PubMed · 10873421
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A Saxon. 2000. Good "negative results".. https://doi.org/10.1006/clim.2000.4885
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OBJECTIVE: To investigate whether detectable C-peptide levels in type 1 diabetes is associated with a lower risk of diabetic ketoacidosis (DKA). RESEARCH DESIGN AND METHODS: We analyzed the Diabetes Control and Complications Trial publicly available data repository for the association between detectable stimulated C-peptide (>0.2 nmol/mL, measured annually) and DKA incidence over an average 6.5-year follow-up. We used crude and adjusted Andersen-Gill models for recurrent DKA events with time-dependent covariates. RESULTS: Of the 1,441 participants (53% male, median age 27 years), 129 (9%) experienced 180 DKA events. Of these events, 179 (99.44%) occurred after C-peptide was ≤0.2 nmol/L in the prior year and only 1 event occurred with C-peptide >0.2 nmol/L. C-peptide >0.2 nmol/L was associated with a DKA hazard ratio of 0.07 (95% CI 0.01-0.48; P = 0.007), consistent across adjusted models. CONCLUSIONS: Endogenous insulin production was significantly associated with lower DKA risk, suggesting that treatments preserving insulin production could decrease long-term DKA risk.
This study applied electrospray ionization-isotope dilution-liquid chromatography-tandem mass spectrometry for the evaluation of five urinary C-peptide immunoassays via split-sample measurements. The immunoassays measured in duplicate in the same run, the comparison method in triplicate over different runs. From the data, the within-run imprecision and the method comparison total RSDs were calculated. Regression analysis revealed on the one hand systematic differences, on the other, an excellent correlation between the test and comparison methods. From the spread of the data around the regression line in comparison with the 95% prediction intervals from the total RSD, sample-related effects and/or specificity problems were apparent and investigated.
Processing of human proinsulin C-peptide and its C-terminal pentapeptide in blood serum was studied using reverse-phase HPLC and electrospray mass spectrometry. The results reveal degradation of both peptides, with a longer half-life for intact C-peptide than for the C-terminal pentapeptide. Products from C-peptide degradation were not distinguishable from the peptide background, suggesting endopeptidase degradation of C-peptide. In contrast, a set of products from the C-terminal pentapeptide were identifiable and corresponded to successive losses from the N terminus, showing that the pentapeptide is degraded by aminopeptidase in serum. Consistent with this finding, a slower degradation was found for the N-acetyl-protected pentapeptide. Removal of serum proteins by acetone precipitation produced N-terminally carbamate-modified C-peptide via a Schiff base intermediate (a ketimine with acetone), to which CO(2) was added and acetone removed, generating a cyclic side chain via anhydride formation. The modification was not seen with the pyroglutamate form of C-peptide, with the N-terminally acetylated C-peptide, or with a control peptide having N-terminal Phe, but was found with human C-peptide, its N-terminal tetrapeptide, and a rat C-peptide fragment (all with N-terminal Glu). Hence, the modification appears to require N-terminal Glu, but this is not the only prerequisite since the C-terminal pentapeptide and another control peptide (also starting with Glu) were not modified. A peptide aldimine Schiff base leading to CO(2) incorporation was detected with formaldehyde in NaHCO(3). The observation that C-peptide forms Schiff bases with ketones/aldehydes, enhancing covalent attachment of CO(2), may have biological implications.