Trypsin activation of porcine procolipase. Kinetics of activation and effects on lipid binding

The effect of pancreatic procolipase and colipase on pancreatic lipase activation

Intestinal fat digestion is carried out by the concerted action of pancreatic lipase and its protein cofactor colipase. Colipase is secreted from pancreas as a procolipase and is transformed into colipase by the trypsin cleavage of the Arg5-Gly6 bond during liberation of an N-terminal pentapeptide. The kinetic parameters for the lipase-colipase system compared to the lipase-procolipase system has been compared using trioctanoin and Intralipid as substrates. It was found that at pH 7.0 the Kmapp using Intralipid as substrate was the same for procolipase and colipase, 0.06 mM and 0.05 mM, respectively. At pH 8.0, however, the Kmapp were different-0.23 mM for procolipase and 0.08 mM for colipase. In a similar way the binding between colipase and lipase had a dissociation constant of 2.4 x 10(-6) M at pH 7.0, while for procolipase--lipase binding the dissociation constant was 4.1 x 10(-6) M with no significant difference. At pH 8.0 the binding between colipase and lipase was stronger, Kd being 2.0 x 10(-7) M, while weaker for procolipase and lipase, Kd being 1.0 x 10(-5) M. It is concluded that at the physiological pH value as is found in the intestine, the activation of procolipase to colipase has no influence on the hydrolysis of trioctanoin or Intralipid in the presence of bile salt.