Guanyl Nucleotide Regulation of the Liver Glucagon-Sensitive Adenylyl Cyclase System
Properties of the GMP-P(NH)P-activated glucagon sensitive adenylyl cyclase from rat liver plasma membranes were studied with respect to reversal of nucleotide imidodiphosphate analog activation. It was found that incubation of the analog-activated state with excess GTP (the natural effector) in the presence of substrate (ATP), divalent cation (Mg++), a chelator (EDTA), the reaction product (cAMP), and an ATP-regeneration system, resulted in reversal of activity to that seen with GTP alone. The reversed state was demonstrated to be both sensitive to re-stimulation by GMP-P(NH)P and to stimulation by glucagon, and was therefore not the result of GTP-induced inactivation of the enzyme. Examination of the time course of GTP-induced reversal of GMP-P(NH)P activation, as well as of GMP-P(NH)P-induced re-stimulation of reversed activity revealed that guanyl nucleotide effects occur with progressively increasing lag times, suggesting the existence or participation of one or more co-factors. These findings are inconsistent with the mechanism of GMP-P(NH)P activation involving the irreversible formation of an enzyme-P(NH)P derivative and suggest a more complex mechanism than the involvement of a simple two step isomerization reaction.
KeywordsCatecholamine Adenine Creatine Nucleoside Coupler
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