Inhibition of bovine trypsin by the phosphorylated N-terminal part beta(1-105) of beta-casein.

Abstract

The sensitivities of the R25-I26 bond on bovine beta-casein and on its N-terminal fragment beta(1-105) to trypsin digestion were compared by monitoring the liberation of the beta(1-25) product. It was shown that this peptide bond was poorly and slowly hydrolysed on beta(1-105), while it is highly susceptible to trypsin attack when whole protein is used as substrate. The marked resistance of beta(1-105) is linked to its inhibitory effect on trypsin activity (apparent K'i = 1.2 x 10(-6) M), as demonstrated by using a related chromogenic substrate. Indeed, a preincubation step of trypsin with beta(1-105) leads to a more pronounced inhibitory effect. The progress curves obtained with and without preincubation show that beta(1-105) acts as a slow binding inhibitor on trypsin activity. These findings promise further insight into the action and the regulation of proteolytic enzymes.