Effect of Temperature and pH on the Esterase Activity of Bile-Salt-Stimulated Human Milk Lipase

The initial rate constants of hydrolysis of 4-nitrophenyl acetate, catalyzed by bile-salt-stimulated human milk lipase, have been measured over the tempera ture range 20 ° C to 37 ° C, in 0.1 M buffer solutions in the pH range 6-10, and in the presence of 2 mM sodium taurocholate. From these data, the values of the energy of activation have been calculated and found to be independent of pH. Analysis of the data has also led to determination of the values of the disso ciation constants, pK ₁ (8.20) and pK ₂ (8.70) (at 25°C), and heats of formation, Δ H° (40.2 and 59.4 kJ mol ^-1 , respectively), of the essential ionizable residues of the enzyme.