Preactivation as a determinant for the size of thyroid adenylate cyclase

The molecular weight of NaF-activated, Triton N-101-solubilized adenylate cyclase from bovine thyroid membranes has been measured by a combination of sucrose density centrifugation and gel exclusion chromatography. The physical parameters are: sedimentation coefficient, 6.6 S; Stokes radium 41 A; partial specific volume, 0.75 ml/g; molecular weight, 119,000. This is in contrast to the molecular weight (159,000) of the enzyme from the same source activated with guanosine 5'-(beta, gamma-imido)triphosphate. Both soluble adenylate cyclase enzymes are subject to cholera toxin-mediated ADP-ribosylation. This implies that the diminished molecular weight of the NaF-activated solubilized adenylate cyclase is a consequence of one of the following: loss of a GTP-binding protein and labeling of some other protein in the cyclase complex; loss of another protein not subject to cholera toxin labeling; or that two GTP-binding proteins normally reside within the catalytic complex and upon NaF activation one is lost.