Protein Release from Biodegradable PolyHPMA-Lysozyme Conjugates Resulting in Bioactivity Enhancement

A novel biodegradable thiazolidine-2-thione functional chain transfer agent was synthesized and employed as a reversible additional fragmentation chain transfer agent to prepare well-defined semitelechelic poly-N-(2-hydroxypropyl) methacrylamides (polyHPMAs) with predetermined molecular weights and narrow polydispersities. The protein reactive group, thiazolidine-2-thione, was located at the polymer chain ends fixed by biodegradable disulfide bonds. The functional polyHPMA chains were subsequently conjugated to protein (lysozyme) by exploiting reactions between the thiazolidine-2-thione functionality and amine residues on the protein surface to form covalent amide linkages. The in vitro bioactivities of the lysozyme-polyHPMA conjugates were assessed by using Micrococcus lysodeikticus cells as substrates. The lysozyme bioactivity was significantly reduced following the conjugation procedure. However, cleavage of the polymer chains from the bioconjugates (under reducing conditions) yielded free protein and a remarkable recovery of bioactivity. In vivo tests were performed by subcutaneous injection into mice and clearly demonstrated decreased proteolytic degradation for the protein-polymer conjugate when compared with native protein, indicating effective protein protection through a conjugation strategy. This bioreversible approach to conjugation allows for a balance to be made between protein protection and effective bioactivity maintenance.