Characteristics of nucleotide-converting enzymes at muscle surfaces with special reference to ion sensitivity

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Abstract
Spectral analyses in the ultraviolet range, paper chromatography, and determinations of the release of inorganic phosphate were used to extend the investigation of the surface enzymes (frog muscle) which catalyze the conversion of ATP→ADP→AMP→IMP. ATPase and 5′-AMP deaminase appear to adhere firmly to the muscle cell surface, even after three successive incubation periods of 4 h each, whereas the adenylate kinase is more loosely bound. The observed loss of the activity of the complete enzyme system with time was shown to be due to a suboptimal substrate concentration and not to the accumulation of products of the reaction. The AMP deaminase activity was inhibited if sodium and potassium chloride were replaced by choline chloride in the incubation media. Cation analyses of media and the determination in muscle of Na, K, water, and the sorbitol-14C space permitted a calculation of the actual intra- and extra-cellular cation concentrations. In choline-containing media the intracellular potassium was in part replaced by choline. The data indicated that the deaminase was influenced by the external, not the internal, cation concentrations. Magnesium ions stimulated the inorganic phosphate release catalyzed by ATPase and adenylate kinase but inhibited the AMP deaminase. Intact frog muscle also converted GTP, CTP, UTP, and ITP in the medium to the corresponding nucleoside monophosphate.