Isolation and partial characterization of prochymosin and chymosin from cat

Activities of gastric, pancreatic, and intestinal brush-border membrane enzymes during postnatal development of dogs

OBJECTIVE

To measure activities of digestive enzymes during postnatal development in dogs.

SAMPLE POPULATION

Gastrointestinal tract tissues obtained from 110 Beagles ranging from neonatal to adult dogs.

PROCEDURE

Pepsin and lipase activities were measured in gastric contents, and amylase, lipase, trypsin, and chymotrypsin activities were measured in small intestinal contents and pancreatic tissue. Activities of lactase, sucrase, 4 peptidases, and enteropeptidase were assayed in samples of mucosa obtained from 3 regions of the small intestine.

RESULTS

Gastric pH was low at all ages. Pepsin was not detected until day 21, and activity increased between day 63 and adulthood. Activities of amylase and lipase in contents of the small intestine and pancreatic tissue were lower during suckling than after weaning. Activities of trypsin and chymotrypsin did not vary among ages for luminal contents, whereas activities associated with pancreatic tissue decreased between birth and adulthood for trypsin but increased for chymotrypsin. Lactase and gamma-glutamyltranspeptidase activities were highest at birth, whereas the activities of sucrase and the 4 peptidases increased after birth. Enteropeptidase was detected only in the proximal region of the small intestine at all ages.

CONCLUSIONS AND CLINICAL RELEVANCE

Secretions in the gastrointestinal tract proximal to the duodenum, enzymes in milk, and other digestive mechanisms compensate for low luminal activities of pancreatic enzymes during the perinatal period. Postnatal changes in digestive secretions influence nutrient availability, concentrations of signaling molecules, and activity of antimicrobial compounds that inhibit pathogens. Matching sources of nutrients to digestive abilities will improve the health of dogs during development.

Homology induction: the use of machine learning to improve sequence similarity searches

BACKGROUND

The inference of homology between proteins is a key problem in molecular biology The current best approaches only identify approximately 50% of homologies (with a false positive rate set at 1/1000).

RESULTS

We present Homology Induction (HI), a new approach to inferring homology. HI uses machine learning to bootstrap from standard sequence similarity search methods. First a standard method is run, then HI learns rules which are true for sequences of high similarity to the target (assumed homologues) and not true for general sequences, these rules are then used to discriminate sequences in the twilight zone. To learn the rules HI describes the sequences in a novel way based on a bioinformatic knowledge base, and the machine learning method of inductive logic programming. To evaluate HI we used the PDB40D benchmark which lists sequences of known homology but low sequence similarity. We compared the HI methodology with PSI-BLAST alone and found HI performed significantly better. In addition, Receiver Operating Characteristic (ROC) curve analysis showed that these improvements were robust for all reasonable error costs. The predictive homology rules learnt by HI by can be interpreted biologically to provide insight into conserved features of homologous protein families.

CONCLUSIONS

HI is a new technique for the detection of remote protein homology--a central bioinformatic problem. HI with PSI-BLAST is shown to outperform PSI-BLAST for all error costs. It is expect that similar improvements would be obtained using HI with any sequence similarity method.

An aspartic proteinase expressed in the yolk sac and neonatal stomach of the mouse

A murine aspartic proteinase, described herein, is intermediate in amino acid sequence identity between the placentally produced pregnancy-associated glycoproteins (PAGs) and gastric pepsins. While PAGs are secreted products of placental trophoblast tissue of ungulates and most are not believed to function proteolytically, pepsins are digestive enzymes. The cDNA for this aspartic proteinase was amplified by reverse transcription-polymerase chain reaction from RNA extracted from murine placentas and neonatal stomachs. The open reading frame encoded a 387-amino acid polypeptide with a 15-residue signal sequence. The enzyme most resembled pepsinogen F (a protein identified in the stomachs of neonatal rabbits and rats) and PAG-like proteins cloned from equine and feline placentae. In the stomach, both its mRNA and protein were expressed in gastric chief cells of preweaned neonates. Within the placenta, its mRNA was present in both the parietal and visceral yolk sacs. However, the protein was most prevalent in the visceral yolk sac, with little detectable in the parietal yolk sac. The recombinant protein was expressed in Escherichia coli. This protein was capable of self-activation and exhibited proteolytic activity toward casein. The presence of this enzyme in two organs involved in the selective transcellular transport of proteins suggests that it has specialized digestive functions.

Development-dependent expression of isozymogens of monkey pepsinogens and structural differences between them

The developmental changes in the expression of monkey pepsinogens and structural differences between the polypeptides were investigated. Monkey pepsinogens included five different components, namely, pepsinogens A-(1-4) and progastricsin. Their respective relative levels and specific activities changed significantly during development. The sequential expression of genes for type-A pepsinogens was particularly noteworthy. Pepsinogen A-3 was the major zymogen at the newborn stage, accounting for nearly half of the total pepsinogens at this stage. Pepsinogen A-2 became predominant at the 4-month stage, and pepsinogen A-1 predominated at the juvenile and adult stages. Enzymatic properties of pepsinogens A-1, A-2 and A-3 were similar but not identical to those of pepsinogen A-4 and progastricsin, in particular with respect to the activation processes. Each pepsin digested various protein substrates but some differences in specificity were evident. cDNA clones for five pepsinogens were isolated, and the nucleotide sequences were determined. Each cDNA contained leader, pro, and pepsin regions that encoded 15, 47, and 326 amino acid residues, respectively, with the exception of the cDNA for progastricsin in which the pro and pepsin regions encoded 43 and 329 amino acid residues, respectively. Type-A pepsinogens exhibited a high degree of similarity, with over 96% of bases in the nucleotide sequences of the protein-coding regions being identical. Northern analysis revealed that the level of expression of genes for type-A pepsinogens and for progastricsin was significant at the fetal stage and increased with development.

Assignment of human prochymosin pseudogene to chromosome 1

Chymosin is an extremely specific aspartatic protease responsible for milk coagulation. Chymosin is expressed in a number of mammalian offspring, yet its presence in the gastric tissue of human infants remains a matter of controversy. In any event, the human genome contains chymosin-related sequences that probably represent a pseudogene. Using DNA obtained from human x hamster somatic cell hybrids as the template and polymerase chain reaction, we have mapped the human prochymosin pseudogene to chromosome 1.

Structure of the human genomic region homologous to the bovine prochymosin-encoding gene

Two human genomic libraries were probed with bovine prochymosin (bPC) cDNA. Recombinant clones covering a genomic region homologous to the entire coding region and flanking sequences of the bPC gene were isolated. Human sequences homologous to exons of the bPC gene are distributed in a DNA fragment of 10 kb. Alignment of the human sequences and the exons of bPC reveals that the human 'exons' 1-3, 5 and 7-9 have sizes identical to the corresponding bovine exons, but a nucleotide (nt) has been deleted in the human exon 4 and two nt in the human exon 6. The aligned human sequence and the coding part of bPC gene share 82% nt homology, the value ranging, in separate exons, from 76 (exon 1) to 84% (exons 5 and 6). 150 bp of 5'-flanking sequence of the human gene has 75% homology to the corresponding region of bPC gene and contains a TATA-box in a similar position. A 1-nt deletion in the human exon 4 would shift the translational reading frame of a putative human PC mRNA relative to bPC mRNA, and result in an in-phase terminator spanning codons 163 and 164 in bPC mRNA. Another terminator in-phase with the amino-acid sequence encoded by the bPC gene occurs in the human exon 5 and the second frameshift mutation in exon 6. Thus, the nt sequence analysis of the human genomic region has revealed the presence of mutations that have rendered it unable to produce a full-length protein homologous to bPC and, therefore, we refer to this gene as a human prochymosin pseudogene (hPC psi). Blot-hybridization analysis of human genomic DNA indicates that hPC psi is a single gene in the human genome.

Comparison between prochymosin and pepsinogen from lamb and calf

1. Prochymosin (EC 3.4.23.4) and pepsinogen A (EC 3.4.23.1) from Mongolian lamb (Ovis platyurea) were purified to homogeneity by salt precipitation, gel filtration and ion-exchange chromatography. 2. Immunoelectrophoresis shows partial immunochemical identity between chymosins and pepsins from lamb and cattle, respectively. 2. Activity determinations, N-terminal amino acid sequences and amino acid compositions also show a close relationship between the proteinases from lamb and cattle. 4. Lamb prochymosin and pepsinogen are both glycosylated.

Close linkage of human chromosomal pepsinogen A genes

We have obtained a clone containing two pepsinogen A genes in a single insert by screening a recombinant cosmid library for human genomic DNA. Restriction endonuclease mappings of this cloned DNA showed that these two genes are very similar, but distinct in structure, and that they are closely linked to one another in the human chromosome DNA. The close arrangement of the genes with very similar structures could facilitate the homologous recombination or the unequal crossing-over which accounts for high frequency of haplotype variation in copy number of pepsinogen A genes as reported by Taggart et al.

Effects of age and weaning on enzyme activities of abomasum and pancreas of the lamb

The effect of age and weaning on enzyme activities of abomasum and pancreas of the lamb was studied. Sixty-nine lambs were either maintained at the preruminant stage until 42 days of age or weaned between 20 and 30 days. They were slaughtered between birth and 100 days, and the mucosa of their abomasum and their pancreas were collected. At 2 days of age there was a threshold of glandular growth and of development of enzyme secretory potentialities. Pancreas showed an extensive hyperplasia without growth until day 2; its weight increased after day 7 but only by hypertrophy. Quantities of gastric enzymes in relation to empty live weight increased between birth and 2 days, but that of chymosin then decreased, whereas pepsin did not change significantly; effects of weaning were similar. The evolution of pancreatic enzyme activity was usually the reverse of that of chymosin; however, trypsin activity was low at birth and that of colipase did not increase at weaning. The ratio of colipase to lipase was always higher than 1, showing that lipase always was saturated in the lamb. The pancreas seemed to take over the secretion of abomasal proteolytic enzymes. Potentialities of enzyme digestion appeared to be influenced mainly by the age or weight of the animal; these potentialities were minimal for at least several days during the first neonatal week and, if there was enzymatic adaptation to the amounts of the ingested substrate, it did not occur rapidly.