Norleucine (Nle)

Norleucine is an alpha-amino acid with an unbranched n-butyl side chain, an isomer of leucine and isoleucine with the same 113-dalton residue mass, and almost the same length and shape as methionine. That is the reason it is used: methionine's thioether oxidises readily to the sulfoxide, adding 16 daltons, whereas norleucine cannot oxidise. It appears as Nle, or as [Nle] in a bracketed sequence block.

The melanocortin analogues show it in approved drugs. Alpha-melanocyte-stimulating hormone has methionine at position four, and the first stabilised analogue, afamelanotide, now approved for erythropoietic protoporphyria, replaced it with norleucine. Bremelanotide, approved in 2019 for hypoactive sexual desire disorder, and its parent melanotan II keep the same Nle4, so their sequence blocks show a bracketed residue where the hormone shows M.

The substitution is not free. Norleucine is more hydrophobic than methionine and lacks the sulfur that can take part in binding, so affinity can fall; in the melanocortin series it rose, part of why those analogues are superpotent. It also has a second life as an impurity: bacteria starved of methionine incorporate it in methionine's place, and recombinant proteins from Escherichia coli can carry measurable misincorporation.

The analytical trap follows from the mass. Because norleucine, leucine and isoleucine weigh the same, a mass spectrum cannot tell a correctly made Nle peptide from one where leucine was coupled at that position, and a certificate showing the expected mass does not prove the right isomer is present. The methionine-to-norleucine change, an 18-dalton difference, is easy to see; the norleucine-to-leucine error takes amino-acid analysis or a chromatographic standard.