Chemical and enzymic studies on pepsinogen D

Abstract

This study has confirmed the existence of pepsinogen D in extracts of pig gastric mucosae. It is present to the extent of about 5% of the potential proteolytic activity in extracts of the body regions of pooled gastric mucosae of a number of pigs and from the gastric mucosa of only one pig.The conditions required for the chromatographic isolation of pepsinogen D have been defined more precisely, and as a result it can be isolated with a purity which is probably greater than 95%.Pepsinogen D is present to the extent of about 15% in extracts of the pyloric regions of pig stomachs but the total yield of zymogen is so small as to make this an unfavourable source of the zymogen.Studies on pepsinogen D have shown it, like pepsinogen, to have a molecular weight of about 41,000 and to have one amino -terminal leucine and one carboxyl - terminal alanine residue per mole. It does not contain phosphorus in phosphodiester linkage and analysis of the peptides obtained after digestion of the performic acid oxidised protein with a- chymotrypsin indicate that it is very similar to or identical with dephosphopepsinogen.Pepsinogen D also behaves in a similar manner to dephosphopepsinogen on chromatography on DEAE- cellulose at pH 6.9 and on electrophoresis in starch gel at pH 3.2. Like pepsinogen, pepsinogen D is activated at pH 3.7 in accordance with autocatalytic kinetics, although at pH 4.6 it is activated more slowly than is the main zymogen,On exposure to alkali, the potential ability of pepsinogen D to digest haemoglobin declines in a way which is similar to that of pepsinogen although its ability to digest N- acetyl -L- phenylalanyl -L -di- iodotyrosine is much more stable.Pepsin D, the enzyme produced by activation of pepsinogen D, has been found to occur in commercial preparations of both crude and crystalline pepsin to the extent of about 10j0, and a method has been developed for its isolation by chromatography on DEAE- cellulose at pH 3.2.Like pepsin, pepsin D has a molecular weight of about 35,000 and, while it is also similar in having the Leu. carboxyl- terminal sequence.... Val. I -leu. AlaCOOH, it differs in having an amino -terminal leucine residue and no, phosphorus which is bound in phosphodiester linkage.Pepsin D is similar to pepsin in its ability to digest haemoglobin, N- acetyl -L- phenylalanyl -L -diiodotyrosine and gelatin but its specific activity in the clotting of milk is twice that of pepsin. Its specificity in digestion of the B -chain of oxidised insulin cannot, however, be distinguished from that of pepsin.Activation of pepsinogen D produces a peptide which inhibits pepsin and pepsin D to the same extent as the peptide inhibitor produced by the activation of pepsinogen.It was concluded that pepsinogen D was very similar to or identical with dephosphopepsinogen and that it gives rise on activation to an inhibitory peptide and an enzyme the latter of which is similar to although not identical with dephosphopepsin.