Human alpha 2-macroglobulin (alpha 2M) was eluted as a single nondispersed peak from a TSK-G4000SW size exclusion chromatography column equilibrated in 20 mM-sodium phosphate/100 mM-NaCl, pH 7.2 (PBS). The void volume and total accessible volume of the column were 6.08 ml and 14.42 ml. The elution volume (Ve) of native alpha 2M was 9.20 +/- 0.04 ml. The Ve was altered minimally by changing the ionic strength or adding ethanol to the equilibration buffer. Ternary alpha 2M-trypsin, containing 2 mol of proteinase/mol of inhibitor, and alpha 2M-methylamine failed to be eluted in well-defined peaks when the column was equilibrated in PBS. The majority of either preparation was recovered slowly at Ve values greater than 14.5 ml, reflecting significant nonideal interactions with the support structure. Addition of 10% ethanol or increased ionic strength in the equilibration buffer independently caused either form of reacted alpha 2M to be eluted in a distinct peak at decreased Ve, suggesting that the nonideal interactions included hydrophobic and electrostatic adsorption. When the equilibration buffer was 80 mM-sodium phosphate/320 mM-NaCl, pH 7.2, partial resolution of ternary alpha 2M-trypsin and alpha 2M-methylamine was obtained with a single column run. The Ve values of ternary alpha 2M-trypsin and alpha 2M-methylamine in this buffer were 13.15 +/- 0.08 ml and 11.94 +/- 0.14 ml, respectively. The Ve of native alpha 2M was 8.84 +/- 0.03 ml. The resolving capacity of TSK-G4000SW was exploited to purify native alpha 2M rapidly and efficiently from solutions that contained significant amounts of either ternary alpha 2M-trypsin or binary alpha 2M-trypsin (1 mol of proteinase/mol of inhibitor). This purification was complete within the limits of sensitivity of denaturing and nondenaturing polyacrylamide-gel electrophoresis. alpha 2M-plasmin was well resolved from native alpha 2M. The Ve of alpha 2M-plasmin was 12.88 +/- 0.32 ml in 80 mM-sodium phosphate/320 mM-NaCl, pH 7.2. A number of procedures were used to prepare solutions with up to 90% binary alpha 2M-trypsin. The Ve of binary alpha 2M-trypsin in these various solutions was intermediate between the values of native alpha 2M and ternary alpha 2M-trypsin. The conformations of binary and ternary complex, as reflected by mobility in nondenaturing electrophoresis, were identical, confirming previous results. Finally, in the binary alpha 2M-trypsin complex, the single trypsin cleaved more than two, and as many as all four alpha 2M subunits.
Size Modulation of Enzymatically Cross-Linked Sodium Caseinate Nanoparticles via Ionic Strength Variation Affects the Properties of Acid-Induced Gels
