REGULATION OF SKELETAL-MUSCLE AMP-DEAMINASE - INVOLVEMENT OF HISTIDINE-RESIDUES IN THE PH-DEPENDENT INHIBITION OF THE RABBIT ENZYME BY ATP

Reaction of rabbit skeletal-muscle AMP deaminase with a low molar exce ss of diethyl pyrocarbonate results in conversion of the enzyme into a species with one or two carbethoxylated histidine residues per subuni t that retains sensitivity to ATP at pH 7.1 but, unlike the native enz yme, it is not sensitive to regulation by ATP at pH 6.5. This effect m imics that exerted on the enzyme by limited proteolysis with trypsin, which removes the 95-residue N-terminal region from the 80 kDa enzyme subunit. These observations suggest involvement of some histidine resi dues localized in the region HHEMQAHILH (residues 51-60) in the regula tory mechanism which stabilizes the binding of ATP to its inhibitory s ite at acidic pH. Carbethoxylation of two histidine residues per subun it abolishes the inhibition by ATP of the proteolysed enzyme at pH 7.1 , suggesting the obligatory participation of a second class of histidi ne residues, localized in the 70 kDa subunit core, in the mechanism of the pH-dependent inhibition of the enzyme by ATP. At a slightly acidi c pH, these histidine residues would be positively charged, resulting in a desensitized form of the enzyme similar to that obtained with the carbethoxylation reaction.