Catecholamine-sensitive guanylate cyclase from human caudate nucleus

Requirement for cGMP in nerve cell death caused by glutathione depletion

Glutathione depletion occurs in several forms of apoptosis and is associated with Parkinson's disease and HIV toxicity. The neurotransmitter glutamate kills immature cortical neurons and a hippocampal nerve cell line via an oxidative pathway associated with glutathione depletion. It is shown here that soluble guanylyl cyclase (sGC) activity is required for nerve cell death caused by glutathione depletion. Inhibitors of sGC block glutamate toxicity and a cGMP analogue potentiates cell death. Glutamate also induces an elevation of cGMP that occurs late in the cell death pathway. The resultant cGMP modulates the increase in intracellular calcium that precedes cell death because sGC inhibitors prevent calcium elevation and the cGMP analogue potentiates the increase in intracellular calcium. These results suggest that the final pathway of glutamate induced nerve cell death is through a cGMP-modulated calcium channel.

Inhibitor of antagonist binding to the muscarinic receptor is elevated in Alzheimer's brain

The 100,000 x g supernatant fraction of human brain contains endogenous inhibitors of antagonist binding to the muscarinic receptor. Significantly greater inhibition was observed with Alzheimer's than non-demented control supernatant fractions. Low molecular weight inhibitor was separated from larger inhibitor species by membrane dialysis (3,500 dalton cut-off). The activity of low molecular weight inhibitor was greatly increased by sulfhydryl reducing agents. While the low molecular weight inhibitor was stable to heat, acid and base for short time periods (< 20 min), it was inactivated by acid hydrolysis (50% loss after 16 h, 100% loss after 96 h). The low molecular weight inhibitor activity is elevated approximately three-fold in Alzheimer's brain. The low molecular weight inhibitor from Alzheimer's brain was found to be a non-competitive inhibitor. This is the first report of endogenous inhibitors in human brain of ligand binding to the muscarinic receptor and of increased inhibitor activity in Alzheimer's disease.

Biochemical and toxicological properties of the oxidation products of catecholamines

The normal catabolism of catecholamines proceeds through enzymatic pathways (monoaminooxidase, catechol-o-methyltranserase, and phenolsulphotransferase). In addition, nonenzymatic oxidative pathways might take place since catechols are readily oxidized. In this review article, the pathways of formation of the oxidation products of catecholamines and their reactions are described. The interactions of these products with different biological systems and their toxicity are examined. Among the reactions known to occur is that with sulfhydryls, which results in either a covalently linked adduct or disulfide production. Another interesting pathway to toxicity involves the oxidation of these catecholamine products by oxygen, with the formation of damaging oxygen-derived species. The action of the oxidation products of catecholamines is outlined, with special attention to the nervous and cardiac systems.

Toxicity of aminochromes

The first part of the present review deals with the chemical and enzymatic synthesis of adrenochrome and other aminochromes from the corresponding catecholamines. A description of the most significant pathways of formation and the reactivity of the aminochromes is presented. In the second part of the toxicity of aminochromes, mainly at the cardiac and CNS level, is described and some of the molecular mechanisms of the toxic action are outlined. The toxicity of the aminochromes appears to depend mainly on the production of reduced oxygen species through redox cycling. The interaction of aminochromes with sulfhydryl groups and the induced depletion of oxygen, ascorbate and glutathione are additional mechanisms resulting in noxious effects at a cellular level.

Amine-mediated toxicity. The effects of dopamine, norepinephrine, 5-hydroxytryptamine, 6-hydroxydopamine, ascorbate, glutathione and peroxide on the in vitro activities of creatine and adenylate kinases in the brain of the rat

The effects of several concentrations of amines and reducing agents on the activity of creatine (CK) and adenylate (AK) kinases were determined in homogenates of the brain of the rat at 0 and 37 degrees C. The order of decreasing irreversible inhibition of the enzymes was peroxide, 6-hydroxydopamine, dopamine, norepinephrine, 5-hydroxytryptamine. At 37 degrees C, approx. 50% of the activity of creatine kinase was lost in 30 min in the presence of 20 microM dopamine. 5-Hydroxytryptamine was several orders of magnitude less toxic. The action of dopamine was not prevented by inhibition of monoamine oxidase, chelation of metals or the addition of a catalase, indicating that formation of peroxide by monoamine oxidase was not the primary cause of the loss of enzyme. Although auto-oxidation of dopamine to a toxic quinone was considered, the degree of inhibition of creatine kinase was not affected when auto-oxidation was prevented under anaerobic conditions. Glutathione (GSH), present during the incubation, protected the enzymes but could not restore activity after exposure to amine. Concentrations of glutathione above 5 mM and of oxidized glutathione as low as 10 microM inhibited creatine kinase. Ascorbate protected the enzymes even when present at a concentration much less than that of the amine, but ascorbate was itself toxic. The findings indicate that dopamine, at concentrations attained after drug-induced release or ischemia, can be toxic to a metabolic enzyme present in the synaptosomal membrane.

Peptides associated with eggs: mechanisms of interaction with spermatozoa

Speract (Gly-Phe-Asp-Leu-Asn-Gly-Gly-Gly-Val-Gly), a peptide obtained from the culture medium of Strongylocentrotus purpuratus eggs, stimulates the respiration and motility of S. purpuratus spermatozoa under appropriate conditions. Resact (Cys-Val-Thr-Gly-Ala-Pro-Gly-Cys-Val-Gly-Gly-Gly-Arg-LeuNH2), a peptide obtained from Arbacia punctulata eggs also stimulates the metabolism and motility of A. punctulata spermatozoa, however, it fails to stimulate S. purpuratus spermatozoa. Early biochemical responses of the spermatozoa to the egg peptides include a net H+ efflux and elevations of cyclic AMP and cyclic GMP concentrations. In addition, in A. punctulata spermatozoa, a major plasma membrane protein is modified in response to resact such that its apparent molecular weight shifts from 160,000 to 150,000. If cells are incubated with 32P, the 160,000 molecular weight form of the protein becomes radiolabeled; subsequent addition of resact causes a rapid loss of 32P from the protein. The plasma membrane protein appears to be the enzyme, guanylate cyclase; coincident with the shift in apparent molecular weight, enzyme activity decreases by as much as 90%. Since speract fails to cause these responses in A. punctulata, it can be concluded that the events are receptor-mediated.

Attempts for light microscopical demonstration of guanylate cyclase activity in rat cerebellum

Guanylate cyclase in rat cerebellum was investigated on the light microscopical level with guanylyl imidodiphosphate as substrate. Several attempts for activation of enzymatic activity and delimitation to other enzymes were made by sodium azide, aminophylline, sodium fluoride and dithioerythrole. The localization was similar but less strong compared to adenylate cyclase (Poeggel and Luppa 1984) and differs in behaviour to the above mentioned substances. Nucleotide pyrophosphatases seem to play an unimportant role in guanylyl imidodiphosphate conversion, while alkaline phosphatase is possibly of more importance. A light microscopical demonstration of guanylate cyclase by its enzymatic activity must be considered with caution. Main reasons are the low activity and therefore the great importance of interfering enzymes with high activities.

Ionic regulation of antagonist binding to the human muscarinic cholinergic receptor of caudate nucleus

In this in vitro study we investigated the effects of a variety of cations on binding of the cholinergic antagonist, (3H)-quinuclidinyl benzilate, to the human caudate. Physiologic sodium concentrations induced a state of positive cooperativity upon the human muscarinic cholinergic receptor (nHill = 1.41). Lithium at therapeutic equivalency (1 mM) was unique in increasing the dissociation constant (KD) for this antagonist from 35.9 to 72.4 pM. This implies a lithium-induced cholinomimetic effect at the human muscarinic receptor consistent with reports of the reversal of mania by physostigmine. These findings are also relevant to previous cholinergic receptor studies in which various cations have been employed.

Postmortem stability of dopamine-sensitive adenylate cyclase, guanylate cyclase, ATPase, and GTPase in rat striatum

The stability of dopamine-sensitive adenylate cyclase, guanylate cyclase, ATPase, and GTPase was measured in homogenates of rat striatal tissue frozen from 0 to 24 h postmortem. ATPase, GTPase, and Mg2+-dependent guanylate cyclase activities showed no significant change over this period. Mn2+-dependent guanylate cyclase activity was stable for 10 h postmortem. Basal and dopamine-stimulated adenylate cyclase activity decreased markedly during the first 5 h. However, when measured in washed membrane preparations, these adenylate cyclase activities remained stable for at least 10 h. Therefore, the postmortem loss of a soluble activator, such as GTP, may decrease the adenylate cyclase activity in homogenates. These results are not consistent with an earlier suggestion that there is a postmortem degradation of the enzyme itself. Other kinetic parameters of dopamine-sensitive adenylate cyclase can also be measured independently of postmortem changes. Thus, it is possible to investigate kinetic parameters of dopamine-sensitive adenylate cyclase, guanylate cyclase, ATPase, and GTPase in human brain obtained postmortem.

Ultrastructural localization of cyclic GMP and cyclic AMP in rat striatum

The subcellular localization of cyclic GMP and cyclic AMP in the rat caudate-putamen has been studied using horseradish peroxidase immunocytochemistry. Both of the putative neurotransmitter second messengers were visualized in neurons and glial cells at light microscopic resolutions, but not all cells of either category gave detectable staining. This was confirmed at the ultrastructural level where both stained and unstained elements of the same cell type were found within the same field. A striking variation was seen in cyclic nucleotide staining intensity within individual neural and glial cells. Both of the cyclic nucleotides were detected within postsynaptic terminal boutons and within astroglial processes. Cyclic GMP postsynaptic staining was stronger than glial staining, whereas the localization pattern was reversed for cyclic AMP. The synaptic localization of cyclic AMP and cyclic GMP immunoreactivity adds support to the idea that these compounds have an influential role in synaptic function within the striatum.

Stimulation of guanylate cyclase by EDTA and other chelating agents

The partially purified soluble guanylate cyclase (GTP pyrophosphatelyase(cyclizing), EC 4.6.1.2) from human caudate nucleus is stimulated from 2 to 4-fold by metal chelating agents. EDTA (K 1/2 - 4.8 microM) is more potent than CDTA (K 1/2 = 13.2 microM) or EGTA (K 1/2 = 21.8 microM) at stimulating activity. Stimulation by chelating agents is apparently not due to removal of inhibitory divalent cations which contaminate the enzyme or reaction mixture. EDTA increases guanylate cyclase activity in part by increasing the affinity of the enzyme for the substrate (MgGTP) 10-fold. Dopamine inhibits partially purified guanylate cyclase in the presence or absence of EDTA. Dopamine increases the Ka of guanylate cyclase for the activator, free Mn2+, more than 50-fold, from 3 to 150 microM.