Cyclase

About: Cyclase is a research topic. Over the lifetime, 10162 publications have been published within this topic receiving 388566 citations.

The RAS-adenylate cyclase pathway and cell cycle control in Saccharomyces cerevisiae.

Abstract: The cell cycle of Saccharomyces cerevisiae contains a decision point in G1 called ‘start’, which is composed of two specific sites. Nutrient-starved cells arrest at the first site while pheromone-treated cells arrest at the second site. Functioning of the RAS-adenylate cyclase pathway is required for progression over the nutrient-starvation site while overactivation of the pathway renders the cells unable to arrest at this site. However, progression of cycling cells over the nutrient-starvation site does not appear to be triggered by the RAS-adenylate cyclase pathway in response to a specific stimulus, such as an exogenous nutrient. The essential function of the pathway appears to be limited to provision of a basal level of cAMP. cAMP-dependent protein kinase rather than cAMP might be the universal integrator of nutrient availability in yeast. On the other hand stimulation of the pathway in glucose-derepressed yeast cells by rapidly-fermented sugars, such as glucose, is well documented and might play a role in the control of the transition from gluconeogenic growth to fermentative growth. The initial trigger of this signalling pathway is proposed to reside in a ‘glucose sensing complex’ which has both a function in controlling the influx of glucose into the cell and in activating in addition to the RAS-adenylate cyclase pathway all other glucose-induced regulatory pathways in yeast. Two crucial problems remaining to be solved with respect to cell cycle control are the nature of the connection between the RAS-adenylate cyclase pathway and nitrogen-source induced progression over the nutrient-starvation site of ‘start’ and second the nature of the downstream processes linking the RAS-adenylate cyclase pathway to Cyclin/CDC28 controlled progression over the pheromone site of ‘start’.

The Guanylyl Cyclase Family at Y2K

Abstract: ▪ Abstract During the 1980s the purification, cloning, and expression of various forms of guanylyl cyclase (GC) revealed that they served as receptors for extracellular signals. Seven membrane forms, which presumably exist as homodimers, and four subunits of apparent heterodimers (commonly referred to as the soluble forms) are known, but in animals such as nematodes, much larger numbers of GCs are expressed. The number of transmembrane segments (none, one, or multiple) divide the GC family into three groups. Those with no or one transmembrane segment bind nitric oxide/carbon monoxide (NO/CO) or peptides. There are no known ligands for the multiple transmembrane segment class of GCs. Mutational and structural analyses support a model where catalysis requires a shared substrate binding site between the subunits, whether homomeric or heteromeric in nature. Because some cyclases or cyclase ligand genes lack specific GC inhibitors, disruption of either has been used to define the functions of individual cyclas...

Insulin generates an enzyme modulator from hepatic plasma membranes: regulation of adenosine 3',5'-monophosphate phosphodiesterase, pyruvate dehydrogenase, and adenylate cyclase.

Abstract: Some of the acute actions of insulin may be mediated by the intracellular generation of a chemical substance that modulates certain enzymes. Such a substance has been identified which is released from liver plasma membranes after exposure to insulin. This substance was purified on sequential ion exchange, reverse phase, and gel permeations columns. The purified substance modulated the activities of cAMP phosphodiesterase, adenylate cyclase, and pyruvate dehydrogenase. The activities that modulated each of these enzymes exhibited singular chromatographic behavior and sensitivity to a variety of chemical reagents. Each activity was also produced by treatment of membranes with a phosphatidylinositol-specific phospholipase C. These results suggested that each of the enzyme-modulating activities was due to a single complex carbohydrate substance which contained inositol, phosphate, glucosamine, and other monosaccharides. The actions of this substance on these three enzymes mimicked those of insulin, suggesting...

Distinct Pathways of Desensitization of A1- and A2-Adenosine Receptors in DDT1 MF-2 Cells

Abstract: Desensitization of adenosine receptors (ARs) was studied in DDT1 MF-2 cells, which possess both A1- and A2AR, differentially coupled to adenylate cyclase. (-)-N6-(R)-Phenylisopropyladenosine (R-PIA), an A1AR-selective agonist at the appropriate concentrations, desensitized A1AR-mediated inhibition of adenylate cyclase activity in a time- (t1/2, 8 hr) and dose-dependent and reversible fashion. This was associated with significant decreases in total A1AR number and in the number of receptors possessing a high affinity for agonist in membrane preparations. The decrease in total A1AR in the membranes from the desensitized cells (approximately 40%) was associated with a 37% increase in A1AR measured in light vesicle preparations, compared with control cells. To test a possible role of phosphorylation in A1AR desensitization, cells were incubated with [32P]orthophosphate, followed by exposure to R-PIA for 18 hr. Subsequent purification of the A1AR indicated a 3-4-fold increase in phosphorylation of A1AR in cells treated with R-PIA, compared with control cells. Desensitization of the A1AR did not alter the levels of alpha s and alpha 12 proteins or affect the ability of stimulatory effectors, such as isoproterenol, sodium fluoride, and forskolin, to activate adenylate cyclase. These results suggest that uncoupling, down-regulation, and phosphorylation of the A1AR contribute, at least in part, to desensitization of this inhibitory receptor. Desensitization of the A2AR was characterized using an A2-selective agonist, 2-[4-(2-(4-aminophenyl]methylcarbonyl)ethyl)phenyl]ethylamino- 59-N-ethylcarboxamidoadenosine (PAPA-APEC). Pretreatment of cells with PAPA-APEC (100 nM) resulted in a rapid loss of agonist stimulation of adenylate cyclase activity (t1/2 of this effect, 45 min). This effect was dose dependent (EC50, approximately 10 nM) and rapidly reversible. Interestingly, desensitization of the A2AR resulted in no change in receptor number, affinity, or mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Taken together, these data suggest distinct mechanisms of desensitization of A1- and A2ARs in a single cell type.

Enhanced expression of inhibitory guanine nucleotide regulatory protein in spontaneously hypertensive rats. Relationship to adenylate cyclase inhibition.

Abstract: We have previously shown that the stimulatory effects of guanine nucleotides, N-ethylcarboxamide-adenosine and other agonists on adenylate cyclase activity were diminished in aorta and heart sarcolemma of spontaneously hypertensive rats (SHR) [Anand-Srivastava (1988) Biochem. Pharmacol. 37, 3017-3022]. In the present studies, we have examined whether the decreased response of these agonists is due to the defective GTP-binding proteins (G-proteins) which couple the receptors to adenylate cyclase, and have therefore measured the levels of G-proteins in aorta and heart from SHR and their respective Wistar-Kyoto (WKY) controls by using pertussis toxin (PT)- and cholera toxin (CT)-catalysed ADP-ribosylations and immunoblotting techniques using specific antibodies against G-proteins. The labelling with [32P]NAD+ and PT identified a 40/41 kDa protein in heart and aorta from WKY and SHR and was significantly increased in the hearts (approximately 100%) and aorta (approximately 30-40%), from SHR as compared with WKY. Immunoblotting revealed an increase in the levels of the G-protein alpha-subunits Gi alpha-2 and Gi alpha-3 in heart and Gi alpha-2 in aorta, whereas no change in Go alpha was observed in heart from SHR and WKY. On the other hand, no differences were observed in CT labelling or immunoblotting of stimulatory G-protein (Gs) in heart and aorta from WKY and SHR. In addition, CT stimulated the adenylate cyclase activity in heart sarcolemma from WKY and SHR to a similar extent. These results were correlated with adenylate cyclase inhibition and stimulation by various hormones. Angiotensin II (AII), atrial natriuretic factor (ANF) and oxotremorine-mediated inhibition was found to be greater in SHR as compared with WKY, whereas the stimulatory effects of adrenaline, isoprenaline, dopamine and forskolin were diminished in SHR aorta as compared to WKY. These results indicate that regulatory protein G(i) is more expressed in SHR, which may be associated with the decreased responsiveness of stimulatory hormones and increased sensitivity of inhibitory hormones to stimulate/inhibit adenylate cyclase activity. It may thus be suggested that the enhanced G(i) activity may be one of the mechanisms responsible for the diminished vascular tone and impaired myocardial functions in hypertension.