Hormonal induction of alkaline phosphatase activity by an increase in catalytic efficiency of the enzyme

Alkaline phosphatase activity of HeLa 65 cells is increased 5- to 20-fold during growth in medium containing cortisol. The kinetics of increase in activity are characterized by a 12- to 24-hour lag period after adding hormone, followed by a linear and rapid increase of activity which reaches a plateau after 60 to 100 hours. Both RNA and protein synthesis are required for "induction". However, immunological methods show that the amount of enzyme protein in induced cells is not increased; suggesting that there is neither an increased rate of alkaline phosphatase synthesis nor a hormone-mediated decrease in the catabolism of the enzyme. When the kinetics of induction are compared to the kinetics of enzyme synthesis using radioactively labeled enzyme and specific precipitation by antiserum, the alkaline phosphatase is maximally labeled long before the increase of enzyme activity reaches a maximum. This finding suggests that the hormone acts by initiating the synthesis of a modifier molecule which interacts with the enzyme to produce an enhanced catalytic efficiency. The physical and chemical characteristics of the base-level and induced enzyme are similar except the V_max of the induced alkaline phosphatase is much greater. The increased catalytic efficiency of induced alkaline phosphatase is not the result of an increase in the zinc ion content of this metalloenzyme. However, the binding of ionic zinc to the induced form of apoenzyme appears to be different from that of the base-level enzyme. It is suggested that the enhanced catalytic activity of induced alkaline phosphatase is the result of an alteration in zinc ion binding which produces an entatic effect, lowering the energy requirements of the enzyme substrate transition state.