The activity of tyrosine 3-monooxygenase is usually suppressed by an endogenous factor in brain.

Tyrosine 3-monooxygenase activity of the crude extract from rat striatum had a sharp pH optimum at pH 5.6 and showed almost no activity at pH 7 and higher. When the crude extract was adjusted to pH 5.2, the resulting precipitate showed high activity in the pH range of 6 to 7.5. Incubation of the acid-precipitated fraction with the acid-soluble fraction resulted in a remarkable decrease in the enzyme activity, as assayed at a neutral pH. These findings suggest that there is an endogenous factor inhibiting the activity of tyrosine 3-monooxygenase in rat brain.