Environmental Health Perspectives Some Studies and Comments on Hepatic and Extrahepatic Microsomal Toxication-Detoxication Systems. A Limited Discussion of Some of the Heterogeneities of These Systems and of Their Responses to Stimulation of Enzyme "Induction" by James R. Fouts, Ph. D.* Introduction and other purposes), or water (such as chem- icals used in purifying the water or pollu- Much of what will be discussed in this tants that cannot be removed), or air (odors, paper can be referred to as drug metabolism, perfumes, and particulates-some added or drug-metabolizing systems and some fac- deliberately, some we can't or don't remove) tors which affect these systems, for example and which we ingest in the process of living by causing enzyme induction. If such terms and working in an environment increasingly as "drug metabolism" and "enzyme induc- filled with effectors of these enzyme sys- tion" are not further defined and explained, tems. To replace "drug metabolizing en- many people will still know what I am talk- zymes" with "xenobiotic or foreign chemical ing about and referring to. However, this metabolizing systems" recognizes part of the sloppy language may needlessly restrict the problem-some of the substrates of these interest and importance of some of the enzymes are not drugs; (b) drug or xeno- research going on in this field. Attempts to biotic metabolizing systems can cause acti- overcome this language-semantics barrier vation and inactivation of chemicals- have been partly successful and need to be sometimes even in sequence with or on the refined and extended. Some points needing same substrate. Thus, changes in activity of emphasis in my opinion are: (a) drug metab- the drug or xenobiotic metabolizing systems olizing systems have a number of substrates can either increase or decrease the toxicity that are not drugs. Some of these are of chemicals serving as substrates of these "normal" substrates such as steroids, fatty systems. Such changes can be specific for acids, vitamins, and hormones. Other sub- one substrate or apply to several substrates strates include chemicals that are present in and at the same time may make one sub- our food (such as preservatives, coloring strate more toxic and another less so. What agents, food additives for taste, odor control often determines toxicity and actions of a chemical is the balance between metabolism to toxic vs. less-toxic metabolites. This *Chief, Pharmacology and Toxicology Branch, National Institute of Environmental Health Sciences, balance may be shifted by increasing any or National Institutes of Health, Box 12233 Research all of these metabolisms or by increasing Triangle Park, North Carolina, 27707 some more than others, as well as by the October 1972 55 simpler examples of increasing one- kind throw up his hands in despair at this point (toxication) of reaction and decreasing since alternatives to "drug metabolizing another (detoxication) or vice versa. Thus, enzymes" are not easy to sell to anyone, let two points come out-drug or xenobiotic alone think up. But the lack of viable alter- metabolizing systems are not just "detoxi- natives has led to a lot of "clubs" who do cation mechanisms" and increases or induc- not communicate well with each other and tion of these systems can lead at the same the whole subject suffers. I will now close or different times to intoxication as well as this part of the introduction by choosing my detoxication relative to the "before induc- label for these systems: toxication- tion" stage; (c) toxication-detoxication sys- detoxication systems, only wishing I could tems include many reactions that are not somehow also add: ... that may use as oxidative in nature, are not cytochrome substrates drugs, foreign chemicals, or endog- P450 requiring, are not restricted to the enous lipid-class members such as steroids, liver, and can occur elsewhere than in the fatty acids, and vitamins, or endogenous microsomes. Therefore, to talk only about non-lipids like thyroxine, and certain amines hepatic microsomal drug oxidations and (even epinephrine or precursors), and may further to consider only those which involve even be systems that neither toxify nor cytochrome P450 is to talk about a very detoxify, but rather only metabolize these small (but vigorously and intensely studied) various substrates, and whose metabolism part of the body's systems for metabolizing can be characterized as oxidative and non- endogenous and exogenous chemicals. oxidative, synthetic and hydrolytic, involving One cannot really deal with these three many organs besides liver, and including a problem areas merely by recycling the lot of non-cytochrome P450 dependent nomenclature. Yet our use of convenient but systems. inexact items can, by repetition, cause some Having spent so much time on broadening dulling of our appreciation of the problems perspectives, I must now hasten to assure we are making for ourselves and colleagues the reader that I cannot possibly deal with in other disciplines who need to inform us all these matters in this paper. Indeed the and vice versa about common research main subjects covered will be few in number efforts with these systems. People who work and terribly selective since I prefer to write on "drug metabolizing enzyme systems" will about my own work and not that of others. tend to use drugs as substrates and think of This will not be a general review of drug or the systems primarily in terms of their roles xenobiotic metabolism, since I think plenty in therapeutics. Scientists who study "xeno- of these already exist. Instead, I wish to biotic metabolizing systems" will think write about three research projects that have largely in terms of non-drug substrates and been under study in my laboratory over the perhaps concentrate on the role of these last few years, and which I feel are of rele- systems and their interactions with model vance to the interface between "drug" substrates, industrial chemicals or environ- metabolism and environmental health science mental pollutants. Research on "steroid or as disciplines having identity problems that fatty acid metabolizing systems" will tend to are not dissimilar. These research projects I focus on the role of these enzymes in regu- wish to concentrate on are: (a) extrahepatic lating homonal functions and body homeo- toxication-detoxication systems; (b) age- stasis. Studies on "vitamin and non-steroid dependent changes in toxication-detoxication hormone metabolisms" will often concern systems in lung vs. liver, and (c) species- itself with nutritional balances, intermediary strain-individual differences in "induction" of metabolism and similar systems. The point hepatic toxication-detoxication systems by being that words do make a difference, the pollutants benzpyrene and DDT. labels do matter, and we ignore this human If the reader wishes a general review of weakness with some costs attached. One can "drug metabolism," or even specifically drug 56 Environmental Health Perspectives metabolism and its role in environmental tifying low levels of component or enzyme health, he should read elsewhere. Several activity especially in the presence of high very good reviews of the subject of "drug blanks due to things like blood - the liver metabolism" and selected aspects of the has high levels of most toxication- systems in liver, especially those requiring detoxication systems and contaminants or cytochrome P450 have appeared quite inhibitors of assays can often be removed recently. Some of these reviews are in book easily or at least lowered by perfusion, form; others are in journals or monographs. washing, etc.; (d) stability of systems - prob- Many conferences are being held on the lems in preparing components or enzyme subject and the conference proceedings often systems for assay before they are lost by appear very shortly thereafter. I cannot cytolysis or autolysis, problems in storage of begin to list all such "reviews" that have systems (freezing), problems in long-time been published even in the last 3 or 4 years. incubation especially with low enzyme activi- Inevitably, I would leave a key reference ties - the liver decomposes slowly, stores well, out, and probably that of a good friend. and is stable for most incubation or assay Having thus dusted off a few old griev- periods especially since content or enzyme ances and given you some idea of what I activity is so high; (e) quantity of tissue will say and will not even attempt to do, I available vs. quantity of tissue needed for can now turn to my subject with a some- assay, statistics, etc. - high activity and plenty what less guilty conscience. of homogeneous tissue makes liver almost ideal in experiments calling for repetitive sampling or biopsy, estimation of individual Extrahepatic Toxication-Detoxication Systems variability, purification of the enzyme The major research efforts in the field of systems, etc. "drug-metabolizing enzymes" have been con- Early studies in "drug metabolism" did cerned with systems in the liver. It is true occasionally include comparisons of liver that the liver of most animal species studied with other tissues, at least in the common is especially rich in enzyme systems that laboratory animals. From such studies it was metabolize chemicals of all kinds. It is also learned that most organs of excretion or true that most workers have been aware that exchange were likely to have at least some other body tissues had the ability to metab- capacity to metabolize chemicals. However, olize chemicals,