Mesotrypsin is an unusual human
trypsin isoform with inhibitor resistance and the ability to degrade
trypsin inhibitors. Degradation of the protective
serine protease inhibitor Kazal type 1 (
SPINK1) by mesotrypsin in the pancreas may contribute to the pathogenesis of
pancreatitis. Here we tested the hypothesis that the regulatory digestive
protease chymotrypsin C (CTRC) mitigates the harmful effects of mesotrypsin by cleaving the
autolysis loop. As human trypsins are post-translationally sulfated in the
autolysis loop, we also assessed the effect of this modification. We found that mesotrypsin cleaved in the
autolysis loop by CTRC exhibited catalytic impairment on short
peptides due to a 10-fold increase in Km , it digested β-
casein poorly and bound soybean
trypsin inhibitor with 10-fold decreased affinity. Importantly, CTRC-cleaved mesotrypsin degraded
SPINK1 with markedly reduced efficiency. Sulfation increased mesotrypsin activity but accelerated CTRC-mediated cleavage of the
autolysis loop and did not protect against the detrimental effect of CTRC cleavage. The observations indicate that CTRC-mediated cleavage of the
autolysis loop in mesotrypsin decreases
protease activity and thereby protects the pancreas against unwanted
SPINK1 degradation. The findings expand the role of CTRC as a key defense mechanism against
pancreatitis through regulation of intrapancreatic
trypsin activity.