Errata

Optimal conditions for activation of adenylate cyclase in membrane particles were studied. Enzyme activation with serotonin (5-hydroxytryptamine), NaF, and guanosine $\text{5′-(3-O-}\text{thio}\text{)-}\text{triphosphate}$ (GTPγS) was time- and temperature-dependent. Mg²⁺ was required for enzyme activation. Adenylate cyclase that was activated by NaF or GTPγS was gradually inhibited by $\text{N-}\text{methylmaleimide}$ while enzyme activated with serotonin and GTP responded faster to inhibition by the same sulfhydryl reagent. The enzyme responded in a similar fashion to a spin-labeled $\text{N-}\text{methylmaleimide}$ analog 3-(maleimidomethyl)-2,2,5,5-tetramethyl-1-pyrolidinyloxyl (i.e., $\text{N-}\text{methylmaleimide}$ nitroxide). Binding of the spin label was enhanced following enzyme activation by serotonin, NaF, or GTPγS in the presence of Mg²⁺. Activation of the enzyme was accompanied by an increase in the strong immobilization peaks in the EPR spectra. Both effects, the increase in binding and in the strong immobilization peaks, can be induced by Mg²⁺ alone. The results indicate that a general conformational change induced by Mg²⁺ may be essential for adenylate cyclase activation.