i·so·leu·cine    Pronunciation Key  (s-lsn)
n.
An essential amino acid, C6H13NO2, that is isomeric with leucine.
Source: The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2000 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.

Isoleucine

n : an essential amino acid; isomeric with leucine

Source: WordNet ® 1.6, © 1997 Princeton University