ABSTRACT
Hydrazine is extensively utilised in industry and is a m inor metabolite of the
clinically used drugs isoniazid and hydralazine. It is toxic, carcinogenic and
mutagenic, but the metabolism and biochemical mechanisms of toxicity are not yet
fu lly understood.
Isolated rat liver microsomes incubated w ith both 2.0 mM and 0.2 m M hydrazine
in the presence of NADPH and oxygen at 37°C resulted in the disappearance of
hydrazine, which was demonstrated to be due to both enzymatic and chemical
oxidation. Boiled microsomes increased the proportion of chemical disappearance
whereas incubating the microsomes on ice effectively eliminated it. Further in vitro
microsomal studies therefore incorporated samples incubated on ice as controls,
allow ing the microsomal enzymatic metabolism of hydrazine to be calculated.
Absence of NADPH and oxygen markedly reduced microsomal hydrazine
metabolism, as did the presence of each of the cytochrome P450 inhibitors carbon
monoxide, piperonyl butoxide and metyrapone, thus indicating that microsomal
hydrazine metabolism is catalysed by cytochrome P450. Methimazole, an inhibitor
of flavin monooxygenase, also diminished hydrazine metabolism, whereas N ADH
in the presence of NADPH, but not alone, increased metabolism.
Microsomes prepared from either p-naphthoflavone, acetone or isoniazid pretreated
rats did not show significantly increased hydrazine metabolism compared to control
microsomes per g protein. However phenobarbitone pretreatment did increase
metabolism.
Hydrazine metabolism was 20-70% lower in human microsomes prepared from 3
individuals compared to control rats.
The dose response for hydrazine hepatotoxicity in vivo, as manifested by triglyceride
increase and depletion of ATP and glutathione (GSH), was measured in control rats
6 hr after an i.p. dose. This was then compared to animals which had been
pretreated w ith various inhibitors and inducers of cytochrome P450. Pretreatment
w ith the inhibitor piperonyl butoxide resulted in an increase in hepatotoxicity, while
induction by phenobarbitone (inducer of P450IIB) or p-naphthoflavone (inducer of
P450IA) decreased hepatotoxicity. In contrast, acetone or isoniazid (inducers of
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PREVIEW
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hydrazine hepatotoxicity by such pretreatments indicates that different isozymes of
cytochrome P450 catalyse the metabolic transformation of hydrazine toxicity by
various mechanisms.
6 hr after an acute i.p. hydrazine dose, certain dose related alterations in hepatic
microsomal enzyme activity were measured, including a depletion in
ethoxyresorufin O-deethylase and p-nitrophenol hydroxylase activity.
Repeated adm inistration of 0.78 mM hydrazine in drinking water (2.5 m g.kg^.day'1)
had a significant effect on several hepatic biochemical parameters and microsomal
enzyme activities after 1,5 and 10 days. This indicated hydrazine to be a probable
inducer of cytochrome P450IIE1.
Hepatic biochemical parameters and activities of microsomal enzymes were virtually
unchanged after repeated administration of 65 pM hydrazine in drinking water
(0.25 m g.kg'1.day’1) for 5 or 10 days.
In the presence of over 5 pM hydrazine, ATP synthesis in isolated mitochondria was
inhibited. Inhibition up to 100 pM hydrazine was found to be dose related and
reached a maximum 20-30% inhibition of control. However, above this concentration
further inhibition did not occur.
Hydrazine was also found to be metabolised by isolated mitochondria, which was
not significantly decreased in the presence of either the monoamine oxidase A
inhibitor, clorgyline or B inhibitor, pargyline
