Pancreatic Secretory Trypsin Inhibitor (PSTI) Has Internalization Basic Triplet
Pancreatitis is an inflammation of the pancreas resulting from lack of adequate inhibition of proteases. Autoantibodies against PSTI would explain some forms of pancreatitis.
I was researching the maintenance of baby gut flora by mother’s
milk, when the reference discussed here was brought to my attention by my wife, who happens to be a lactation consultant. The paper showed that PSTI is present in colostrom, the first milk that a baby gets, before the true milk comes in. PSTI protects the new gut from digestion by its own pancreatic proteases, since PSTI is a protease inhibitor that sticks to the gut.
I naturally assumed that PSTI stuck to the gut by heparin-binding domains that would stick to the heparan sulfate proteoglycans on the gut surface. [Recall that it is via these HSPGs that viruses and bacteria infect the gut and the HSPGs in turn are protected during infections by the release of heparin from mast cells. The heparin in the guts of cattle and pigs are used to make commercial heparin to block blood clotting.] So I looked up the structure (above, with basic amino acids in blue and basic triplet on right) sequence of human PSTI at NCBI:
>gi|190694|gb|AAA36522.1| PSTI
MKVTGIFLLSALALLSLSGNTGADSLGREAKCYNELNGCTKIYD
PVCGTDGNTYPNECVLCFEN
RKRQTSILIQKSGPC
The basic triplet (RKR,arg-lys-arg), from my perspective, should result in presentation to the immune system during high levels of inflammation, and as a consequence result in autoantibodies against PSTI. The result would be the neutralization of the protease inhibitor and damaging production of active protease to attack the pancreas, i.e. pancreatitis.
It would be fairly easy to test this hypothesis by looking for the anti-PSTI antibodies in some people with pancreatitis. Other autoantibodies, e.g. against tissue transglutaminase, might also be checked, because the inflammation that produced one autoantibody may produce others and both PSTI and tTG are produced in the intestines. In fact, celiac may be the cause of some cases of autoimmune pancreatitis.
Note added in proof:
I just checked the literature on PubMed and found that PSTI is in fact an autoantigen in pancreatitis and produces antibodies against PSTI:
Raina A, Greer JB, Whitcomb DC. Serology in autoimmune pancreatitis. Minerva Gastroenterol Dietol. 2008 Dec;54(4):375-87.
and
I found that pancreatitis is often found associated with celiac (gluten intolerance):
Patel RS, Johlin FC Jr, Murray JA. Celiac disease and recurrent pancreatitis. Gastrointest Endosc. 1999 Dec;50(6):823-7.
ref:
Marchbank T, Weaver G, Nilsen-Hamilton M, Playford RJ. Pancreatic secretory trypsin inhibitor is a major motogenic and protective factor in human breast milk. Am J Physiol Gastrointest Liver Physiol. 2009 Apr;296(4):G697-703.