PANCREATIC STONE PROTEIN (LITHOSTATHINE), A PHYSIOLOGICALLY RELEVANT PANCREATIC CALCIUM-CARBONATE CRYSTAL INHIBITOR

Apart from digestive enzymes, pancreatic juice contains several protei ns that are not directly involved in digestion. One of these, lithosta thine, has been reported to exhibit calcite crystal inhibitor activity in vitro. As pancreatic juice is supersaturated with respect to calci um carbonate, it was hypothesized that lithostathine stabilizes pancre atic juice. Lithostathine is cleaved by trace amounts of trypsin, resu lting in a C-terminal polypeptide and an N-terminal undecapeptide, whi ch has been identified as the active site of lithostathine regarding c rystal inhibition. We produced rat lithostathine in a baculovirus expr ession system. In order to test its functional activity, the protein w as purified using a nondenaturing multistep procedure. In the low micr omolar range, recombinant rat lithostathine in vitro exhibited calcite crystal inhibitor activity, confirming earlier reports. Limited trypt ic proteolysis of recombinant lithostathine was performed, and the two cleavage products were separated; the C-terminal polypeptide was prec ipitated by centrifugation, and the N-terminal undecapeptide was purif ied by high performance liquid chromatography. Only the C-terminal pep tide displayed measurable calcite crystal inhibitory activity. Further more, synthetic undecapeptides with identical sequence to the N-termin al undecapeptides of rat or human lithostathine were inactive. However , when tested in the same in vitro assays, other pancreatic or extra-p ancreatic proteins show inhibitory activity in the same concentration range as lithostathine, and inorganic phosphate is active as well. Bas ed on these findings it seems unlikely that lithostathine is a physiol ogically relevant calcite crystal inhibitor. The name 'lithostathine'' is therefore inappropriate, and the protein's key function remains to be elucidated.