Studies in bile salt solutions

Abstract

The pseudo‐first‐order rate constants of hydrolysis of p ‐nitrophenylacetate, catalyzed by human milk lipase, have been measured in solutions of 0.01 mol dm −3 Bistris(2‐[bis(2‐hydroxyethyl)amino]‐2‐(hydroxymethyl)‐propane‐1,3‐diol) buffer at 310.5 K, containing a range of concentrations of sodium taurocholate and sodium cholate, at pH 8.00 and of sodium cholate at pH 6.5. The effect of pH on the activity of the enzyme has been investigated and the stimulation factors of taurocholate and cholate ions and of cholic acid have been calculated to be equal to 5.3, 3.7 and 10.7, respectively. The essential residues for catalytic activity of the enzyme have ionization constants equal to 6.45–6.46 for p K 1 and 8.33–8.40 for p K 2 . The former value is attributed to the presence of a histidine imidazolium group but the identity of the residue leading to p K 2 is not proven.