Investigation of protein-surfactant interactions by analytical ultracentrifugation and electron paramagnetic resonance: The use of recombinant human tissue factor as an example

Purpose. The purpose of this work is to utilize electron paramagnetic reson ance (EPR) spectroscopy in conjunction with analytical ultracentrifugation (AUC) to investigate the binding of surfactants to proteins with a transmem brance domain. As an example these methods have been used to study the inte raction of a nonionic surfactant, C12E8, to recombinant human tissue factor (rhTF) in liquid formulations. The complementary nature of the two techniq ues aids in data interpretation when there is ambiguity using a single tech nique. in addition to binding stoichiometries, the possibility of identifyi ng the interacting domains by using two forms of rhTF is explored. Methods. Two recombinant, truncated forms of human tissue factor were formu lated in the absence of phospholipids. Neither of the recombinant proteins, produced in E. coli, contains the cytoplasmic domain. Recombinant human ti ssue Factor 243 (rhTF 243) consists of 243 amino acids and includes the tra nsmembrane sequences. Recombinant human tissue factor 220 (rhTF 220), howev er, contains only the first 221 amino acids of the human tissue factor, lac king those of the transmembrane region. EPR and AUC were used to investigat e the interactions between these two forms of rhTF and polyoxyethylene 8 la uryl ether, C12E8. Results. Binding of C12E8 to rhTF 243 is detected by both EPR spectroscopy and AUG. Although a unique binding stoichiometry was not determined, EPR sp ectroscopy greatly narrowed the range of possible solutions suggested by th e AUC data. Neither technique revealed an interaction between rhTF 220 and C12E8. Conclusions. The complementary nature of EPR spectroscopy and AUC make the combination of the two techniques useful in data interpretation when studyi ng the interactions between rhTF and C12E8. By utilizing these techniques i n this study, the binding stoichiometry of rhTF 243 to C12E8 ranges from 1. 2:1 to 1.3:0.6 based on an aggregation number of 120. This binding is consi stent with previously reported activity data that showed an increase in clo tting rate when rhTF 243 is in the presence of C12E8 micelles. From the rhT F 220 data, it can further be concluded that the transmembrane domain of rh TF is necessary for interactions with C12E8.