Activation of rat brain adenylate cyclase by copper plus dithiothreitol

Modulation of adenylyl cyclase activity in young and adult rat brain cortex. Identification of suramin as a direct inhibitor of adenylyl cyclase

Adenylyl cyclase (AC) in brain cortex from young (12-day-old) rats exhibits markedly higher activity than in adult (90-day-old) animals. In order to find some possibly different regulatory features of AC in these two age groups, here we modulated AC activity by dithiothreitol (DTT), Fe(2+), ascorbic acid and suramin. We did not detect any substantial difference between the effects of all these tested agents on AC activity in cerebrocortical membranes from young and adult rats, and the enzyme activity was always about two-fold higher in the former preparations. Nevertheless, several interesting findings have come out of these investigations. Whereas forskolin- and Mn(2+)-stimulated AC activity was significantly enhanced by the addition of DTT, increased concentrations of Fe(2+) ions or ascorbic acid substantially suppressed the enzyme activity. Lipid peroxidation induced by suitable combinations of DTT/Fe(2+) or by ascorbic acid did not influence AC activity. We have also observed that PKC- or protein tyrosine kinase-mediated phosphorylation apparently does not play any significant role in different activity of AC determined in cerebrocortical preparations from young and adult rats. Our experiments analysing the presumed modulatory role of suramin revealed that this pharmacologically important drug may act as a direct inhibitor of AC. The enzyme activity was diminished to the same extent by suramin in membranes from both tested age groups. Our present data show that AC is regulated similarly in brain cortex from both young and adult rats, but its overall activity is much lower in adulthood.

Effects of tricyclohexylhydroxytin on the kinetics of adenosine triphosphatase system and protection by thiol reagents

Tricyclohexylhydroxytin, commonly known as Plictran, inhibited Na+, K+-ATPase activity of rat brain synaptosomes in a concentration-dependent manner with median inhibitory concentration (IC-50) of 2 microM. Both K+-stimulated para-nitrophenylphosphatase and [3-H]-ouabain binding to synaptosomes were also inhibited by Plictran with IC-50 values of 11 and 30 microM, respectively. Altered pH and Na+, K+-ATPase activity curves demonstrated comparable inhibition in buffered neutral and alkaline pH ranges, and no inhibition was observed in acidic pH. The inhibition of Na+, K+-ATPase was independent of temperature. Kinetic studies of substrate (ATP) activation of Na+, K+-ATPase indicated uncompetitive inhibition. Results also showed noncompetitive inhibition for p-nitrophenylphosphate and uncompetitive inhibition for K+ activations of p-nitrophenylphosphatase. Preincubation of synaptosomes with dithiothreitol, a sulfhydryl (SH) agent, resulted in the complete protection of Plictran inhibition of Na+, K+-ATPase, K+-para-nitrophenylphosphatase, and [3-H]-ouabain binding. The protection was specific and concentration dependent since cysteine and glutathione did not afford protection. These results indicate that Plictran inhibited Na+, K+-ATPase by interacting with dephosphorylation of the enzyme-phosphoryl complex and exerted a similar effect to that of SH-blocking agents.

Solubilization of adenylate cyclase of brain membranes by lipid peroxidation

Adenylate cyclase in the membrane fractions of bovine and rat brains, but not in rat liver plasma membranes, was solubilized by treatment with Fe2+ (10 microM) plus dithiothreitol (5 mM). Solubilization of the enzyme by these agents was completely prevented by simultaneous addition of N,N'-diphenyl-p-phenylenediamine (DPPD), an inhibitor of lipid peroxidation. Ascorbic acid also solubilized the enzyme from the brain membranes. Lipid peroxidation of the brain membranes was characterized by a selective loss of phosphatidylethanolamine. Solubilization of membrane-bound enzymes by Fe2+ plus dithiothreitol was not specific for adenylate cyclase, because phosphodiesterase, thiaminediphosphatase and many other proteins were also solubilized. Solubilized adenylate cyclase had a high specific activity and was not activated by either NaF, 5'-guanylyl imidodiphosphate (Gpp[NH]p) or calmodulin. These results suggested that lipid peroxidation of the brain membranes significantly solubilized adenylate cyclase of high specific activity.