MPP+

MPP+ (1-methyl-4-phenylpyridinium) is a positively charged molecule with chemical formula C12H12N+. It is toxic and acts by interfering with oxidative phosphorylation in mitochondria, causing depletion of ATP and cell death. It also inhibits the synthesis of catecholamines, reduces levels of dopamine and cardiac norepinephrine, and inactivates tyrosine hydroxylase.

The neurotoxin MPTP is converted in the brain into MPP+ by the enzyme MAO-B, causing parkinsonism in primates by killing certain dopamine-producing neurons in the substantia nigra.

The chloride of MPP+ has been used as a herbicide under the trade name cyperquat and is structurally similar to the herbicide paraquat.