Association of phosphoenolpyruvate phosphatase activity with the cytosolic pyruvate kinase of germinating mung beans.

The procedure of Malhotra and Kayastha ([1990] Plant Physiology 93: 194-200) for the purification to homogeneity of a phosphoenolpyruvate-specific alkaline phosphatase (PEP phosphatase) from germinating mung beans (Vigna radiata) was followed. Although a higher specific activity of 1.4 micromoles pyruvate produced per minute per milligram protein was obtained, the final preparation was less than 10% pure as judged by polyacrylamide gel electrophoresis. Attempts to further purify the enzyme resulted in loss of activity. The partially purified enzyme contained significant pyruvate kinase activity (0.13 micromole pyruvate produced per minute per milligram protein) when assayed at pH 7.2, but not at pH 8.5. The PEP phosphatase activity of the final preparation exhibited hysteresis; a lag time of 5 to 6 minutes was required before a steady-state reaction rate was attained. A western blot of the final preparation revealed an immunoreactive 57 kilodalton polypeptide when probed with monospecific rabbit polyclonal antibodies prepared against germinating castor bean cytosolic pyruvate kinase. No antigenic cross-reaction of the final preparation was observed with antibodies against castor bean leucoplast pyruvate kinase, or black mustard PEP-specific acid phosphatase. Nondenaturing polyacrylamide gel electrophoresis of the final preparation resulted in a single PEP phosphatase activity band; when this band was excised and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and western blotting, a 57 kilodalton silver-staining polypeptide was obtained that strongly cross-reacted with the anti-(cytosolic pyruvate kinase) immunoglobulin G. It is suggested that mung bean PEP-specific alkaline phosphatase activity is due to cytosolic pyruvate kinase, in which pyruvate and ortho-phosphate are formed in the absence of ADP.