MOLECULAR AND KINETIC ALTERATIONS OF MUSCLE AMP-DEAMINASE DURING CHRONIC CREATINE DEPLETION

We examined a possible mechanism to account for the maintenance of pea k AMP deamination rate in fast-twitch muscle of rats fed the creatine analog beta-guanidinopropionic acid (beta-GPA), in spite of reduced ab undance of the enzyme AMP deaminase (AMPD). AMPD enzymatic capacity (d etermined at saturating AMP concentration) and AMPD protein abundance (Western blot) were coordinately reduced similar to 80% in fast-twitch white gastrocnemius muscle by beta-GPA feeding over 7 wk. Kinetic ana lysis of AMPD in the soluble cell fraction demonstrated a single Micha elis-Menten constant (K-m; similar to 1.5 mM) in control muscle extrac ts. An additional high-affinity K-m (similar to 0.03 mM) was revealed at low AMP concentrations in extracts of beta-GPA-treated muscle. The kinetic alteration in AMPD reflects increased molecular activity at lo w AMP concentrations; this could account for high rates of deamination in beta-GPA-treated muscle in situ, despite the loss of AMPD enzyme p rotein. The elimination of this kinetic effect by treatment of beta-GP A-treated muscle extracts with acid phosphatase in vitro suggests that phosphorylation is involved in the kinetic control of skeletal muscle AMPD in vivo.