Adenylate Cyclase in Skeletal Muscle

The regulation of adenylate cyclase in skeletal muscle may be classified as a “V” allosteric system since metal ions, F-, and epinephrine all result in increased maximal velocity of the enzyme reaction.

Abstract

Abstract Adenylate cyclase was studied in plasma membranes prepared from rabbit skeletal muscle. Such preparations represented an increase in specific activity of 10- to 20-fold over the whole homogenate with a yield of activity of approximately 30%. Various parameters, such as phase contrast microscopy, marker enzyme activities, and chemical composition, suggested that the preparation constituted sarcolemma in a high degree of purity. The Km for substrate (MgATP) was 0.3 to 0.5 mm. The Ka for Mg2+ was 3 to 5 mm. Binding of Mg2+ to a second site resulted in increased reactivity of the catalytic site for substrate. Stimulation by fluoride resulted from an increase in maximal velocity; the Ka for Mg2+ and the Km for substrate were not appreciably altered. The effect of F- was markedly temperature-sensitive and partially irreversible. Fluoride-stimulated activity was particularly sensitive to inhibition by pyrophosphate and this inhibition was competitive with respect to ATP (Ki, 0.45 mm). Catecholamines stimulated the enzyme in a typical β adrenergic fashion. The prominent kinetic effect of epinephrine was (like F-) to increase reaction velocity without affecting affinity for Mg2+ or ATP. The regulation of adenylate cyclase in skeletal muscle may be classified as a V allosteric system since metal ions, F-, and epinephrine all result in increased maximal velocity of the enzyme reaction.