POLYPEPTIDE-N-ACETYLGALACTOSAMINYLTRANSFERASE ACTIVITIES TOWARDS THE MUCIN MUC5AC PEPTIDE MOTIF USING MICROSOMAL PREPARATIONS OF NORMAL ANDTUMORAL DIGESTIVE MUCOSA

The selected-acceptor substrate peptide (TTSAPTTS), deduced from the h uman mucin gene MUC5AC (expressed essentially in the human gastric and tracheobronchial mucosa), was used to assay polypeptide:N-acetylgalac tosaminyltransferases (GalNAc transferases) of different microsomal pr eparations, obtained from gastric and colonic mucosa in normal and tum oral situations. The O-glycosylated products, analyzed by capillary el ectrophoresis and electrospray mass spectrometry, showed a variable nu mber of GalNAc O-linked to the different hydroxy amino acids of TTSAPT TS, depending on the tissue studied. Our observations were consistent with the existence of more than one form of GalNAc transferases which were expressed differentially in the gastrointestinal tract (stomach a nd/or colon). The levels of enzyme activities showed a tissue-specific pattern as they were high in normal colonic tissue and low in colon c ancer. On the other hand, in the tumoral gastric tissue (displaying in testinal metaplasia) a high level of GalNAc transferase activities was obtained, similar to that found in the normal colon. Moreover, slight discrepancies (activities and number of O-linked GalNAc) were only de tected between normal gastric and tumoral colonic preparations. Thus, the data indicated that the dedifferentiation of the gastric cancer ti ssue may induce GalNAc transferase activities similar to those in the normal colonic, tissue and that colonic and gastric tissues map contai n families of glycosyltransferases involved specifically in reaction t owards particular peptide or protein substrates. In addition, the anal ysis by capillary electrophoresis and electrospray mass spectrometry r evealed, in tumoral gastric as well as in normal colonic tissues, a hi gh dipeptidylaminotransferase activity inducing an elongation of TTSAP TTS by dithreonine. This activity was low in normal gastric and tumora l colonic tissues. ((C) Societe francaise de biochimie et biologie mol eculaire/Elsevier, Paris).