Leucine
Properties
| State | Solid (white crystalline powder) |
| Color | White |
| Solubility | Slightly soluble in water (21.5 g/L at 25°C); soluble in dilute acids and bases |
| Melting Point | 293°C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Leucine
Leucine is a branched-chain amino acid with an isobutyl side chain that does two things no other proteinogenic amino acid quite matches. First, biochemically: leucine is the strongest single activator of the mTORC1 signaling complex, the master regulator of cap-dependent translation initiation. The sensing happens through Sestrin2, which releases its inhibition of GATOR2 when leucine binds, and the downstream cascade flips on S6K1 and 4E-BP1 phosphorylation within minutes — meaning a 2-3 g oral dose can measurably bump muscle protein synthesis rates in humans even without other amino acids present. Second, structurally: the bulky hydrophobic side chain drives the formation of leucine zipper motifs in transcription factors (the Fos/Jun AP-1 dimer, GCN4, the bZIP family generally), where leucines at every seventh position along an alpha-helix interlock through hydrophobic packing to form a coiled-coil DNA-binding dimer. In folded globular proteins leucine almost always sits in the buried hydrophobic core; surface-exposed leucines are statistically rare for a reason. Humans cannot synthesize leucine — the carbon skeleton lacks any biosynthetic route in animals — so the 14 mg/kg/day requirement comes from chicken breast, beef, eggs, milk, soy, and whey. Catabolism runs through alpha-ketoisocaproate and ultimately to acetyl-CoA and acetoacetate, making leucine purely ketogenic (the only other purely ketogenic amino acid is lysine).
Where you'll encounter it
If you've ever scooped whey protein after a workout or eaten a steak and felt the post-meal anabolic rebound, the leucine content is what flipped the mTOR switch — the rest of the amino acids were the building blocks, but leucine was the signal. Sports-nutrition formulators target the 2.5-3 g 'leucine threshold' per serving because tracer studies measuring fractional synthesis rate by infusing 1-13C leucine and biopsying vastus lateralis show muscle protein synthesis flat-lines below that dose and saturates above it. Cell-biology labs probing mTORC1 dose isolated cells with 800 µM L-leucine and Western-blot for phospho-S6K1 (Thr389) within 30 minutes to confirm pathway activation. Pharmaceutical formulators of dry-powder inhalers add micronized L-leucine at 5-10 wt% as an anti-aggregation excipient that lets fine drug particles disperse cleanly out of the device.
Common Uses
- Free-form supplement for triggering muscle protein synthesis (2-3 g per dose)
- Component of branched-chain amino acid (BCAA) blends with valine and isoleucine in 2:1:1 ratios
- mTORC1 pathway agonist in cell biology research and Sestrin2 binding studies
- Parenteral nutrition formulations for hospitalized and critically ill patients
- Substrate in stable-isotope tracer studies (1-13C leucine for whole-body protein turnover)
- Flavor enhancer and Maillard reaction precursor in food processing
- Excipient lubricant in dry-powder inhaler formulations for pulmonary drug delivery
- Reference standard in HPLC amino acid analysis
Safety Information
GRAS for dietary use, non-toxic at normal intakes. Not classified hazardous under GHS. Chronic supplementation above ~10 g/day can compete with valine, isoleucine, tryptophan, and the other large neutral amino acids for the LAT1 transporter at the blood-brain barrier, potentially lowering brain serotonin precursor availability. Patients with maple syrup urine disease (BCKDH deficiency) accumulate leucine to neurotoxic levels and require strict dietary restriction.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.