Rat peptidylglycine alpha-amidating enzyme: the relation between activities at neutral and alkaline pH Values

A substantially high level of alpha-amidating activity at an alkaline pH (8-9.5), often seen as another pH optimum peak in addition to the neutral one, has been observed in various rat tissues. We have also found that crude enzymes from rat brain, pituitary, and small intestine showed a pH profile with two pH optima at neutral pH (6.5-7) and alkaline pH (8.5-9) when D-Tyr-Val-Gly was used as substrate. With a combination of ion-exchange and gel filtration chromatographies, we obtained two fractions, S-1 and S-2, from rat brain; S-1 contained an alpha-amidating enzyme of an apparent molecular weight of 36,000 (36K enzyme) exhibiting a single pH optimum at 8.5. On the other hand, S-2 apparently showed almost no or only marginal activity at either pH 7 or 8.5, but when S-2 was combined with S-1, a neutral pH optimum at 7 could be elicited. The factor in S-2 that was responsible for this combined action was a protein of an apparent molecular weight of 41,000 (41K protein). Both proteins were found to be colocalized in the same subcellular organelle, probably in the secretory granule. It seems likely, then, that the pH profiles characterized by two optimal peaks seen in crude rat enzymes can be attributed to the presence at an appropriate ratio of the 41K protein and 36K enzyme.