Molecular and kinetic alterations of muscle AMP deaminase during chronic creatine depletion

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Abstract
We examined a possible mechanism to account for the maintenance of peak AMP deamination rate in fast-twitch muscle of rats fed the creatine analog β-guanidinopropionic acid (β-GPA), in spite of reduced abundance of the enzyme AMP deaminase (AMPD). AMPD enzymatic capacity (determined at saturating AMP concentration) and AMPD protein abundance (Western blot) were coordinately reduced ∼80% in fast-twitch white gastrocnemius muscle by β-GPA feeding over 7 wk. Kinetic analysis of AMPD in the soluble cell fraction demonstrated a single Michaelis-Menten constant ( Km; ∼1.5 mM) in control muscle extracts. An additional high-affinity Km(∼0.03 mM) was revealed at low AMP concentrations in extracts of β-GPA-treated muscle. The kinetic alteration in AMPD reflects increased molecular activity at low AMP concentrations; this could account for high rates of deamination in β-GPA-treated muscle in situ, despite the loss of AMPD enzyme protein. The elimination of this kinetic effect by treatment of β-GPA-treated muscle extracts with acid phosphatase in vitro suggests that phosphorylation is involved in the kinetic control of skeletal muscle AMPD in vivo.