Quantitative analysis of derivatized proteins prepared with pyridyl disulfide-containing cross-linkers by high-performance liquid chromatography.

Determination of the introduced moieties into derivatized proteins is an essential step in the preparation and quality control of chemically defined immunoconjugates. For the derivatized proteins using pyridyl disulfide-containing cross-linkers such as N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP) and 4-succinimidyloxycarbonyl-alpha-methyl-alpha-(2-pyridyldithio)tolu ene (SMPT), the derivatization degree (ratio of pyridyl disulfide moieties to protein) has been traditionally determined by measuring the absorbance of both the derivatized protein and 2-thiopyridone (2-TP) released from the dithiothreitol (DTT) treatment (spectrophotometric method). This method, however, causes several problems including false high and low determinations of the protein and 2-TP, respectively, low selectivity, poor reproducibility, and relatively large amounts of sample consumption. A quantitative determination method of the derivatization ratios using bovine serum albumin derivatized with SPDP and SMPT as the model system has been developed. The concentration of protein and 2-TP released from the DTT treatment of derivatized proteins was determined directly without consideration of different reagents used and their concentrations. The present HPLC method was proved to be better in terms of accuracy, selectivity, and reproducibility with micro sample consumption. Moreover, this HPLC method can be directly applied to all derivatized proteins prepared with pyridyl disulfide-containing cross-linkers.