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. [ 1913 Webster ]

n. [ Gr. prwtagonisth`s; prw^tos first + 'agonisth`s an actor, combatant, fr. 'agw`n a contest. ] One who takes the leading part in a drama; hence, one who takes lead in some great scene, enterprise, conflict, or the like. [ 1913 Webster ]

Shakespeare, the protagonist on the great of modern poetry. De Quincey. [ 1913 Webster ]