Stabilisation of cathepsin E by ATP

The hydrolysis of 3 distinct substrates by cathepsin E from human red blood cells and gastric mucosa was measured in the presence and absence of physiologically relevant concentrations of ATP. At pH values below about 5.0, the nucleotide was without effect. However, at pH 5.8, whereas cathepsin E was virtually inactive by itself, it was restored to full activity (k cat) by ATP and the non‐hydrolysable methylene‐ATP analogue. At still higher pH values, k cat progressively diminished but significant levels of cathepsin E activity were readily detectable at pH 7.0. The specificity of this stabilisation effect was examined.