Vitamin<scp>B<sub>12</sub></scp>: Disorders of Absorption and Metabolism
Bibliographic record
Abstract
Abstract Derivatives of vitamin B12(cobalamin) are required for activity of two enzymes: methylmalonyl‐CoA mutase, which catalyses conversion of methylmalonyl‐CoA to succinyl‐CoA, and methionine synthase, which converts homocysteine to methionine. Cobalamin deficiency results in the accumulation of methylmalonic acid and homocysteine in blood and urine. Clinically, deficiency results in megaloblastic anaemia and subacute combined degeneration of the spinal cord, loss of sensation, ataxia, dementia and psychosis. Dietary cobalamin deficiency is rare in developed countries, except for individuals consuming a vegan diet. A more common cause of cobalamin deficiency is pernicious anaemia, an autoimmune disease that results in inability to absorb cobalamin from the intestine. A number of rare genetic disorders have been identified that result in either failure to absorb dietary cobalamin, to transport cobalamin from the intestine to cells that require it for metabolism or to convert intracellular cobalamin to one or both of its coenzyme derivatives, adenosylcobalamin and methylcobalamin. Key Concepts Cobalamin (vitamin B12) is required for activity of two enzymes, methylmalonyl‐CoA mutase and methionine synthase, in mammalian cells. Cobalamin deficiency results in accumulation of methylmalonic acid and homocysteine in blood and urine. Clinically, deficiency is characterised by megaloblastic anaemia and/or neurological problems. Uptake of dietary cobalamin requires the binding protein intrinsic factor, secreted by parietal cells of the stomach, and the intestinal receptor cubam. Autoimmune destruction of parietal cells results in pernicious anaemia. Mutations in the genes encoding intrinsic factor or the components of cubam (cubilin and amnionless) result in heritable cobalamin malabsorption and signs of cobalamin deficiency. Peripheral cells take up cobalamin bound to the transport protein transcobalamin by carrier‐mediated endocytosis. Cells convert cobalamin to adenosylcobalamin, required by methylmalonyl‐CoA mutase, and methylcobalamin, required by methionine synthase. Rare mutations affecting genes involved in cobalamin metabolism result in decreased synthesis of either or both cobalamin derivatives. Mutations in certain transcription regulators can affect expression of genes encoding cobalamin metabolising enzymes, resulting in altered cobalamin coenzyme synthesis.
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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) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.018 | 0.007 |
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".