Bovine pancreatic trypsin inhibitor and related isoinhibitors in bovine liver. A biochemical and histochemical study

Characterisation of Ag1, the major species-specific contaminant of bovine crude heparin, and its identification as an aprotinin/heparin complex

Heparin is a potent anticoagulant polysaccharide purified for decades from ruminants or porcine tissues. However, with the emergence of bovine spongiform encephalopathy (BSE), the source of pharmaceutical heparin is currently restricted to porcine intestinal mucosa. A major species-specific contaminant, called Ag1, has recently been identified in bovine crude heparin [Rivera et al., J. Pharm. Biomed. Anal., submitted] and used to develop an enzyme-linked immunosorbent assay (ELISA) for the species origin control of crude heparins [Levieux et al., J. Immunoassay, submitted]. In this report, we describe the different investigations, which were carried out to identify Ag1. This antigen was first localised by immunohistological studies essentially in the connective tissue of the bovine small intestine. After extraction from an intestinal extract by immuno-affinity chromatography, Ag1 was isolated as a single band by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Ag1 was then partly sequenced and identified as an aprotinin/heparin complex. Aprotinin, also known as the bovine pancreatic trypsin inhibitor (BPTI), is present with heparin in mast cells, and is very resistant to heat, pH, chemical treatments and proteolytic digestion. The stability of Ag1 towards the different treatments performed during heparin extraction process allows this protein to remain in sufficient amounts in crude heparin and makes it an ideal target for the immunochemical control of the absence of bovine material in crude heparins.

Differential in vitro translation of the precursors of bovine pancreatic trypsin inhibitor and its isoinhibitor II is controlled by the 5'-end region of their mRNAs

Bovine spleen inhibitor (SI II), a 58-amino-acid protein present in several bovine tissues, is an isoinhibitor of bovine pancreatic trypsin inhibitor (BPTI or aprotinin). These two proteins, which differ in seven amino-acidic residues, have very similar inhibitory activity against serine proteinases and are biosynthesized as two separate precursors of 100 residues. Higher levels of BPTI, compared to SI II, are found in bovine lung, as well as in other bovine tissues, in contrast to the level in vivo of the corresponding mRNAs. SI mRNA possesses a 90-nt 5'-end region, absent in BPTI mRNA, with an additional 5' AUG in a different open reading frame (ORF). We have used an in vitro transcription/translation system to determine the effect of this upstream region on the efficiency of SI precursor translation. Full-length SI mRNA is translated in vitro 6-fold less efficiently than BPTI mRNA. However, when SI mRNA lacks the 5' non-coding region, the translational efficiency of the 'truncated' transcript is significantly increased, reaching the same level as that of BPTI mRNA. In all cases the 10,500 Da precursor is the product of the in vitro translation. Our results indicate that the dramatic differences in translational efficiency of the mRNAs encoding BPTI and SI II in vitro parallel the different levels of the two proteins in vivo, and could be attributed to the features of the 5' non-coding region of SI mRNA.

Bovine pancreatic trypsin inhibitor and homologous polypeptide inhibitors in nephron cells

Bovine pancreatic trypsin inhibitor (BPTI, aprotinin) is a fifty-eight amino acid polypeptide, which is present together with related molecular isoforms in various bovine organs. In the present study these protease inhibitors were isolated from bovine kidney by affinity chromatography on immobilized trypsin and a subsequent FPLC step. Due to their electrophoretic, structural, and inhibitory properties, the inhibitors were strictly similar to the polypeptides identified previously in other bovine organs. Immunohistochemical experiments showed a widespread localization of these polypeptides in nephron epithelial cells (proximal and distal tubules, loop of Henle, collecting tubules).

Proteinase isoinhibitors from bovine spleen: primary structure of an intermediate in the processing of the precursor

The complete amino acid sequence of the proteinase inhibitor III from bovine spleen is reported. It consists of 62 amino acid residues and is identical to that of spleen inhibitor II (an isoinhibitor of the bovine pancreatic trypsin inhibitor, which shares with the latter 89% of sequence identity), except for four extra residues at the C-terminal side. Inhibitor III appears to be an intermediate in the processing of the putative 100-residue primary expression product, which leads to the mature inhibitor II. These results and those previously obtained for another intermediate, isoinhibitor I, are indicative of the following order for the last steps of the precursor processing inhibitor I----inhibitor III----inhibitor II. The mature protein and the two intermediates isolated have a very similar antiproteolytic activity. However, their in vivo target enzyme(s) are not yet known, as also the target enzyme of the bovine pancreatic trypsin inhibitor is not known. Thus, the available data would indicate that either the three isoinhibitors have a distinct functional role, by inhibiting different target enzymes, or inhibitors I and III are obligatory intermediates for directing the final targeting of the mature, functionally relevant inhibitor II.