NADH oxidase activity of plasma membranes from rat
hepatoma and HeLa cells responded to
thiol reagents in a manner different from that of plasma membranes of liver. Specifically, the
NADH oxidase activity of plasma membranes of HeLa cells was inhibited by submicromolar concentrations of the
thiol reagents p-chloromercuribenzoate (
PCMB),
N-ethylmaleimide (NEM), or 5,5'-dithiobis-(2-nitrophenylbenzoic acid) (
DTNB), whereas that of the rat liver plasma membranes was unaffected or stimulated over a wide range of concentrations extending into the millimolar range. With some
hepatoma preparations, the
NADH oxidase activity of
hepatoma plasma membranes was stimulated rather than inhibited by
PCMB, whereas with all preparations of
hepatoma plasma membranes, NEM and
DTNB stimulated the activity. In contrast,
NADH oxidase activity of rat liver plasma membrane was largely unaffected over the same range of
PCMB concentrations that either stimulated or inhibited with rat
hepatoma or HeLa cell plasma membranes.
Dithiothreitol and
glutathione stimulated
NADH oxidase activity of plasma membranes of rat liver and
hepatoma but inhibited that of HeLa plasma membranes. The findings demonstrate a difference between the
NADH oxidase activity of normal rat liver plasma membranes of rat
hepatoma and HeLa cell plasma membranes in addition to the differential response to
growth factors and
hormones reported previously (Bruno et al., 1992). Results are consistent with a structural modification of a
NADH oxidase activity involving
thiol groups present in plasma membranes of rat
hepatoma and HeLa cells but absent or inaccessible with plasma membranes of rat liver.