Radiometric assay of tyrosine hydroxylase and tryptophan hydroxylase by Kalignost extraction procedures

Neurochemical development of the striatum in a precocial (guinea pig) and an altricial (rat) species

In this study, the developmental patterns of key neurotransmitter marker enzymes, namely choline acetyltransferase (ChAT), glutamate decarboxylase (GAD), and tyrosine hydroxylase (TH) in the corpus striatum were compared between the guinea pig and rat. In an attempt to assess cell acquisition and differentiation, striatal DNA and protein content were also determined during development in the two species. The most outstanding feature apparent from these investigations was a sequential maturation of the three neurotransmitter-synthesizing enzymes: TH and GAD first, followed by ChAT in striata of both species. The developmental increase in DNA content suggests that cell acquisition in the rat striatum is approximately 50% at birth, whereas in the guinea pig, the total population is present before birth, and thereafter cell numbers decline. The accretion of protein, reflecting the increase in region size, continued into adulthood in both species. With emphasis on cholinergic neurons, the neurochemical findings are assessed in relation to the morphological descriptions of cellular changes during pre- and postnatal development of the striatum.

A rapid and sensitive assay for tyrosine-3-monooxygenase based upon the release of 3H2O and adsorption of [3H]-tyrosine by charcoal

A rapid, simple and sensitive assay has been developed for tyrosine-3-monooxygenase, the enzyme catalyzing the rate-limiting step in catecholamine biosynthesis. The assay is based upon the release of 3H2O from 3H-[3,5]-L-tyrosine with adsorption of the isotopic substrate (and its metabolites) by an aqueous slurry of activated charcoal. This method routinely yields low blank values and is simpler than the procedure requiring the use of cation exchange columns to separate the isotopic substrate from the 3H2O formed during the hydroxylation reaction.

Decreased neurotransmitter receptor binding in striatum and cortex from adult hypothyroid rats

The consequences of propylthiouracil-induced thyroid deficiency on pre- and postsynaptic biochemical markers in the corpus striatum and the visual cortex were investigated in adult rats. Hypothyroidism in adult rats significantly impaired (by 30%) striatal [3H]spiroperidol binding capacity. In the visual cortex, small but significant reductions in [3H]WB.4101 and in [3H]muscimol binding were evident, whereas glutamate decarboxylase activity was increased by 18% in the thyroid-deficient rats. Compatible with previous morphological evidence (Ruiz-Marcos et al.), the present observations may imply alterations in dendritic morphology of neurones in the striatum and cortex during long-term hypothyroidism in adult rats. However, the molecular changes observed suggest a differential sensitivity of transmitter-specific neurones in the central nervous system to thyroid hormone.

A sensitive and inexpensive high-performance liquid chromatographic assay for tyrosine hydroxylase

We describe a highly sensitive assay method for tyrosine hydroxylase (TH) using high-performance liquid chromatography with amperometric determination. This assay method could be applicable to any tissues with low enzyme activity, such as rat cerebellum. We also describe the kinetic properties of TH in rat cerebral cortex.

Tyrosine hydroxylase activation and inactivation by protein phosphorylation conditions

Tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, catalyzes the conversion of tyrosine to DOPA, Cyclic AMP-dependent protein phosphorylation conditions alter tyrosine hydroxylase activity in rat striatal homogenates. In agreement with other laboratories, we find that short-term pre-incubation (3 min) of extracts under phosphorylating conditions (Mg . ATP, cAMP) increases enzyme activity two- to tenfold over control as measured during a subsequent 15-min assay. We now report that preincubation under phosphorylating conditions for longer periods (30 min) results in a loss of activity to levels equal to or below that of the control enzyme. Addition of purified bovine brain protein kinase catalytic subunit and Mg . ATP enhances activation and increases the rate of inactivation. To demonstrate that inactivation is not associated with proteolytic degradation or irreversible denaturation, the inactivated form of the enzyme can be reactivated. The protein kinase inhibitor protein decreases the activation process and prevents inactivation of the enzyme to below control values. The sedimentation coefficient is not changed by phosphorylation conditions (S = 8.8 +/- 0.1). Although the apparent Km of the enzyme for the 6-methyltetrahydropterine (6-MPH4) cofactor is reduced (0.86 mM, control; 0.32 mM, activated), it is also reduced in the inactivated form (0.38 mM). The Ki for dopamine is increased from 4.5 microM for the control to 28 microM for the activated enzyme, whereas the inactivated form of the enzyme exhibits a Ki of 10 microM. Removal of catecholamines by gel filtration fails to alter activity and the apparent cofactor Km. Moreover, both the activated and the inactivated states persist following gel filtration. It therefore appears that the activation-inactivation process is not mediated solely by the modulation of enzyme feedback inhibition or changes in the Km for 6-MPH4. We also describe a coupled decarboxylase assay in which labeled dopamine is resolved from the precursors tyrosine and DOPA by low-voltage paper electrophoresis.