SUBCELLULAR-LOCALIZATION OF PEPTIDASE ACTIVITY IN THE HUMAN JEJUNUM

Abstract. The subcellular localization of peptidase activity in the normal human jejunum has been investigated. Subcellular organelles were fractionated by density gradient centrifugation. The localization of peptidases was determined by comparing the distributions of peptidase activities with those of organelle ‘marker’ enzymes. The organelles and their markers were: cytosol—lactate dehydrogenase; brush border—neutral α‐glucosidase, γ‐glutamyl transferase and leucyl‐2‐naphthylamidase; plasma membrane—5′‐nucleotidase; lysosomes—N‐acetyl‐β‐glucosaminidase; mitochrondria—malate dehydrogenase; endoplasmic reticulum—alkaline α‐glucosidase; peroxisomes—catalase. Thirteen dipeptides, seven tripeptides, two tetrapeptides, two pentapeptides and a hexapeptide were used as substrates. The distribution of dipeptidyl peptidase IV was also determined. Irrespective of whether the NH2‐terminal or COOH‐terminal amino acid was neutral, basic or acidic, the major or exclusive locus of dipeptidase activity was cytosolic. All of the activity against a dipeptide with the amino acid proline at the NH2‐terminus was in the cytosol. The distribution of tripeptidase activity was quite different. Although the cytosol hydrolysed all tripeptides, as much as 50% of tripeptidase activity was particulate. For both tetrapeptides, one of the pentapeptides and the hexapeptide, the major or exclusive locus of activity was the brush border membrane. Pentaphenylalanine, however, was hydrolysed by both the cytosol and the brush border. Dipeptidyl peptidase IV was localized in the brush border. Copyright © 1979, Wiley Blackwell. All rights reserved