Dysregulation of
cathepsin B, which involves the translocation
of the enzyme from acidic pH lysosomes to the neutral pH cytosol,
followed by the initiation of cell death and inflammation, occurs
in numerous brain disorders. The wide difference in the acidic pH
(4.6) of lysosomes compared to the neutral pH (7.2) of the cytosol
suggests that screening at different pH conditions may identify pH-selective
modulators of cathepsin B. Therefore, a collection of pure marine
and plant natural product (NP) compounds, with synthetic compounds,
was screened at pH 4.6 and pH 7.2 in cathepsin B assays, which led
to the identification of GER-12 (Crossbyanol B) and GER-24 ((7
Z
,9
Z
,12
Z
)-octadeca-7,9,12-trien-5-ynoic
acid) marine NP inhibitors at acidic pH but not at neutral pH. GER-12
was effective for the reversible inhibition of cathepsin B, with an
IC
50
of 3 μM. GER-24 had an IC
50
of 16
μM and was found to be an irreversible inhibitor. These results
show that NP screening at distinct biological pH conditions can lead
to the identification of pH-selective cathepsin B modulators. These
findings suggest that screening efforts for molecular probes and drug
discovery may consider the biological pH environment of the target
in the disease process.