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 degrees C. The first phase is rapid (t1/2 = 2.3 +/- 0.3 min) and results in the loss of 60-70% of basal activity. The second phase is slow (t1/2 approximately 100 min) during which time the catalytic units denature irreversibly. The GTP analogue guanosine-5' (beta-gamma imino) triphosphate (pNHppG) 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 pNHppG is rapid at 37 degrees C, but the binding of pNHppG 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 degrees C all of the catalytic units can be stabilized by saturating pNHppG concentrations. At 0 degree C, 35% of the catalytic units can be stabilized by saturating pNHppG concentrations within 30 s. The half-saturation constant for the binding of pNHppG at 0 degree C is identical to that derived in an assay at 37 degrees C, or after an incubation of the membranes for 10 min at 45 degrees C, when the process of thermal denaturation is 80% complete (K1/2 approximately 3 +/- 2 microM).(ABSTRACT TRUNCATED AT 400 WORDS)