Target Molecular Size of the Red Beet Plasma Membrane ATPase

Radiation inactivation of the red beet (Beta vulgaris L.) plasma membrane ATPase was carried out using gamma-ray radiation from a (137)Cs source. Inactivation of vanadate-sensitive ATPase activity by gamma-ray radiation followed an exponential decline with increasing total dose, indicating a single target size calculated to have a molecular weight of about 228,000. Since the catalytic subunit of the red beet plasma membrane ATPase has been demonstrated to have a molecular weight of about 100,000 by dodecyl-sulfate gel electrophoresis following (32)P-phosphorylation, it is suggested that the native enzyme may exist, at least, as a dimer of catalytic subunits.