Studies were carried out to determine the effects of preincubation of
4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone (
MAIQ) with hepatic microsomes on the ability of
MAIQ to inhibit
CDP reductase activity in vitro. An aliquot from the 100,000 x g supernatant fraction from this incubation was used in the
CDP reductase assay.
MAIQ incubated in the absence of microsomes inhibited
CDP reductase activity in a dose-dependent manner. At high
MAIQ concentration (5 microM)
CDP reductase activity was inhibited 95%. When
MAIQ (5 microM) was first incubated in the presence of hepatic microsomes and
NADPH,
CDP reductase activity was inhibited only 30%. This attenuation of
MAIQ inhibition was dependent on time of incubation and microsomal
protein concentration and showed an obligatory requirement for
NADPH or
NADH. Significant attenuation was observed at
pyridine nucleotide concentrations as low as 0.1 mM. Heat denaturation of microsomal
proteins inactivated their ability to attenuate the
MAIQ inhibition. Microsomes prepared from Ehrlich
tumor cells were ineffective as inactivators of
MAIQ. Results of our studies show that hepatic microsomes contain an
enzyme(s) which can inactive
MAIQ as an inhibitor of
CDP reductase.