Dextrin–trypsin and ST-HPMA–trypsin conjugates: Enzyme activity, autolysis and thermal stability

Using monomethoxy poly(ethylene glycol) (mPEG)–trypsin conjugates we recently showed that both PEG molecular weight (1100–5000 g/mol) and linker chemistry affect the rate of protein autolysis and thermal stability. These important factors are often overlooked but they can guide the early choice of optimal polymer/chemistry for synthesis of a lead polymer therapeutic suitable for later formulation development. As we are currently developing dextrin- and semi-telechelic poly[N-(2-hydroxypropyl)methacrylamide] (ST-HPMA)–protein conjugates as new therapeutics, the aim of this study was to examine the effect of polymer on activity, autolysis and its thermal stability using trypsin conjugates as a model and compare to the data obtained for mPEG conjugates. Trypsin conjugates were first synthesized using succinoylated dextrin (Mw ∼ 8000 g/mol, dextrin I; or ∼61,000 g/mol, dextrin II), and a ST-HPMA–COOH (Mw ∼ 10,100 g/mol). The conjugates had a trypsin content of ∼54, 17 and 3 wt% respectively with <5% free protein. When amidase activity (KM, Vmax and Kcat) was determined by using N-benzoyl-l-arginine p-nitroanilide (BAPNA) as substrate, trypsin KM values were not altered by conjugation, but the Vmax was ∼6–7-fold lower, and the substrate turnover rate (Kcat) decreased by ∼5–7-fold. The dextrin II–trypsin conjugate was more stable than the other conjugates and native trypsin at all temperatures between 30 and 70 °C, and also exhibited improved thermal stability in the autolysis assays at 40 °C.