The Collaborative International Dictionary
Protagon
Protagon \Pro"ta*gon\, n. [Proto- + Gr. 'agw`n a contest. See. Protagonist. So called because it was the first definitely ascertained principle of the brain.] (Physiol. Chem.) A nitrogenous phosphorized principle found in brain tissue. By decomposition it yields neurine, fatty acids, and other bodies.
Wiktionary
protagon
n. (context biochemistry English) A nitrogenous phosphorized principle found in brain tissue. By decomposition it yields neurine, fatty acids, and other bodies.