Transient States of Adenylate Cyclase in Brain Membranes

Basal activity of adenylate cyclase from the amygdala of sheep brain and the neostriatum of turkey brain decays in two phases at 37°C. The first phase is rapid (t 1/2 = 2.3 ± 0.3 min) and results in the loss of 60–70% of basal activity. The second phase is slow (t 1/2 ± 100 min) during which time the catalytic units denature irreversibly. The GTP analogue guanosine‐5’(β‐γ‐imino) triphosphate (p[NH]ppG) prevents the rapid decay by stabilizing the enzyme at its initial level of activity and also reactivates the enzyme to initial levels during or immediately following the early phase, indicating that denaturation of neither the guanylnucleotide units nor the catalytic units causes the rapid decline in basal activity. Activation by p[NH]ppG is rapid at 37°C, but the binding of p[NH]ppG to the guanylnucleotide subunit also occurs at nonactivatory temperatures. This is determined by the protection of catalytic units from thermal or N ‐ethylmaleimide inactivation after extensive washing. Thus, at 25°C all of the catalytic units can be stabilized by saturating p[NH]ppG concentrations. At 0°C, 35% of the catalytic units can be stabilized by saturating p[NH]ppG concentrations within 30 s. The half‐saturation constant for the binding of p[NH]ppG at 0°C is identical to that derived in an assay at 37°C, or after an incubation of the membranes for 10 min at 45°C, when the process of thermal denaturation is 80% complete ( K 1/2 ∼ 3 ± 2 μ M ). Taken together these results suggest that the catalytic units of adenylate cyclase in brain membranes are coupled to a freely accessible, preactive state of guanylnucleotide units when the membranes are prepared, leading to an initial state of high activity.