The bifunctional rat pancreatic secretory trypsin inhibitor/monitor peptide provides protection against premature activation of pancreatic juice.

BACKGROUND: In the rat, two forms of the pancreatic secretory trypsin inhibitor, PSTI-I and PSTI-II, are secreted into pancreatic juice. It is assumed that their role is to protect the pancreas from premature activation of the protease-rich pancreatic juice. In the small intestine, PSTI-I, also called 'monitor peptide', is thought to have a different role: PSTI-I competes with protein for activated trypsin. In the presence of a protein-rich meal, free PSTI induces a release of cholecystokinine from the intestine. METHODS: To investigate whether its role as monitor peptide is compatible with the inhibitory, protective function in the pancreas, PSTI-I was chemically synthesized and then renatured. RESULTS: The peptide was almost completely trypsin resistant and exhibited a dose-dependent inhibitory activity to bovine and partially purified rat trypsin. Furthermore, experiments with trypsin- and endopeptidase-activated pancreatic juice demonstrated that its inhibitory capacity was sufficient to prevent premature activation. Binding studies of (125)I-labeled PSTI-I with the putative intestinal receptor using isolated membranes indicated the presence of high-affinity binding sites (k(d) = 5 x 10(-8)M). Binding of PSTI-I could be competed with excess PSTI-I or trypsin. In a biological assay system, injections of PSTI-I displayed monitor peptide activity by inducing a dose-dependent trypsinogen release from the pancreas. CONCLUSION: Our experiments support a dual function of PSTI-I: monitoring protein in the gut due to its 'moderate' affinity for trypsin and a protective role in the pancreas.

Characterization and functional expression of the cDNA encoding human brain quinolinate phosphoribosyltransferase.

Mammalian quinolinate phosphoribosyltransferase (QPRTase) (EC 2.4.2.19) is a key enzyme in catabolism of quinolinate, an intermediate in the tryptophan-nicotinamide adenine dinucleotide (NAD) pathway. Quinolinate acts as a most potent endogenous exitotoxin to neurons. Elevation of quinolinate levels in the brain has been linked to the pathogenesis of neurodegenerative disorders. As the first step to elucidate molecular basis underlying the quinolinate metabolism, the cDNA encoding human brain QPRTase was cloned and characterized. Utilizing partial amino acid sequences obtained from highly purified porcine kidney QPRTase, a human isolog was obtained from a human brain cDNA library. The cDNA encodes a open reading frame of 297 amino acids, and shares 30 to 40% identity with those of bacterial QPRTases. To confirm that the cDNA clone encodes human QPRTase, its functional expression was studied in a bacterial host. Introduction of the human cDNA into a QPRTase defective (nadC) E. coli strain brought about an abrupt increase in QPRTase activity and allowed the cells to grow in the absence of nicotinic acid. It is concluded that the cloned cDNA encodes human QPRTase which is functional beyond the phylogenic boundary.

Identification and expression of the cDNA-encoding human mesotrypsin(ogen), an isoform of trypsin with inhibitor resistance.

The cDNA encoding a novel isoform of human trypsinogen was identified. The isoelectric points of the proenzyme and active forms calculated from the deduced amino acid sequence are consistent with those of mesotrypsin(ogen), known to be an inhibitor-resistant trypsin isoform. The cDNA attached with a bacterial signal peptide sequence was expressed in Escherichia coli. The recombinant proenzyme purified from periplasm showed enterokinase-dependent activation similar to a major isoform of human trypsinogen. The enzyme was far less inhibited by trypsin inhibitors such as soybean trypsin inhibitor, aprotinin, or pancreatic secretory trypsin inhibitor than the control trypsin. A gel filtration assay showed that the enzyme and aprotinin did not form a stable complex. It is noteworthy that the amino acid at position 198, which is in close vicinity to the active Ser, is Arg while those of other major trypsins are all Gly. It is concluded that the cloned cDNA encodes human mesotrypsinogen, a unique isoform of trypsinogen with inhibitor resistance.

Pancreatic dysfunction in cystic fibrosis occurs as a result of impairments in luminal pH, apical trafficking of zymogen granule membranes, and solubilization of secretory enzymes.

Recent progress in understanding the luminal biochemistry of regulated pancreatic exocrine secretion, including acid-base interactions between acinar and duct cells and pH-dependent processes that regulate membrane trafficking (endocytosis) at the apical plasma membrane, have led to the development of in vitro models of cystic fibrosis in the rat exocrine pancreas. Based on investigations in these model systems, a unifying hypothesis is presented that proposes that pancreatic dysfunction in cystic fibrosis occurs as a result of progressive acidification of the acinar and duct lumen, which leads to secondary defects in (i) apical trafficking of zymogen granule membranes and (ii) solubilization of secretory (pro)enzymes. By directly acidifying the pH of the acinar lumen in cholescystokinin-stimulated acini, the early cytological findings observed in cystic fibrosis, including (i) massive dilatation of the acinar lumen, (ii) decreased appearance of zymogen granules, (iii) loss of the apical pole of the acinar cell, and (iv) persistent aggregation of secretory (pro)enzymes released into the luminal space, have been reproduced in primary cultures of pancreatic tissue.