Spontaneous in vivo isomerization of bovine serum albumin as a determinant of its normal catabolism

Abstract

By combination of isoelectric focusing and immunoelectrophoresis of fresh bovine plasma it is shown that 10% of the albumin in plasma has isoionic points equal to the intramolecular SS interchanged isomers of bovine serum albumin (BSA). It is also shown that albumin with the isoionic point of SS interchanged BSA is produced in the cow from radioiodinated BSA depleted from SS interchanged albumin before injection, and purified radiolabeled SS interchanged BSA can be converted in vivo to albumin with the native isoionic point. On this basis, it is proposed that SS interchanged albumin in vivo is in postsynthetic equilibrium with the 'native' albumin conformation. The SS interchanged isomers purified either from commercial BSA or from BSA submitted to SH SS interchange was, after radioiodination with 125I, compared metabolically with 'native' albumin labeled with 131I in the same calf. Both species of SS interchanged albumins have fast initial rates but obtain a normal rate of degradation after the reversion to native albumin. If the isomers formed in vivo have the same properties as the ones present in commercial BSA, at least 50% of the physiological degradation of albumin can be accounted for by the 6-7 times faster catabolic rates of these isomers.