Mary F. Argus

Repressible and inducible enzymic forms of dimethylnitrosamine-demethylase

Two enzymic forms, with different kinetic characteristics and responding in opposite ways to in vivo “enzyme inducer” pretreatment, underlie hepatic dimethylnitrosamine(DMN)-demethylase activity. Determination of the Hofstee plot of DMN-demethylase using a DMN substrate concentration range of 0.5 to 200 mM yields three intersecting line segments from which widely different K m and V max values may be calculated. Identically patterned Hofstee plots are obtained with rat and mouse postmitochondrial supernatant fractions, as well as with the isolated microsomes, yielding for the respective segments, similar K m values. The low-substrate-range line segment (0–4 mM) and the high-substrate-range line segment (50–200 mM) correspond, in both the rat and the mouse, to two different enzymic forms of DMN-demethylase (DMN-demethylase I and II, respectively) which have different regulatory characteristics: (A) Pretreatment of rats and mice with the polychlorinated biphenyl, Aroclor 1254, brings about repression of DMN-demethylase I (determined at 4 mM DMN) and induction of DMN-demethylase II (determined at 200 mM DMN); both responses are stronger in the rat than in the mouse. Kinetic experiments show that the differential responses of the two DMN-demethylases to Aroclor pretreatment are paralleled by corresponding changes in the V max values. (B) Pretreatment with phenobarbital represses enzyme I in both species; however, it induces enzyme II only in the rat

Comparative effects of indole and aminoacetonitrile derivatives on dimethylnitrosamine-demethylase and aryl hydrocarbon hydroxylase activities

The effect of in vivo administration of indole and five 3-indolyl derivatives including L-tryptophan, as well as of aminoacetonitrile and 3 of its derivatives, were studied on the carcinogen-metabolizing hepatic mixed-function oxidases dimethylnitrosamine (DMN)-demethylase I and II and aryl hydrocarbon hydroxylase (AHH). Indole, 3-indolylmethanol, 3-indolyl-acetonitrile, 3-indolylacetone and L-tryptophan induce AHH activity from 3- to 6-fold of the control level, whereas beta-3-indolylethanol has no effect; the latter compound produces a 21% decrease of the endoplasmic reticulum content in the tissue. Only L-tryptophan induces DMN-demethylase I and only L-tryptophan and 3-indolylmethanol induce DMN-demethylase II, representing a doubling of enzyme activity in all 3 instances. Aminoacetonitrile is a potent repressor of DMN-demethylase I. Substitutions on the amino group bring about strong decrease or abolishment of mixed-function oxidase repressor activity; thus, iminodiacetonitrile has only about 1/5th the repressor activity of the parent compound, whereas nitrilotriacetonitrile and dimethylaminoacetonitrile appear to be inactive. Aminoacetonitrile and its derivatives studied have no effect on DMN-demethylase II and AHH activities. The mixed-function oxidase-modifying effects of the indole compounds and of aminoacetonitrile and its derivatives illustrate the potential complexity of effects of dietary constituents on the carcinogenic responses.

Structural limits of specificity of methylcholanthrene-repressible nitrosamine N-dealkylases. inhibition by analog substrates

The dealkylation of dsimethyl-, diethyl- and dipropylnitrosamine by hepatic microsomes of Sprague-Dawley rats is repressed by pretreatment of the animals with 3-methylcholanthrene (MC), and this repression progressively decreases with the increase of alkyl chain length. In contrast to its effect on the demethylation of dimethylnitrosamine (DMN), in vivo phenobarbital induces rather than represses the deethylation of diethylnitrosamine. The rates of demethylation of the DMN analog substrates (dimethylformamide, dimethylacetamide, dimethylpropionamide, and dimethylbutyramide), although low as compared to DMN, increase with the acyl chain length. These analogs are potent in vitro inhibitors of DMN demethylation when used in combination with DMN as substrates, and the inhibition decreases with the length of the acyl chain. Dimethylaminoacetone, which corresponds to the insertion of a CH2 group between the N atom and the carbonyl group in dimethylacetamide, is not an in vitro inhibitor of DMN demethylation; the demethylation rates are additive when this compound is used as substrate in combination with DMN. The rate of demethylation of dimethylaminoacetone is substantially higher than the rates of the dimethylacylamides, and is significantly repressed by MC-pretreatment. The rate of demethylation of methylphenylnitrosamine is not influenced by MC-pretreatment; the compound is, however, a potent inhibitor of demethylation when used as substrate in combination with DMN. The demethylation rates of 1,1-dimethylhydrazine (the reduction product of DMN) and dimethylaniline are not influenced by MC-pretreatment; neither do they affect the overall rate of demethylation when used as substrate in combination with DMN.