Foaming reduces the working volume and limits the biosynthesis of macrolide immunosuppressant ascomycin(FK520) in the batch fermentation process of Streptomyces hygroscopicus FS-35 in a 7.5 L bioreactor. To find the relation between FK520 production and foaming, effects of 10 fermentation parameters including organic acids and membrane permeability were investigated. The results suggest that acetate accumulation caused by short period oxygen de ficiency and fast consumption of glucose is the reason for increased foaming and declined FK520 production. Therefore, a fed-batch fermentation strategy was developed to reduce the accumulation of acetate. After optimization, the maximum acetate concentration dropped from 320 mg·L-1to 157 mg·L-1, decreased by 50.8%, and the maximum foam height reduced from 5.32 cm to 3.74 cm, decreased by 29.7%, while the maximum FK520 production increased from 375 mg·L-1to 421 mg·L-1, improved by 12%.
Our previous studies have reported the presence of "chain delivery" effects of protein adsorption onto ion exchangers with polymer-grafted ion-exchange groups, such as dextran-grafted and poly(ethylenimine)-modified Sepharose gels. However, it is unclear if the "chain delivery" occurs on affinity adsorption with specific interactions. This work is designed to address this issue. A dextran-grafted Sepharose gel was prepared, and then the matrix was modified using diethylaminoethyl, a typical ion-exchange group, or octapeptide(FYCHWQDE), an affinity ligand for human immunoglobulin G(h Ig G) to prepare ion-exchange or affinity adsorbents, respectively.Results of h Ig G adsorption showed that the uptake rate represented by the effective diffusivity of h Ig G onto the dextran-grafted ion exchangers was obviously enhanced by the dextran grafting, indicating the presence of"chain delivery" of the bound proteins on the charged groups on the dextran chains. By contrast, the effective diffusivity of h Ig G changed little as ligand density increased on the dextran-grafted FYCHWQDE adsorbents.Their adsorption capacities decreased and effective diffusivities were not accelerated by the dextran grafting.Thus, this work clarified that grafted dextran could not accelerate h Ig G uptake rate on the affinity resins, or in other words, chain delivery did not occur on the specific interaction-based affinity adsorption.
In this work, we have synthesized two polymer-grafted cation exchangers: one via the grafting-from approach, in which sulfopropyl methacrylate (SPM) is grafted through atom transfer radical polymerization onto Sepharose FF (the thus resulting exchanger is referred as Sep-g-SPM), and another via the grafting-to approach, in which the polymer of SPM is directly coupled onto Sepharose FF (the thus resulting exchanger is called as Sep-pSPM). Protein adsorption on these two cation exchangers have been also investigated. At the same ligand density, Sep-g-SPM has a larger accessible pore radius and a smaller depth of polymer layer than Sep-pSPM, due to the controllable introduction of polymer chains with the regular distribution of the ligand. Therefore, high-capacity adsorption of lysozyme and γ-globulin could be achieved simultaneously in Sep-g-SPM with an ionic capacity (IC) of 308 mmol·L^-1. However, Sep-pSPM has an irregular chain distribution and different architecture of polymer layer, which lead to more serious repulsive interaction to proteins, and thus Sep-pSPM has a lower adsorption capacity for γ-globulin than Sep-g-SPM with the similar IC. Moreover, the results from protein uptake experiments indicate that the facilitated transport of adsorbed γ-globulin occurs only in Sep-pSPM and depends on the architecture of polymer layers. Our research provides a clear clue for the development of high-performance protein chromatography.
