Characterization of microsomal aryl hydrocarbon hydroxylase of rat brain
Das, M.; Seth, P.K.; Mukhtar, H.
Journal of Pharmacology and Experimental Therapeutics 216(1): 156-161
1981
ISSN/ISBN: 0022-3565 PMID: 7452502 Document Number: 169989
Characterization of cytochrome P-450-dependent aryl hydrocarbon hydroxylase (AHH) activity of rat brain was undertaken. Rat brain shows 0.8 and 0.02% of the liver AHH activity when expressed as specific activity (pmol of 3-hydroxybenzo(a)pyrene per min per mg of protein) and total organ activity (pmol of 3-hydroxy-benzo(a)pyrene per min per organ), respectively. AHH of rat brain microsomes required oxygen and NADPH for optimal expression of activity. Brain AHH activity was linear with respect to protein concentration and incubation time up to 30 min. A sharp pH optima at 7.6 was observed. Among the species investigated, brain microsomal AHH specific activity was in the following order: rag > mice > guinea pig > kite > crow > vulture > pigeon. As far as rat is concerned, the maximum activity is localized in the cerebellum followed by cerebrum and brain stem. Brain AHH was confined to mitochondrial and microsomal fractions. Mitochondrial specific enzyme activity was comparable to the microsomal enzyme activity. Rat brain microsomal AHH was induced by 3-methylcholanthrene (142%) and benzo(a)pyrene (132%), whereas phenobarbital produced .apprx. 70% induction. Brain AHH was inhibited when the incubation mixture containing microsomes was flushed either with CO or N. The apparent Km value of rat brain AHH was 6.66 .mu.M. This value was only 1/6 that of the liver enzyme (40 .mu.M). 1-Benzylimidazole, diethylmaleate, SKF 525A and metyrapone inhibited both noninduced and 3-methylcholanthrene-induced AHH activity. .alpha.-Naphthoflavone was more effective in inhibiting 3-methylcholanthrene-stimulated rat brain AHH as compared to the noninduced enzyme. Age-dependent increase in AHH activity of rat brain microsomes was observed. The activity increases gradually between 10-56 days. Activity of AHH in brain of 90 day old rats did not significantly differ from that of 56 day old rats.