Semipreparative isoelectrophoresis of DTPA-conjugated antibody leads to isolation of antibody isoforms with different immunoreactivities.

Diethylenetriaminepentaacetic acid (DTPA)-conjugated purified rabbit anti-human serum albumin antibody was subjected to free-flow, semipreparative isoelectrophoresis using an apparatus that fractionates protein molecules based on isoelectric point (pI). The results indicate that (1) the apparatus was capable of developing linear pH gradients and separating proteins, (2) > or = 86% of the protein was recovered following fractionation, and (3) the protein concentration of the individual fractions varied. Focusing the fractions on an isoelectric slab gel revealed that the pIs of the isoforms were modified. The isoforms were radiolabeled with indium-111, their DTPA to antibody molar ratios determined, and their immunoreactivities evaluated by a solid-phase radioimmunoassay. The results demonstrate that (1) the molar ratio of DTPA to antibody varied among the fractions, and (2) the immunoreactivity of the majority of fractions was higher than that of unfractionated DTPA-antibody conjugate. Semipreparative isoelectric focusing may therefore improve the potential of radioimmunoconjugates in the radio-diagnosis and therapy of disease.