Some properties of adenosine 3',5'-cyclic monophosphate phosphodiesterase in the superior cervical ganglion of the guinea pig

Adenosine 3',5'-cyclic monophosphate (cAMP) phosphodiesterase activity in crude guinea-pig superior cervical ganglion homogenates was assayed under a variety of experimental conditions. Two forms of cAMP phosphodiesterase were found, one with high and the other with low affinity for the substrate. The Km values were about 1 and 110 microM respectively. Imidazole slightly but constantly stimulated the former enzyme form over a wide range of concentrations and 1-methyl-3-isobutylxanthine was a weak competitive inhibitor with a Ki value of 90 microM. Low affinity cAMP phosphodiesterase activity was increased by calmodulin and Ca2+. This stimulation was not observed in the presence of trifluoperazine, a specific inhibitor of calmodulin. On the other hand, neither [D-Ala2]met-enkephalinamide nor prostaglandin E2, alone or in combination, influenced high affinity cAMP phosphodiesterase.

Alpha-adrenoceptor-mediated inhibition of acetylcholine release in guinea-pig superior cervical ganglion

The effect of different substances on the electrically evoked acetylcholine (ACh) output from guinea-pig superior cervical ganglion (SCG) preincubated with [3H]choline was investigated. Forskolin significantly reduced neurotransmitter release and dose-dependently enhanced cyclic AMP (cAMP) levels. Phenylephrine and clonidine, alpha 1-and alpha 2-adrenergic agonists respectively, caused a significant inhibition of ACh output from presynaptic nerve terminals. In the presence of RMI 12330A, phenylephrine but not clonidine failed to reduce neurotransmitter release. These results suggest that cAMP and different alpha-adrenergic receptors are involved in the regulation of cholinergic transmission.

[Preliminary observations on in vitro biosynthesis of prostaglandins E in tumor cells and virus transformed cells]

Biosynthesis of prostaglandins of E series in BK virus-transformed rabbit-kidney cells (RKBK) and mouse hepatoma cells cultured in vitro with and without exogenous arachidonic acid was determined by radioimmunoassay. Elevated concentrations of prostaglandin E2 were observed in both cell types; moreover, tumor and transformed cells were capable to synthesize higher levels of prostaglandin E2 than normal cells. Mouse hepatoma cells produced more prostaglandins compared to RKBK cells; in addition they seemed less sensitive to inhibition by indomethacin. These data suggest that arachidonic acid metabolism may be another one of the intrinsic biochemical properties that differentiate tumor and virus transformed cells.

Energy transduction in photosynthetic bacteria. VII. Inhibition of the coupling ATPase by N-ethylmaleimide related to the energized state of the membrane

N-Ethylmaleimide, at millimolar concentrations, irreversibily inhibits photophosphorylation and ATPase activity of photosynthetic membranes from Rhodopseudomonas capsulata. The inhibitory effect of N-ethylmaleimide is evident only the membranes are preincubated with the inhibitor in the light and in the absence of phosphorylation substrates. ADP and orthophosphate (or arsenate) exert a protective effect against the inhibition if they are present during the preillumination stage. The energization of the membrane by ATP hydrolysis, measured as ATP-induced quenching of 9-aminoacridine fluorescence, also is inhibited irreversibly by N-ethylmaleimide. Uncouplers protect the ATPase from inhibition by N-ethylmaleimide at concentrations at which they inhibit photophosphorylation. The ATPase, as measured either in the dark or in the light, is also inhibited by carbonylcyanide p-trifluoromethoxypenylhydrazone in parallel with photophosphorylation. These results are interpreted as evidence that the high-energy state of the membrane induces a conformational change of the ATPase, making it sensitive to attack by N-ethylmaleimide; this conformational change might be related to the active state of the ATPase.

Energy transduction in photosynthetic bacteria. VIII. Activation of the energy-transducing ATPase by inorganic phosphate

ATPase activity and ATP-induced energization of photosynthetic membranes from Rhodopseudomonas capsulata are stimulated by phosphate; the maximum stimulatory effect occurs at a concentration between 1 and 2 mM. The sensitivity of the ATPase to oligomycin increases in the presence of phosphate since all the Pi-stimulated activity is inhibited by this antibiotic. Aurovertin, which has no effect on ATPase in the absence of phosphate, inhibits completely the activity elicited by this anion. The addition of Pi induces a substantial increase in the V of ATPase activity without changing the affinity of the enzyme for ATP or ADP. Arsenate, at the same concentrations, produces effects very similar to those of phosphate. The stimulation by arsenate of the transfer of energy from ATP to the membrane suggests a non-hydrolytic role of this anion as a modifier of the ATPase activity.