AMP and IMP dephosphorylation by soluble high- and low-Km 5'-nucleotidases

Modelling muscle energy-metabolism in anaerobic muscle

A mathematical model of anaerobic muscle energy-metabolism was developed to predict pH and the concentrations of nine muscle metabolites over time. Phosphorous-31 Nuclear Magnetic Resonance was used to measure time-course data for some phosphate metabolites and pH in anoxic M. semitendinosus taken from three slaughtered sheep. Muscles were held at 35 degrees C during the experiment. Measurement commenced 25 min post mortem and concluded before rigor mortis. The model was fitted to these data within experimental error, by simultaneously varying model parameter values and initial substrate concentrations. The model was used to simulate the period from death until metabolic activity ceased, in order to predict the different stages of metabolic response to anoxia. The model suggested that alkalinisation would occur in all three muscles in the first few minutes after the onset of anoxia, followed by a steady decline in pH. For two of the muscles this decline continued until rigor, with final pH values of 5.60 and 6.07. For the other muscle, pH reached a low of 5.60 near rigor but then increased to a final value of 5.73. A rise in pH after rigor has been observed but not previously explained in the literature. The modelling results suggest it was caused by the alkalising effect of adenosine monophosphate deamination being greater at low pH than the acidifying effect of inosine monophosphate dephosphorylation.

5'-Nucleotidase: molecular structure and functional aspects

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The soluble 'low-Km' 5'-nucleotidase of rat kidney represents solubilized ecto-5'-nucleotidase

A soluble 'low-Km' 5'-nucleotidase has been described previously in several organs. It has been presumed to be of cytosolic origin and thus to play a role in the intracellular production of adenosine. Its catalytic properties are similar to those of the ecto-5'-nucleotidase of cell membranes. In the present study we compared molecular properties of the two enzymes in the kidney of the rat. The Mr of the main peak of soluble 'low-Km' 5'-nucleotidase in gel-filtration chromatography was similar to that of the ecto-5'-nucleotidase solubilized by a phosphatidylinositol-specific phospholipase C from renal brush-border membranes. In phase-partition experiments using Triton X-114, the soluble enzyme appeared to be hydrophobic. Its hydrophobicity was decreased on treatment with a phosphatidylinositol-specific phospholipase C, suggesting that the soluble 'low-Km' 5'-nucleotidase contains the phosphatidylinositol anchor which is characteristic for the ecto-enzyme. An anti-ecto-5'-nucleotidase antiserum provoked an almost complete inhibition of the soluble enzyme. Immunoblotting using anti-ecto-5'-nucleotidase antiserum revealed in the high-speed supernatants a polypeptide with a similar Mr to the subunit of the ecto-5'-nucleotidase. The soluble 'low-Km' 5'-nucleotidase, like the ecto-5'-nucleotidase, bound specifically to concanavalin A. We conclude that the soluble 'low-Km' 5'-nucleotidase is not a cytosolic enzyme, but that it most probably originates from the solubilization of the ecto-5'-nucleotidase, and that it therefore cannot participate in the intracellular production of adenosine.