L-Tyrosine ((S)-Tyrosine) is a naturally occurring tyrosine and is synthesized in vivo from L-phenylalanine, considered a non-essential amino acid. L-Tyrosine(L-p-Tyrosine) is the levorotatory isomer of the aromatic amino acid tyrosine.

L-tyrosine in vitro inhibits citrate synthase activity in the posterior cortex. Succinate dehydrogenase activity is increased in the posterior cortex, hippocampus, striatum and liver. Complex I activity is inhibited in the hippocampus, complex II activity is also inhibited in the hippocampus and in the posterior cortex and liver, complex II–III activity is not altered by L-tyrosine and complex IV activity decreased in the posterior cortex[1].

Acute administration of L-tyrosine inhibits malate dehydrogenase and complex II, II–III and IV of the mitochondrial respiratory chain activity in the posterior cortex and liver of rats. The succinate dehydrogenase enzyme and complex I activity are inhibited in the posterior cortex and increased in the striatum[1].