Purification and characterization from guniea-pig intestinal mucosa of two peptide hydrolases which preferentially hydrolyse dipeptides

Intestinal mucosal hydrolysis of proteins and peptides

The major products of intraluminal hydrolysis of dietary proteins appear to be small peptides and amino acids. Recent studies indicate that the distal part of the small intestine may play an important role in the digestion and absorption of dietary proteins. Intestinal mucosal cellular hydrolysis of peptides and proteins represents the terminal stage of digestion of dietary proteins and appears to be carried out predominantly by amino-oligopeptidases in brush border membranes and cytoplasm. These enzymes in the two main subcellular loci are distinct since they exhibit different electrophoretic mobilities, physicochemical properties, substrate specificities and responses to starvation and dietary manipulation. Two amino-oligopeptidases have been purified from the intestinal brush border of the rat. The enzymes are remarkably similar to each other in many respects. They have an apparent molecular weight of 280 000 and are composed of two subunits of equal molecular weight. Both enzymes are glycoproteins having similar chemical compositions, common antigenic properties, substrate specificities and kinetic properties.

Two broad-specificity dipeptide hydrolysing activities from cytoplasm of guinea pig brain, both of which contain prolinase but neither of which contain carnosinase activity

Two dipeptide hydrolysing activities were purified from cytoplasm of guinea pig brain. Dipeptide hydrolase I has been shown to be a strict dipeptidase requiring a free amino and a free carboxy terminus, while dipeptide hydrolase II displays very low activity against Leu-Leu-Leu. Of the 41 dipeptides presented to both enzymes, 25 were hydrolysed by both enzymes, while six (including carnosine) were hydrolysed by neither. Six were hydrolysed solely by dipeptide hydrolase I and four were hydrolysed solely by Pro-Leu hydrolase II. Kinetic analysis suggested that dipeptides which were hydrolysed with unfavourable kinetics or which were not hydrolysed by one dipeptide hydrolase were generally hydrolysed with more favourable kinetics by the other dipeptide hydrolase. Dipeptide hydrolase I displays optimum activity at pH 9.0, while dipeptide hydrolase II was optimally active at pH 8.0. Both enzymes were inhibited by 1,10-phenanthroline, p-chloromercuribenzoate and bestatin. Dipeptide hydrolase II was more strongly inhibited by arphamenine B than was dipeptide hydrolase I. Dipeptide hydrolase II was also inhibited by N-ethyl maleimide, while dipeptide hydrolase I was inhibited by dithiothreitol. Native M(r) values of 70,000 and 67,000 were computed for dipeptide hydrolase I and dipeptide hydrolase II, respectively. Sodium dodecyl sulphate polyacrylamide gel electrophoresis conducted with each enzyme under denaturing conditions suggested that both enzymes were comprised of a single polypeptide chain.

Prolinase and non-specific dipeptidase of human kidney

Human kidney prolinase, assayed with Pro-Ala, and non-specific dipeptidase, assayed with Gly-Leu, were purified by using DEAE-cellulose, gel-filtration, metal-ion-chelate, hydrophobic and adsorption chromatography and chromatofocusing. Both enzymes gave single peaks of activity that were congruent and the ratio of their activities was constant throughout the purification. Gel filtration indicated an Mr of 100 000 and chromatofocusing a pI of 5.4. Ni2+-chelate chromatography demonstrated the presence of exposed histidine residues on the enzyme and was an effective separative procedure. Polyacrylamide-gel electrophoresis of the final preparation showed the two enzyme activities to be coincident. Both enzyme activities decayed at the same rate at 53 degrees C and were inhibited to the same extent by p-hydroxymercuribenzoate. Of six non-specific dipeptidase substrates tested Gly-Leu gave the highest activity, and of six prolinase substrates Pro-Leu had the highest activity. Gly-Leu was hydrolysed at double the rate of Pro-Leu. Pro-Ala was a competitive inhibitor of activity towards Gly-Leu, and Gly-Leu was a competitive inhibitor of activity towards Pro-Ala. Mixed-substrate studies strongly suggested that Gly-Leu and Pro-Ala were hydrolysed at a common active site. The data are consistent with prolinase and non-specific dipeptidase activity in human kidney being due to a single enzyme.

Purification and characterization of two aminopeptidases from guinea-pig small-intestinal mucosa. Cavian intestinal tripeptide hydrolases

Two electrophoretically distinct cytosolic peptide hydrolases from guinea-pig small-intestinal mucosa have been highly purified by a six-step procedure comprising extraction from mucosal homogenate, ammonium sulphate fractionation, DEAE-cellulose chromatography, chromatofocusing, calcium phosphate chromatography and Sephadex G-100 gel filtration. They have similar apparent molecular masses as determined by gel filtration (Mr = 68 000) or by sodium dodecyl sulphate gel electrophoresis (Mr = 72 000). Both are aminopeptidases with optimum activity at pH 7.6. They are strongly inhibited by p-hydroxymercuribenzoate, o-phenanthroline and bestatin. Although both hydrolyse some dipeptides they have a distinctive kinetic preference for tripeptides composed of aromatic or non-polar residues. Their affinities for some tripeptides are particularly high and also the hydrolysis of some substrates exhibits biphasic kinetics. These two aminotripeptidases are similar but they can be differentiated from each other and from a number of other aminopeptidases.

Peptide hydrolases of the human small intestinal mucosa: distribution of activities between brush border membranes and cytosol

Brush border membranes from frozen human small intestine have been purified using a method which did not involve the use of EDTA-containing buffers or the disruption of brush border fragments with high concentrations of Tris. On average a 24-fold increase in specific activity of alpha-glucosidase (brush border marker) was obtained in the final preparation which contained insignificant traces of enzyme marker activities from cytosol and lysosomes. The homogenates of human small intestinal mucosa were shown to contain enzymes capable of hydrolysing di-, tri-, and tetrapeptides as well as amino acid- and peptide-2-nephthylamides. Assuming a 100% location of alpha-glucosidase in the brush border membrane, distribution studies indicated that activities against tetrapeptides and leucyl-2-naphthylamide were located exclusively in the brush border membrane. A large proportion of activity against alpha-glutamyl-2-naphthylamide, gamma-glutamyl-2-naphthylamide and glycyl-prolyl-2-naphthylamide were also recovered in the brush border membrane fraction. Depending on the substrate utilized, 33-87% of tripeptidase activity was located in the brush border membrane. However, 58-87% of dipeptidase activity was recovered in the soluble fraction.

Similarities between a dipeptide hydrolase from brush-border and cytosol fractions of guinea-pig intestinal mucosa

A dipeptide hydrolase from the brush border of guinea-pig intestinal mucosa was purified. The enzyme resembles another dipeptide hydrolase isolated from the cytosol fraction of intestinal mucosa. Studies on the binding of cytosol peptide hydrolase to brush-border membranes indicate that the enzyme found in the brush border may be a cytoplasmic contaminant.