Regulation of skeletal muscle AMP deaminase: lysine residues are critical for the pH-dependent positive homotropic cooperativity behaviour of the rabbit enzyme

Reaction of rabbit skeletal muscle AMP deaminase with a low molar excess of trinitrobenzene sulfonic acid (TNBS) results in conversion of the enzyme into a species with about six trinitrophenylated lysine residues per molecule which no longer manifests positive homotropic cooperativity at pH 7.1 or at the optimal pH value of 6.5 in the presence of low K+ concentrations. Substitution of the reactive thiol groups with 5,5'-dithiobis-(2-nitrobenzoic acid) does not protect the enzyme from the TNBS-induced changes of the catalytic properties, indicating that cysteine residues modification is not at the basis of the effects of TNBS treatment on AMP deaminase and strongly suggesting the obligatory participation of lysine residues to the constitution of a regulatory anionic site to which AMP must bind to stimulate the enzyme at alkaline pH. The TNBS-treated enzyme is also completely desensitized to inhibition by ATP, but not to inhibition by GTP and stimulation by ADP. This observation suggests a connection between the operation of the hypothesized anionic activating site, responsible for positive homotropic cooperativity, and the inhibition exerted by anionic compounds that compete for the same site, among them the most efficient metabolite being probably ATP.