Abstract
The effect of various neuroleptics with different chemical structure on tyrosine hydroxylase isolated from synaptosomes of the rat hypothalamus was studied. A direct spectrophotometric method was used to determine the activity of the enzyme, based on measurement of absorption at 335 nm, at the isobestic point of oxidized forms of 6,7-dimethyl-5,6,7,8-tetrahydropterine. In the presence of 0.15 mM tyrosine, the initial velocity of the tyrosine-hydroxylase reaction was increased by haloperidol, haloanisone, and fluphenazine, but reduced by triperidol, droperidol, and carbidine. All the neuroleptics studied abolished the phenomenon of substrate inhibition of the enzyme normally arising when the tyrosine concentration is increased to 0.3 mM. The value of Km for tyrosine (0.04 mmole) was unchanged by the action of the neuroleptics. The effect of the neuroleptics is considered to be allosteric in nature.
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