Ala-Cys-Cys-Ala dipeptide dimer alleviates problematic cysteine and cystine levels in media formulations and enhances CHO cell growth and metabolism

Cysteine and cystine are essential amino acids present in mammalian cell cultures. While contributing to biomass synthesis, recombinant protein production, and antioxidant defense mechanisms, cysteine poses a major challenge in media formulations owing to its poor stability and oxidation to cystine, a cysteine dimer. Due to its poor solubility, cystine can cause precipitation of feed media, formation of undesired products, and consequently, reduce cysteine bioavailability. In this study, a highly soluble cysteine containing dipeptide dimer, Ala-Cys-Cys-Ala (ACCA), was evaluated as a suitable alternative to cysteine and cystine in CHO cell cultures. Replacing cysteine and cystine in basal medium with ACCA did not sustain cell growth. However, addition of ACCA at 4 mM and 8 mM to basal medium containing cysteine and cystine boosted cell growth up to 15% and 27% in CHO-GS and CHO-K1 batch cell cultures respectively and led to a proportionate increase in IgG titer. 13C-Metabolic flux analysis revealed that supplementation of ACCA reduced glycolytic fluxes by 20% leading to more efficient glucose metabolism in CHO-K1 cells. In fed-batch cultures, ACCA was able to replace cysteine and cystine in feed medium. Furthermore, supplementation of ACCA at high concentrations in basal medium eliminated the need for any cysteine equivalents in feed medium and increased cell densities and viabilities in fed-batch cultures without any significant impact on IgG charge variants. Taken together, this study demonstrates the potential of ACCA to improve CHO cell growth, productivity, and metabolism while also facilitating the formulation of cysteine- and cystine-free feed media. Such alternatives to cysteine and cystine will pave the way for enhanced biomanufacturing by increasing cell densities in culture and extending the storage of highly concentrated feed media as part of achieving intensified bioproduction processes.

Single-vial preconcentration and cold vapor generation for the determination of Hg(II) in water samples of different salinities

In this work, a single-vial methodology for the extraction and cold vapor generation of mercury(II) was developed, followed by the determination of the analyte by atomic absorption spectrometry, with application in water samples of different salinities. L-cystine-modified Fe3O4 nanoparticles (2LcysMNP) were used as sorbent material in the magnetic solid phase extraction (MSPE) in the same flask in which the mercury vapor generation step was performed using a handmade gas-liquid separator developed in our laboratory. The main conditions for extraction, pre-concentration, and cold vapor generation of mercury were optimized. Under the optimized conditions, detection and quantification limits of 0.04 and 0.12 μg L-1, respectively, were achieved with a relative standard deviation of 7.5%. The single-vial system allowed for a preconcentration factor of 30 and an enrichment factor of 24. The accuracy of the method was evaluated by applying it to certified reference materials, and the obtained values were not significantly different from the expected values according to the Student's t-test. Verification of non-specific interferences was assessed by recovery tests, resulting in recoveries ranging from 81 to 111% for water samples of different salinities.

A tumor-sensitive biological metal-organic complex for drug delivery and cancer therapy

Metal-organic complexes (MOCs) or metal-organic frameworks (MOFs) have attracted increasing interest for constructing nanoscale drug delivery systems for cancer therapy. However, conventional MOC/MOF materials usually contain toxic metals or low-biocompatible organic ligands. Also, current approaches for creating tumor-sensitive nanocarriers are always based on the instability of coordination bonds under acidic conditions, or through post-synthetic modification with sensitive molecules. As a matter of fact, it is more facile to fabricate tumor-sensitive MOCs/MOFs based on the stimuli-responsiveness of organic ligands. In this study, a novel tumor-sensitive biological MOC (bioMOC-Zn(Cys)) was created through the assembly of endogenous Zn2+ ions and the small biological molecule (l-cystine, Cys). The disulfide bond in l-cystine is cleavable by the overexpressed GSH in tumor cells, thus achieving rapid release of drugs from nanocarriers. By encapsulating doxorubicin (DOX) in bioMOC-Zn(Cys), DOX@bioMOC-Zn(Cys) displayed higher cellular uptake and cytotoxicity in cancer cells than free DOX. In vivo investigations indicated that DOX@bioMOC-Zn(Cys) largely inhibited tumor growth and reduced side effects. Remarkably, since both metal ions and organic ligands were obtained from biological sources, bioMOC-Zn(Cys) exhibited superior biocompatibility. This study presents a new method for fabricating MOC-based nanodrugs with high tumor-sensitivity and low toxicity.

Mercury(II) binds to both of chymotrypsin's histidines, causing inhibition followed by irreversible denaturation/aggregation

The toxicity of mercury is often attributed to its tight binding to cysteine thiolate anions in vital enzymes. To test our hypothesis that Hg(II) binding to histidine could be a significant factor in mercury's toxic effects, we studied the enzyme chymotrypsin, which lacks free cysteine thiols; we found that chymotrypsin is not only inhibited, but also denatured by Hg(II). We followed the aggregation of denatured enzyme by the increase in visible absorbance due to light scattering. Hg(II)-induced chymotrypsin precipitation increased dramatically above pH 6.5, and free imidazole inhibited this precipitation, implicating histidine-Hg(II) binding in the process of chymotrypsin denaturation/aggregation. Diethylpyrocarbonate (DEPC) blocked chymotrypsin's two histidines (his40 and his57 ) quickly and completely, with an IC50 of 35 ± 6 µM. DEPC at 350 µM reduced the hydrolytic activity of chymotrypsin by 90%, suggesting that low concentrations of DEPC react with his57 at the active site catalytic triad; furthermore, DEPC below 400 µM enhanced the Hg(II)-induced precipitation of chymotrypsin. We conclude that his57 reacts readily with DEPC, causing enzyme inhibition and enhancement of Hg(II)-induced aggregation. Above 500 µM, DEPC inhibited Hg(II)-induced precipitation, and [DEPC] >2.5 mM completely protected chymotrypsin against precipitation. This suggests that his40 reacts less readily with DEPC, and that chymotrypsin denaturation is caused by Hg(II) binding specifically to the his40 residue. Finally, we show that Hg(II)-histidine binding may trigger hemoglobin aggregation as well. Because of results with these two enzymes, we suggest that metal-histidine binding may be key to understanding all heavy metal-induced protein aggregation.