Effect of Sodium Cholate on the Catalytic and Structural Properties of Phosphorylase b

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Abstract
Sodium cholate at millimolar concentration is able to induce activity in rabbit muscle phosphorylase b in the absence of AMP. The maximum activation of the enzyme in presence of 7 mM sodium cholate was 24% of that achieved by 1 mM AMP. Other bile salts tested showed a negligible activating effect. The Ka for AMP was lowered fivefold by 5 mM of the steroid detergent, while the cooperative binding of the nucleotide was abolished Phosphorylase b′, a modified form of phosphorylase in which the phosphorylation site has been removed by limited tryptic attack, presented an activation profile similar to that of phosphorylase b. In contrast, phosphorylase a was inhibited by the bile salt, while the activity of liver phosphorylase b was not significantly affected. Modification of the AMP site of the enzyme with 2,3‐butanedione could not inhibit sodium‐cholate‐induced activity. tert‐Butanol, an organic solvent activator of phosphorylase b, was found to enhance the activity induced by sodium cholate. The interaction of sodium cholate and phosphorylase b was also followed by difference spectroscopy using a fluorescein isothiocyanate–phosphorylase b conjugate. Furthermore, measurements of electron spin resonance demonstrated that the mobility of a spin‐label bound at buried ‐ NH2 groups of phosphorylase b decreases cooperatively with increasing bile salt concentration.