Adenyl cyclase in human platelets: activity and responsiveness

A clinical study was conducted whereby the activity of adenyl cyclase in the human platelet was demonstrated. The study showed that this activity can be stimulated and inhibited in vitro. Platelets were isolated from normal donors. The laboratory procedures involved in the study are described in detail. It seems that many of the biologic processes which occur in the human platelet are dependent on the breakdown of ATP (adenosine-tri-phosphate) to, among other substances, AMP (adenosine-3',5' monophosphate). Activity of the adenyl cyclase was stimulated by fluoride, prostaglandin E1, and glucagon; it was inhibited by thrombin, epinephrine, norepinephrine, and serotonin. PG (prostaglandin) E1 at concentrations of 20 ng/ml and above increased adenyl cyclase in 7 experiments by 3-5 times. Even at concentrations as low as 2 ng/ml., PGE2 caused perceptable stimulation. The PGE, while stimulating adenyl cyclase activity, also inhibited aggregation of platelets by a variety of substances. Results of the study suggest that adenosine-3',5' monophosphate may be important in the regulation of platelet adhesiveness.     

Adenyl cyclase in cell-free extracts ofEscherichia coli

The adenyl cyclase enzyme system was detected in the cells ofEscherichia coli disrupted by sonic treatment. This enzyme activity is located mainly in the cell fraction sedimenting at 2,000×g, i.e. in the cytoplasmic membrane fraction. A prolonged sonication treatment of the cell suspension was followed by the disappearance of activity in the membrane preparation. The pH optimum of the adenyl cyclase inEscherichia coli was on the alkaline side, around pH 9.     

Adenyl cyclase in the haemocytes of the American cockroach

Incubation of ¹⁴C-adenosinetriphosphate with lysed haemocytes produces ¹⁴C-cyclic 3′,5′-adenosinemonophosphate (cAMP). The addition of phosphodiesterase to similar reaction mixtures results in the conversion of cAMP to 5′-adenosinemonophosphate.Differential centrifugation of the haemocyte preparation revealed that adenyl cyclase activity occurred in both membrane bound and soluble fractions, although the evidence suggests that the enzyme activity was originally particulate.The significance of these data is discussed in regard to the sclerotization process and the mode of action of bursicon.     

Dopamine-stimulated adenylate cyclase in human term placenta

The effect of dopamine on adenylate cyclase activity was investigated in slices of human term placentas. Dopamine elicited a dose-dependent stimulation of cAMP formation with a ED50 value of about 1 X 10(-6)M dopamine and an increase of 110% over the control with 1 X 10(-4)M dopamine. (-)-Epinephrine and (-)-norepinephrine also increased placental cAMP formation. Apomorphine displayed a slight but non-significant stimulatory effect while bromocriptine was not effective. SCH 23390, a selective antagonist of dopamine D1 receptors caused a dose-dependent decrease of the dopamine activation. In contrast, the dopamine increase of cAMP was unaffected by beta- and alpha-adrenergic blocking drugs and by the D2 selective antagonist, (-)-sulpiride. These data indicate that dopamine stimulates cAMP formation in human term placenta through a specific mechanism via D1 dopaminergic receptors positively coupled to adenylate cyclase.     

Adenyl cyclase in normal and denervated skeletal muscles

Adenyl cyclase (AC) has been studied in homogenates and crude plasma membranes from normal and denervated red and white skeletal muscle from male rats. Basal-, NaF- and epinephrine-stimulated activities were increased in homogenates of both types of muscles after nerve transection, supporting a possible role of the cAMP-AC system in the neurotrophic control of skeletal muscle. AC-specific activity was increased 10 times in crude plasmic membranes from normal muscle if compared to that of homogenate. It was decreased in crude plasmic membrane from denervated muscle. The correlation of our results with other results on cAMP concentrations and cAMP phosphodiesterase (PDE) activities in denervated muscle suggests that factors other than AC and PDE might control the synthesis and degradation of cAMP.     

Ultracytochemical localization of guanylate cyclase in early human placenta

Previous biochemical and cytochemical studies have indicated that in human term placenta the enzyme guanylate cyclase (GC) is associated mostly with the cytosolic fraction of homogenates and localized on the syncytiotrophoblast microvillous border. In the present study we have shown cytochemically the GC particulate form in early human placenta using guanylyl-imidodiphosphate [Gpp(NH)p] as substrate and NaN3 as activator. In samples of placental villi taken from the 6th to 12th week of pregnancy, the GC reaction product was always found on the apposing Langhans cytotrophoblast and syncytiotrophoblast plasma membranes. Furthermore, GC was present on cells in mitosis of the Langhans cytotrophoblast. From the 11th week GC was also visible on basal plasma membranes of Langhans cytotrophoblast and on endothelial cells of fetal capillaries. In samples of human term placenta GC was detectable on the syncytiotrophoblast microvillous border. This suggests a shift of enzyme localization during pregnancy.     

Studies on cytosolic guanylate cyclase from human placenta.

We have purified the soluble form of guanylate cyclase from human placenta greater than 2400-fold. The enzyme shared several characteristics with the enzyme purified from other sources including molecular mass and subunit composition, activation by divalent cations, inhibition by ATP and Michaelis constants. The enzyme, however, had a lower absorption maximum in the Soret region (417 +/- 1 nm) than the enzyme from other sources and was activated only one-fifth as much by nitric oxide as the bovine lung enzyme. It appears that the heme prosthetic group in the human placental enzyme may be hexa-coordinate and in the bovine lung enzyme the heme group may be penta-coordinate.