Distribution of Tyrosine Hydroxylase in Human and Animal Brain¹

The activity of tyrosine hydroxylase (EC 1.10.3.1) when assayed under ideal conditions in young human brains, was comparable to that in brains of other species in level of activity and distribution. The highest levels of activity were in the putamen, caudate nucleus and substantia nigra, in keeping with data on other species. The caudate activity in human brain appeared to decrease substantially with increasing age. In both humans and baboons, the enzyme in the neostriatum was particle‐bound and inhibited by the 2‐amino‐4‐hydroxy‐6,7‐dimethyltetrahydropteridine cofactor system. In the substantia nigra it was soluble and stimulated by the 2‐amino‐4‐hydroxy‐6,7‐dimethyltetrahydropteridine cofactor system. The data suggest that tyrosine hydroxylase may be produced in a soluble form in the cell bodies of the substantia nigra but become bound as it moves toward the nerve endings in the putamen and caudate nucleus. The bound form of the enzyme was unstable but the soluble form exhibited considerable stability.