Leucine Unit Converter
Leucine Unit Converter
Convert leucine between mg/dL and µmol/L, with the maple syrup urine disease treatment target — and why a rising leucine in MSUD is a neurological emergency rather than a number to repeat tomorrow.
Leucine converter
mg/dL ⇄ µmol/LLeucine 1.6 mg/dL, read against the adult reference interval
Formula and conversion factor
mg/dL = µmol/L ÷ 76.2369
- 76.2369
- derived from the molecular weight of L-leucine, 131.17 Da — the largest of the three factors on this page because leucine is the smallest of the three molecules
- isoleucine shares it
- isoleucine is a structural isomer of leucine with an identical molecular mass, so the same factor converts it exactly. Valine does not — it is 117.15 Da and needs its own factor
- alloisoleucine
- normally below 5 µmol/L, and the most sensitive and specific marker of every form of maple syrup urine disease. It is what confirms the diagnosis, not the leucine
- the rate, not the level
- in an MSUD crisis the severity tracks how quickly leucine is rising rather than the absolute concentration, which is why a single value is never read on its own
Worked example
Leucine 1.6 mg/dL, read against the adult reference interval
1.6 × 76.2369 = 121.98 µmol/L, reported as 122 µmol/L
Within the 75–175 µmol/L adult interval used here
The same factor applied to an isoleucine of 1.6 mg/dL also gives 122 µmol/L — identical molecular mass, identical arithmetic
Read against the maple syrup urine disease maintenance target of 150–300 µmol/L, 122 µmol/L is below range, and in a treated child that would prompt a review of protein intake rather than reassurance
Which is the point of choosing the group first: the same number is normal in one frame and low in the other
Leucine, isoleucine and valine are not interchangeable
| Amino acid | Molecular weight (Da) | mg/dL → µmol/L factor | Role in MSUD |
|---|---|---|---|
| Leucine | 131.17 | 76.2369 | The neurotoxic one. Drives encephalopathy and cerebral oedema, and is the analyte monitored |
| Isoleucine | 131.17 | 76.2369 — identical, an isomer of the same mass | Raised, but far less neurotoxic. Too low an isoleucine on treatment causes a skin rash |
| Valine | 117.15 | 85.3607 — a different molecule, a different factor | Raised, far less neurotoxic. Also supplemented if it falls too low on treatment |
| Alloisoleucine | 131.17 | 76.2369 | Normally under 5 µmol/L. The most sensitive and specific diagnostic marker of all forms of MSUD |
Reading a leucine in maple syrup urine disease
| Situation | What the leucine is doing | What it means |
|---|---|---|
| Maintenance, well | Steady within roughly 150–300 µmol/L | The maintenance target. Read alongside isoleucine and valine, which are supplemented if they fall too low |
| Intercurrent illness, any level | Rising | A metabolic emergency in the making. Protein stopped, high-energy feeds started, the metabolic service contacted the same day — the rate of rise predicts the crisis better than the value |
| Acute decompensation | High and climbing, with a rising alpha-ketoisocaproate | Progressive lethargy, ataxia, stupor and the risk of cerebral oedema. Managed in hospital with branched-chain-free amino acid mixture, energy and sometimes dialysis |
| Newborn screening, undiagnosed | Raised, with alloisoleucine above 5 µmol/L | The pattern of MSUD. Classical forms present within the first week, before or alongside the screening result |
| Treated, below target | Low | Over-restriction. Leucine is essential; too little causes poor growth, a rash and a catabolic state that releases more leucine |
The branched-chain amino acid that matters
Leucine, isoleucine and valine are the three branched-chain amino acids, all essential, and all broken down through a common step catalysed by the branched-chain alpha-ketoacid dehydrogenase complex. When that complex is deficient the three amino acids and their ketoacids accumulate, and the resulting disorder — maple syrup urine disease, named for the smell of the ketoacids in urine and cerumen — is one of the classic newborn screening conditions. Converting the numbers is simple: 1 mg/dL of leucine is 76.24 µmol/L, from a molecular weight of 131.17 Da.
Isoleucine is a structural isomer of leucine with exactly the same molecular mass, so the same factor converts it precisely. Valine is a different molecule at 117.15 Da and needs its own. That distinction gets lost when the three are spoken of together as the branched-chain amino acids, and applying the leucine factor to a valine overstates it by about 12 per cent. Alloisoleucine, formed from accumulated isoleucine, is also an isomer of the same mass; it is normally below 5 µmol/L and is the most sensitive and specific diagnostic marker of every form of MSUD, which is why the diagnosis rests on it rather than on the leucine.
What makes leucine the analyte to watch is that it is the neurotoxic one. Isoleucine and valine rise too, but leucine and its ketoacid, alpha-ketoisocaproate, are what produce the progressive lethargy, ataxia, stupor and cerebral oedema of an MSUD crisis. Leucine is therefore what is monitored, with a maintenance target of roughly 150 to 300 µmol/L, while isoleucine and valine are more often supplemented than restricted, because letting them fall too low causes its own problems including an acrodermatitis-like rash.
The single most important thing about a leucine in someone with known MSUD is that a rising value during an intercurrent illness is a neurological emergency, not a result to repeat tomorrow. Severity in a crisis tracks the rate at which leucine is climbing rather than the absolute concentration, and treatment — stopping protein, giving generous energy, starting a branched-chain-free amino acid mixture, and sometimes dialysis — begins on the clinical picture rather than on waiting for the number. Any family with an MSUD emergency plan should follow it and contact their metabolic service the same day; a converted value on a screen is not a substitute for that call.
Frequently asked questions
How do I convert leucine from mg/dL to µmol/L?
Multiply by 76.2369, derived from the molecular weight of leucine, 131.17 Da. A leucine of 1.6 mg/dL is 122 µmol/L. The same factor converts isoleucine exactly, because it is an isomer of identical mass, but not valine.
Can I use the leucine factor for isoleucine and valine?
For isoleucine yes — it is a structural isomer of leucine with the same molecular mass of 131.17 Da, so the factor is identical. For valine no: it is 117.15 Da and converts at 85.3607, and using the leucine factor overstates a valine by about 12 per cent.
What is a normal leucine level?
Roughly 75–175 µmol/L in an adult on this page, though published adult intervals run from about 55–205 to 79–217 µmol/L, and the intervals for children and infants are different again. Use the interval your own laboratory prints for that age.
Why does leucine matter in maple syrup urine disease?
Because it is the neurotoxic one. Isoleucine and valine also accumulate, but leucine and its ketoacid alpha-ketoisocaproate drive the encephalopathy and cerebral oedema, so leucine is the amino acid monitored, with a maintenance target of roughly 150–300 µmol/L.
Is a rising leucine an emergency?
In someone known to have MSUD, yes. A leucine that is climbing during an intercurrent illness signals impending metabolic decompensation, and the severity of a crisis tracks the rate of rise rather than the absolute value. Follow the emergency regimen and contact the metabolic service the same day rather than waiting for a repeat result.
Related calculators
References
- Strauss KA, Carson VJ, Puffenberger EG. Maple Syrup Urine Disease. In: GeneReviews. Seattle: University of Washington; 2006, updated 2020.
- Frazier DM, Allgeier C, Homer C, et al. Nutrition management guideline for maple syrup urine disease: an evidence- and consensus-based approach. Mol Genet Metab. 2014;112(3):210–217.
- Lepage N, McDonald N, Dallaire L, Lambert M. Age-specific distribution of plasma amino acid concentrations in a healthy pediatric population. Clin Chem. 1997;43(12):2397–2402.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.