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Adenosine Triphosphate-Adenosine 5'-Monophosphate Phosphotransferase of Bovine Liver Mitochondria

II. GENERAL KINETIC AND STRUCTURAL PROPERTIES

From the ¹ From the Department of Physiological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

Some of the kinetic and structural properties of purified adenylate kinase derived from bovine liver mitochondria have been investigated and compared with the corresponding properties of adenylate kinase from rabbit muscle. The results of studies of the isotope exchange reaction between each of the substrates at equilibrium are consistent with the two-site mechanism proposed for the skeletal muscle enzyme. Other studies show that the liver enzyme is similar to the muscle enzyme with respect to maximum catalytic rate, K_m values of the several substrates, product inhibition, cation activation, and pH optimum, and therefore provide evidence that both enzymes utilize a similar reaction mechanism.

Studies of the structural properties of the liver enzyme have confirmed earlier suggestions that it does not contain an essential sulfhydryl group but does contain at least one and probably two disulfide bridges which are essential for enzymatic activity. The presence of essential disulfide groups was inferred from the loss of activity when the enzyme was treated with reagents such as sodium sulfite or ß-mercaptoethanol. The inactivation observed with ß-mercaptoethanol could be reversed by exposure to air, which presumably results in reoxidation of the reduced disulfide bridge, or bridges.

The results described in this and the preceding communication indicate that different, possibly tissue-specific, forms of adenylate kinase do exist in mammalian tissues and, therefore, that this enzyme constitutes an addition to a growing list of enzymes that exist in multiple molecular forms.

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