Restoration of guanine nucleotide- and fluoride-stimulated activity to an adenylate cyclase-deficient cell line with affinity-purified guanine nucleotide regulatory protein

Activation of fluoride-stimulated adenylate cyclase by phospholipase A2 in the caudate nucleus of the rat brain

Phospholipase A2 (PLA2) increases adenylate cyclase (AC) activity in the rat caudate nucleus in a dose-dependent manner. After maximal stimulation by fluoride, PLA2 treatment further increases AC activity 2.4 fold. Adenylate cyclase activity is maximal after 45% hydrolysis of the phospholipids. Of the products of PLA2 treatment only lysophosphatidylcholine (LPC) produces such an increase in AC activity. In contrast to PLA2 treatment, LPC solubilizes the enzyme, decreases the Km value for ATP, and requires much larger amounts of LPC than that produced by lipase treatment. After maximal stimulation with fluoride and PLA2, removal of most of the LPC does not reduce the activity of adenylate cyclase. These findings suggest that removal of membrane lipid rather than generation of LPC is responsible for the activation of brain adenylate cyclase by phospholipase A2.

Roles of GTP and GDP in the regulation of the thyroid adenylate cyclase system

Effects of guanine nucleotides on the adenylate cyclase activity of thyroid plasma membranes were investigated by monitoring metabolism of the radiolabeled nucleotides by thin-layer chromatography (TLC). When ATP was used as substrate with a nucleotide-regenerating system, TSH stimulated the adenylate cyclase activity in the absence of exogenous guanine nucleotide. Addition of GTP or GDP equally enhanced the TSH stimulation. Effects of GTP and GDP were indistinguishable in regard to their inhibitory effects on NaF-stimulated activities. The results from TLC suggested that GDP could be converted to GTP by a nucleotide-regenerating system. Even in the absence of a nucleotide-regeneration system, addition of GDP to the adenylate cyclase assay mixture resulted in the parallel decrease in ATP levels and formation of GTP indicating that thyroid plasma membrane preparations possessed a transphosphorylating activity. When an ATP analog, App[NH]p, was used as substrate without a nucleotide-regenerating system, no conversion of GDP to GTP was observed. Under such conditions, TSH did not stimulate the adenylate cyclase activity unless exogenous GTP or Gpp[NH]p was added. GDP no longer supported TSH stimulation and caused a slight decrease in the activity. GDP was less inhibitory than Gpp(NH)p to the NaF-stimulated adenylate cyclase activity. These results suggest: (1) TSH stimulation of thyroid adenylate cyclase is absolutely dependent on the regulatory nucleotides. (2) In contrast to GTP, GDP cannot support the coupling of the receptor-TSH complex to the catalytic component of adenylate cyclase. (3) The nucleotide regulatory site is more inhibitory to the stimulation of the enzyme by NaF when occupied by Gpp[NH]p than GDP.

Dopamine receptors in canine caudate nucleus

Physiological, pharmacological, histochemical and biochemical studies indicate that dopamine receptors are heterogenous in the central nervous system with each individual functions. This review describes pharmacological and biochemical characteristics of dopamine receptors, particularly in canine caudate nucleus, which have been studied in our laboratory with a brief comparison to the current studies by other workers in similar research fields. Two distinct dopamine receptors have been characterized by means of [3H]dopamine binding to the synaptic membranes from canine caudate nucleus. One of the receptors with a Kd of about 3 muM for dopamine may be associated with adenylate cyclase and referred to as D2 receptor. The other receptor with a Kd of about 10 nM for dopamine is independent of adenylate cyclase and referred to as D2. A photochemical irreversible association of [3H]dopamine with the membraneous receptors makes it possible to separate D1 and D2 receptors from one another by gel filtration on a Sephadex G-200 column after solubilization with Lubrol PX. On the basis of selective inhibition of [3H]dopamine binding to D1 and D2 receptors, dopamine antagonists can be classified into three classes: D1-selective (YM-09151-2), D2-selective (sulpiride) and nonselective (haloperidol, chlorpromazine). Effects of these typical antagonists on the metabolism of rat brain dopamine suggest that D1 receptor is more closely associated with the neuroleptic-induced increase in dopamine turnover. Studies with 28 benzamide derivatives and some classical neuroleptics reveal that apomorphine-induced stereotypy displays a greater association with D1 than with D2 receptors. Dopamine-sensitive adenylate cyclase in canine caudate nucleus can be solubilized with Lubrol PX in a sensitive form to either dopamine. Gpp(NH)p or fluoride Sephadex G-200 gel filtration separates adenylate cyclase from D1 receptors with a concomitant loss of dopamine sensitivity. Addition of the D1 receptor fraction to the adenylate cyclase restores the responsiveness to dopamine. The solubilized dopamine-unresponsive adenylate cyclase can be further separated into two distinct fractions by a batch-wise treatment with GTP-sepharose: a catalytic unit which does not respond to fluoride, and a guanine nucleotide regulatory protein. The regulatory protein confers distinct responsiveness to Gpp(NH)p and fluoride upon adenylate cyclase. These results indicate that dopamine-sensitive adenylate cyclase is composed of at least three distinct units; D1 receptor, guanine nucleotide regulatory protein and adenylate cyclase.