The oxidation of formaldehyde by rat liver mitochondria in the presence of 50 mM phosphate was enhanced 2-fold by exogenous NAD+. Absolute requirement of NAD+ for formaldehyde oxidation was demonstrated by depleting the mitochondria of their NAD+ content (4.6 nmol/mg of protein), followed by reincorporation of the NAD+ into the depleted mitochondria. Aldehyde (formaldehyde) dehydrogenase activity was completely abolished in the depleted mitochondria, but the enzyme activity was restored to control levels following reincorporation of the pyridine nucleotide. Phosphate stimulation of formaldehyde oxidation could not be explained fully by the phosphate-induced swelling which enhances membrane permeability to NAD+, since stimulation of the enzyme activity by increased phosphate concentrations was still observed in the absence of exogenous NAD+. The Km for formaldehyde oxidation by the mitochondria was found to be 0.38 nM, a value similar to that obtained with varying concentrations of NAD+; both Vmax values were very similar, giving a value of 70 to 80 nmol/min/mg of protein. The pH optimum for the mitochondrial enzyme was 8.0. Inhibition of the enzyme activity by anaerobiosis was apparently due to the inability of the respiratory chain to oxidize the generated NADH. The inhibition of mitochondrial formaldehyde oxidation by succinate was found to be due to a lowering of the NAD+ level in the mitochondria. Succinate also inhibited acetaldehyde oxidation by the mitochondria. Malonate, a competitive inhibitor of succinic dehydrogenase, blocked the inhibitory effect of succinate. The respiratory chain inhibitors, rotenone, and antimycin A plus succinate, strongly inhibited formaldehyde oxidation by apparently the same mechanism, although the crude enzyme preparation (freed from the membrane) was slightly sensitive to rotenone. The mitochondria were subfractionated, and 85% of the enzyme activity was found in the inner membrane fraction (mitoplast). Furthermore, separation into inner membrane and matrix components indicated a distribution of aldehyde dehydrogenase activity similar to malic dehydrogenase.

Aldehyde Oxidoreductases↗

PubMed1976

Induction of liver microsomal mixed-function oxidases by volatile hydrocarbons.

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Aminopyrine N-Demethylase↗

PubMed1976

Hepatic organelle interaction. IV. Mechanism of succinate enhancement of formaldehyde accumulation from endoplasmic reticulum N-dealkylations.

Further evidence for organelle interaction during drug metabolism by the liver is presented. The apparent stimulation by succinate of formaldehyde accumulation in the medium, which was reported to occur with liver slices and homogenates as well as with mitochondria plus microsomes, has been shown to be the result of succinate inhibition of mitochondrial aldehyde dehydrogenase. The mechanism of succinate inhibition is shown to be by reverse electron transport, and an increase in the NADH to NAD+ ratio in the mitochondria; the aldehyde dehydrogenase requires the oxidized form of the pyridine nucleotide as its cofactor. Studies on in vitro N-demethylation by liver microsomes and endoplasmic reticulum segments which cosediment with the mitochondria indicate that formaldehyde produced by the mixed function oxidase is handled differently from formaldehyde added to the medium. The latter is mainly retained in the medium containing 5 mM semicarbazide, while the generated formaldehyde is more than 50% consumed by the mitochondria. Electron microscopy has indicated that the microsomes and the endoplasmic reticulum fragments have a tendency to align themselves close to the mitochondria when present in the same medium. Consequently, it is possible that formaldehyde released to the medium adjacent to the mitochondria, as by N-demethylation, would be exposed to semicarbazide for shorter periods than that added directly to the medium. In agreement with this suggestion, complexing of formaldehyde with semicarbazide was observed spectroscopically not to be an extremely rapid reaction even at 37 degrees C. This is believed to be the reason for the greater extent of consumption of formaldehyde generated by the endoplasmic reticulum.

Aldehyde Oxidoreductases↗

PubMed1975

Binding of homogeneous cytochrome b5 to rat liver microsomes. Effect on N-demethylation reactions.

Incubation of rat homogeneous detergent-solubilized cytochrome b5 with rat liver microsomes resulted in specific binding of the hemoprotein which was rapidly reduced by NADH. The NADH cytochrome c reductase activity in these preparations increased in proportion to the amount of cytochrome bound. However, the extra-bound detergent-solubilized cytochrome b5 did inhibit NADPH-dependent N-demethylations, the NADH synergism and NADPH cytochrome P-450 reductase activity. Manganese protoporphyrin-apocytochrome complex when bound to microsomes in amounts equivalent to detergent-solubilised cytochrome b5 showed no effect on N-demethylation activity. Furthermore, the binding of cytochrome b5 preparations reconstituted from heme and apocytochrome b5 had no effect on either the NADPH-dependent N-demethylation of aminopyrene or ethylmorphine or the NADH synergism observed with rat liver microsomes. In addition, homogeneous cytochrome b5 eluted from three additional Sephadex G-100 columns showed no inhibitory effects when bound to liver microsomes. Spectral analyses of the acid-acetone extract of the hemoprotein showed an absorption peak at 278 nm suggesting that the homogeneous b5 contains contaminating amounts of tightly bound detergent which is responsible for the observed inhibition of mixed function oxidase activity and which is removed during extraction of the heme from the apocytochrome and during further gel filtration applications.

Animals↗

PubMed1975

The role of cytochrome b5 in mixed function oxidations: effect of microsomal binding of the hemoprotein on hepatic N-demethylations.

Incubation of rat cytochrome b5 (D-b5) with rat liver microsomes resulted in specific binding of the hemoprotein. The bound hemoprotein was rapidly reduced by NADH. The NADH cytochrome c reductase activity in these preparations increased in proportion to the amount of cytochrome. In contrast to D-b5, which inhibited N-demethylation and the NADH synergism, the binding of cytochrome b5 preparations, reconstituted from heme and apocytochrome b5 had no effect on either the NADPH-dependent N-demethylation of aminopyrine or ethylmorphine or the NADH synergism observed with rat liver microsomes. In addition, manganese protoporphyrin-apocytochrome complex, when bound to microsomes in amounts equilvalent to D-b5, showed no effect on N-demethylation activity. These results suggest that homogeneous cytochrome b5 contains contaminating amounts of tightly bound detergent which presumably is removed during the extraction of the heme from the apocytochrome.

Aminopyrine↗

PubMed1975

Explanation of the stimulation of microsomal N-demethylation reactions by soluble supernatant fraction.

Addition of the cell soluble supernatant fraction to an assay medium containing NADPH generating system, mixed function oxidase substrate and microsomes, resulted in a stimulation of drug metabolism ranging from 12-75%. This stimulation was observed only when the supply of DADPH generating system (isocitric dehydrogenase or glucose 6 phosphate dehydrogenase) was insufficient, leading to a NADPH oxidation rate which was greater than the rate of reduction of NADP+ during the oxidation of a drug. Hence, under our assay conditions, the soluble supernate (SS) is only providing sufficient NADPH generator, and possibly relieving inhibition by the generated NADP+. Finally, microsomal lipid peroxidation measurements under these same conditions indicate negligible to no peroxidation activity in the absence of SS.

Aminopyrine N-Demethylase↗