Modification of Peptide and Protein Cysteine Thiol Groups by Conjugation with a Degradation Product of Ascorbate
Bibliographic record
Abstract
Ascorbate is an important water-soluble antioxidant, which when oxidized by reactive oxygen species is converted into dehydroascorbate (DHA). If not rapidly reduced back to ascorbate, DHA decomposes to a reactive 5-carbon compound (DHA*, +130 Da) that can modify reduced cysteinyl residues in peptides and proteins in vitro. The formation of cysteine adducts by DHA* was characterized by mass spectrometry using reduced insulin B-chain, α-lactalbumin, and hemoglobin. Mass spectrometry of DHA* modified insulin B-chain revealed the presence of one and two DHA* adducts. Enzymatic cleavage and tandem mass spectrometry of modified peptides allowed unambiguous localization of DHA* to the two cysteine residues in positions 7 and 19 of the insulin B-chain. Incubations of DHA with α-lactalbumin revealed that approximately 25% of the protein population was in a reduced state and could be modified by DHA*. The adduct was assigned to the N-terminally located cysteinyl residue in position 6. Incubation of hemoglobin with DHA followed by pepsin digestion and electrospray ionization tandem mass spectrometry (ESI-MSMS) of the peptide mixture allowed for the identification of three modified peptides. Tandem mass spectrometry of the modified peptides, two from the hemoglobin A-chain with identical mass and one from the hemoglobin B-chain, gave a complete series of y-type fragment ions, which were assigned to the cysteine containing peptides (100)LLSHCL(105) (A-chain), (101)LSHCLL(106) (A-chain), and (111)VCVLAHHFGKE(121) (B-chain). Although the DHA* adduct was lost from the peptides derived from α-lactalbumin and hemoglobin before fragmentation of the peptide bond, carbamidomethylation of the proteins prior to incubation with DHA abolished the formation of DHA*-protein adducts and confirmed that the target was indeed the cysteine thiol group. Future studies are focused on the modification of proteins by DHA* in cells and in vivo.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".