Physico-chemical properties and evidence for electrophoretic variants of rat transcortin

Isolation of rat plasma transcortin was carried out by affinity chromatography, as previously described for human. The protein was shown to be pure by PAGE and one single N-terminal amino acid was identified (Ser), which suggested that the protein molecule has a single polypeptide chain. This assumption is supported by SDS-PAGE. The amino acid composition was reported and compared with the one of human transcortin. The purified protein always migrated in PAGE (with or without SDS) as a double band; the faster component being more intense than the slower one. Whether transcortin was free or bound to corticosterone, the same aspect was observed. Molecular weight of these two variants were determined by SDS-PAGE as 65,900 and 75,800. Polymers only appeared after irreversible denaturation of the protein, as previously described for human transcortin. Various other physical parameters were determined: a sedimentation coefficient of 3.71 S +/- 0.18 was calculated by ultracentrifugation in sucrose gradient, association constants at 4 degrees C for corticosterone and cortisol (2.7 X 10(9) M-1 and 4.2 X 10(8) M-1, respectively).