Catalysis of Covalent Lp(a) Assembly: Evidence for an Extracellular Enzyme Activity that Enhances Disulfide Bond Formation
Bibliographic record
Abstract
The assembly of lipoprotein(a) (Lp(a)) particles occurs via a two-step mechanism in which noncovalent interactions between apolipoprotein(a) (apo(a)) and the apolipoproteinB-100 component of low density lipoprotein precede the formation of a single disulfide bond. Although we have previously demonstrated that the rate constant for the covalent step of Lp(a) assembly can be enhanced by altering the conformational status of apo(a), the resultant rates of covalent Lp(a) particle formation measured in vitro are relatively slow. The large excess of Lp(a) (over apo(a)) observed in vivo can be accounted for by a preferential clearance of apo(a) over Lp(a) and/or a sufficiently high rate of covalent Lp(a) assembly. In the present study, we report that cultured human hepatoma cells secrete an oxidase activity that dramatically enhances the rate of covalent Lp(a) assembly. This activity is likely possessed by a protein because it is heat-sensitive and is retained in the concentrate following ultrafiltration through a 5 kDa cutoff filter. However, a small molecule cofactor for the activity is suggested by the observation that the activity is lost upon dialysis. Plots of Lp(a) assembly rate versus input apo(a) concentration gave rectangular hyperbolae; the reaction displayed an unusual dependence on the concentration of apoB-100, with increasing concentrations of apoB-100 resulting in slower rates of Lp(a) assembly at low concentrations of apo(a), an effect that was alleviated by higher apo(a) concentrations. Interestingly, V(max(app))/K(m(app)) ratios were insensitive to apoB-100 concentration, which is diagnostic of a ping-pong reaction mechanism. In this way, the putative Lp(a) oxidase may be functionally analogous to protein disulfide isomerase, which exhibits a similar mechanism during the catalysis of disulfide bond formation during protein folding, although we have ruled out a role for this enzyme in Lp(a) assembly.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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 source (direct Gemma or distilled Codex), 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".