Arch Toxicol (1989) 63 : 221 - 225 Archives of Toxicology 9 Springer-Verlag 1989 Changes in the inducibility of a hepatic aldehyde dehydrogenase by various effectors V. Vasiliou and M. Marselos Department of Pharmacology, Medical School, University of Ioannina, P.O. Box 1186, Ioannina 451 10, Greece Abstract. A hepatic soluble aldehyde dehydrogenase (ALDH), inducible by polycyclic aromatic hydrocarbons, was studied in Wistar rats in connection with substances known to affect drug metabolism or aldehyde dehydroge- nase activity, such as phenobarbital (PB), disulfiram (DS), 13-diethylaminoethyl diphenylpropylacetate (SKF 525A) and calcium cyanamide (CC). 3-Methylcholanthrene (MC) was given as a model inducer of ALDH (100 mg/kg, i.p., as a single dose) and the animals were killed after 3 days. Pretreatment with PB (1 g/l drinking water, for 2 weeks) enhanced the inducing effect of MC. On the contrary, pre- treatment with DS (100 mg/kg, i.p., daily x 4) reduced by 70% the expected increase in ALDH activity. Neither SKF 525A (25 mg/kg, i.p., daily x 4), nor CC (5 mg/kg, i.p., daily x 4) could affect the action of the inducer. At the above doses, basal ALDH activity was inhibited by DS (30%) and CC (70%), but was not affected at all by PB or SKF 525A. The results were somewhat different when the various effectors tested were administered to animals al- ready treated with MC (20 mg/kg, i.p., daily x6). In this case, DS did not affect the already induced ALDH activi- ty. On the contrary, CC was still an effective inhibitor. Un- expectedly, post-treatment with SKF 525A further en- hanced the initial induction brought about by MC. Our findings show that substances affecting microsomal drug metabolism can interfere with the process of ALDH induc- tion by MC. The additive result of PB pretreatment is probably due to the enhanced accumulation of an active metabolite of MC. The opposite effect of DS on drug me- tabolism could explain the decreased ability of MC to in- duce ALDH activity. The MC-inducible ALDH isozyme can be effectively inhibited with CC, but not with DS. Key words: Aldehyde dehydrogenase - Rat liver - In- duction - Phenobarbital - Disulfiram - Methylcholan- threne - Calcium cyanamide - SKF 525A Introduction In the normal rat liver, only a minor fraction of the total aldehyde dehydrogenase activity (ALDH, EC 1.2.1.3) is confined to the cytosol. Despite this fact, the cytosol con- tains at least two isozymes which can be induced several times by various xenobiotics (Deitrich 1971 ; Marselos and Offprint requests to: Marios Marselos Hfinninen 1974; Deitrich et al. 1977). Phenobarbital (PB) and other type I inducers of the drug-metabolizing enzymes increase the activity of an ALDH isozyme in cer- tain rat strains with a genetic predisposition (Deitrich 1971; Deitrich et al. 1972; Marselos 1976; Nakanishi et al. 1978). Administration of polycyclic aromatic hydrocar- bons (PAHs), or other type II inducers, can induce anoth- er isozyme which has been detected in all rat strains tested to date (Deitrich et al. 1978; Marselos et al. 1979). An in- crease in ALDH activity was also found in primary cul- tures of human or rat hepatocytes, after in vitro exposure to PB or 3-methylcholanthrene (MC) (Marselos and Mi- chalopoulos 1986; Marselos et al. 1986). The PB-inducible ALDH (q~-isozyme) prefers propion- aldehyde or phenylacetaldehyde as substrates and NAD as the coenzyme (Deitrich et al. 1972; Koivula and Koivu- salo 1975; Lindahl and Evces 1984). This isozyme is mea- sured as P/NAD activity and is apparently different from that induced by PAHs (x-isozyme) (Deitrich et al. 1978; Marselos et al. 1979), which is best measured with benz- aldehyde and NADP (B/NADP activity) (T6rr6nen et al. 1981). The mechanism of ALDH induction by PAHs is not fully understood. Even with large doses of MC, given to the rat in vivo (T6rr~nen et al. 1981), or applied to hepato- cyte cultures in vitro (Marselos et al. 1986), a lag time of at least 3 days is necessary before the induction of B/NADP activity becomes maximal. These observations suggest that the eventual effect of MC on the ALDH activity is mediat- ed through an active metabolite. The purpose of this study was to examine the possible modification of the ALDH inducibility by MC, after treat- ment with substances known to affect drug metabolism. For the induction of the microsomal enzymes, we used PB. On the other hand, disulfiram (DS) and l~-diethylamino- ethyl diphenylpropylacetate (SKF 525A) were chosen for their well-known inhibitory effect on the same enzymatic system (La Du et al. 1972; Lang et al. 1978). Since basal ALDH activity in the liver is inhibited by DS and calcium cyanamide (CC), we tested these compounds for their pos- sible inhibitory effect on the PAHs-inducible isozyme. Materials and methods Chemicals. All chemicals used were reagent grade. Pyra- zole was obtained from Fluka AG (W. Germany), prop- ionaldehyde from Ferak (W. Germany), calcium cyana-