ATP sulfurylase from higher plants : purification and preliminary kinetics studies on the cabbage leaf enzyme

ATP sulfurylase was purified extensively from green cabbage (Brassica capitata L.) leaf. The enzyme appears to be an asymmetric dimer composed of 57,000 dalton subunits. Initial velocity and product inhibition studies of the forward and reverse reactions point to an obligately ordered kinetic mechanism with MgATP adding before MoO(4) (2-) (or SO(4) (2-)). and MgPPi leaving before AMP + MoO(4) (2-) (or adenosine-5'-phosphosulfate [APS]). The addition of excess purified fungal APS kinase to assay mixtures increased the rate of (35)SO(4) (2-) incorporation and MgPPi formation and extended the linearity of the forward reaction. This effect can be ascribed to the continual removal of APS, a potent product inhibitor of ATP sulfurylase. The specific activities of the enzyme in the APS synthesis, molybdolysis, MgATP synthesis, and sulfate-dependent [(32)P]-MgPPi-MgATP exchange assays were 3.3, 38, 38, and 4.3 micromole product formed per minute per milligram protein, respectively.