The activities of crude adenosine and AMP aminohydrolases isolated from skeletal muscle (apparent Km: 68 and 72 μM, respectively), and crude cardiac adenosine aminohydrolase (Km: 48) are spectrophotometrically determined in the presence of some chemical factors. The activity of cardiac AMP aminohydrolase is practically negligible. We apply a simple assay system, which consists of 1.5 ml of 0.05M Tris (hydroxymethyl)-aminomethane-HCl buffer (pH 7.38), 1 ml of substrate solution (10-500 μM of adenosine or AMP, final concentration), and solution of each chemical factor, the concentration of which is assumed to be those in the skeletal muscle during ischemia or contractions. Among such factors, ammonia (0.5-5mM, final concentration), lactate (50-200mM) and phosphoric acid (PO
4---: 25-50 mEq/liter) depress in this order, and potassium chloride (K
+: 150-250 mEq/liter) activates these enzymes. Bubbling of carbon dioxide and nitrogen gases depresses the activities markedly and slightly, respectively. It is unclear, however, whether carbon dioxide itself has inhibitory action, because there is marked decrease in pH of the medium when its effect was obvious. Bubbling oxygen gas induces an activation of the enzymes. The enzyme activities show no detectable changes in near physiological range of pH (7.0-7.8). Changes in the factors, which occur in the skeletal muscle during ischemia or contractions, induce depression of the enzyme activities except pH. The disputable reports on the deamination of AMP are reviewed and discussed.
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