a denaturation phenomenon.