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Membrane of Mammary Gland. II. 5'-Nucleotidase Activity of Bovine Mammary Plasma Membranes

Department of Animal Sciences, Purdue University, Lafayette, Indiana 47907

ABSTRACT

The 5'-nucleotidase activity of a plasma-membrane rich fraction from bovine mammary gland was characterized. In the absence of added metal ions a broad plateau of maximum activity was observed between pH 7.8 and 9.8. In the presence of 5 mM Mg⁺⁺, the enzyme was activated and displayed pH optima at 7.8 and 9.5. Michaelis constants were 4.0 and 1.1 mar for cytidine-5'-monophosphate and adenosines-5'monophosphate (AMP). With AMP as substrate the activation energy was 10,300 cal/mole. At 5 mM relative rates of hydrolysis of 5'-monophosphates were cytidine > uridine > inosine > guanosine > adenosine > deoxyguanosine > xanthine > deoxycytidine. The membrane fraction also contained large amounts of adenosine triphosphatase as well as some nonspecific phosphatase activity.