1. Because of the possibility that the proteinases of gastric adenocarcinomata may differ from those of healthy gastric mucosa, an investigation of these enzymes by means of pH-activity studies, agar-gel electrophoresis, column chromatography and the mode of action on the B-chain of oxidized insulin has been undertaken.
2. Agar-gel electrophoresis of extracts at pH 5 revealed a single zone of proteinase activity situated at the equivalent position to the zone 7 of normal gastric mucosal extracts and not activated at pH 2. The typical pepsins of normal mucosal extracts were not present.
3. Agar-gel electrophoresis at pH 8·2 resolved this single zone into three zones. One was located slightly cathodally (proteinase 1) and the other two anodally (a slower proteinase 2A and a faster 2B).
4. Proteinases 2A and 2B were separated by column chromatography and shown to have identical asymmetrical broad pH-activity curves with the maximum at pH 3·3–3·4.
5. Proteinase 1 had a symmetrical narrow pH-activity curve with the maximum at pH 3·7–3·9. Proteinase 1 was purified and resolved into two highly active major components, 1A and 1B, by column chromatography, first on DEAE-cellulose and then on CM-cellulose.
6. The sites of cleavage of the B-chain of oxidized insulin were determined for proteinases 1A and 1B. The same bonds were split by each, with one exception, but several were split at differing rates indicating that the two enzymes had related, but different, modes of action.
7. The tumour proteinases 1 resemble the cathepsins D in certain respects and the pepsins in others. They may represent an enzyme structure of an intermediate form. There is insufficient evidence to indicate whether they are elaborated by partially differentiated cells or whether they are derived from cells which have become dedifferentiated. No enzymes exactly like them have yet been found in normal tissues.