Rush J W, Tullson P C, Terjung R L
Department of Physiology, State University of New York Health Science Center, Syracuse 13210, USA.
Am J Physiol. 1998 Feb;274(2):C465-71. doi: 10.1152/ajpcell.1998.274.2.C465.
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 beta-guanidinopropionic acid (beta-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 approximately 80% in fast-twitch white gastrocnemius muscle by beta-GPA feeding over 7 wk. Kinetic analysis of AMPD in the soluble cell fraction demonstrated a single Michaelis-Menten constant (Km; approximately 1.5 mM) in control muscle extracts. An additional high-affinity Km (approximately 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 low AMP concentrations; this could account for high rates of deamination in beta-GPA-treated muscle in situ, despite the loss of AMPD enzyme protein. The elimination of this kinetic effect by treatment of beta-GPA-treated muscle extracts with acid phosphatase in vitro suggests that phosphorylation is involved in the kinetic control of skeletal muscle AMPD in vivo.
我们研究了一种可能的机制，以解释尽管大鼠快肌中AMP脱氨酶（AMPD）的丰度降低，但喂食肌酸类似物β-胍基丙酸（β-GPA）的大鼠快肌中AMP脱氨峰值速率仍能维持的原因。通过7周的β-GPA喂养，大鼠快肌白色腓肠肌中的AMPD酶活性（在AMP饱和浓度下测定）和AMPD蛋白丰度（蛋白质免疫印迹法）协同降低了约80%。对可溶性细胞组分中的AMPD进行动力学分析表明，对照肌肉提取物中的米氏常数（Km）为单一值（约1.5 mM）。在β-GPA处理的肌肉提取物中，低AMP浓度下还显示出一个额外的高亲和力Km（约0.03 mM）。AMPD的动力学改变反映了低AMP浓度下分子活性的增加；这可以解释在β-GPA处理的肌肉原位中，尽管AMPD酶蛋白减少，但脱氨速率仍然很高的现象。体外使用酸性磷酸酶处理β-GPA处理的肌肉提取物可消除这种动力学效应，这表明磷酸化参与了体内骨骼肌AMPD的动力学控制。
