4 The Metabolism, Storage, and Excretion of Highly
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Chlorinated Compounds by Mammals H. B. MATTHEWS, P. R. CHEN, H. M. MEHENDALE, and M. W. ANDERSON National Institute of Environmental Health Sciences, P.O. Box 12233, Research Triangle Park, N.C. 27709 Abstract The distribution and excretion of 4-chloro-,4,4'dichloro-, and 2,4,5,2',5'-pentachlorobiphenyl have been studied i n the rat. The half-lives of these compounds in the blood, l i v e r , muscle, skin and particularly the fat were correlated with their degree of chlorination, whereas their overall rate of metabolism and excretion was inversely correlated with chlorination. In f a t , the h a l f - l i f e of these compounds showed an exponential relationship to their degree of chlorination by weight. In the rat, this exponential relationship indicates that the h a l f - l i f e of chlorinated biphenyls increases by approximately two-fold with each 10 percent increase in chlorination. The effect of chlorination on the biolog i c a l h a l f - l i f e of these compounds has been related to the problems, both anticipated and realized, arising from the use of these and other lipophylic chlorinated hydrocarbons. Chlorinated hydrocarbons are a diverse group of compounds which have a variety of molecular structures, degrees of c h l o r i nation, uses and associated problems. Their structures vary from simple benzene rings with varying degrees of chlorination to complex mixtures of chlorinated compounds. Their uses range from various types of pesticides to hydrolic fluids and electric insulators. The value of their use i s undeniable; DDT has saved millions from malaria (1) and the polychlorinated biphenyls (PCB's) have become almost invaluable industrial tools (2). However, the problems associated with their use i s also equally undeniable, since they range from environmental contamination (1,2,3,4) to outbreaks of human disease i n which numerous people died (5). Thus, depending upon your source of information, the chlorinated hydrocarbons are a group of compounds which we cannot l i v e with or we cannot l i v e without. 54 Kohn; Mechanism of Pesticide Action ACS Symposium Series; American Chemical Society: Washington, DC, 1974.
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4.
MATTHEWS
ET AL.
Highly Chlorinated Compounds in Mammals
55
Chlorinated hydrocarbons, with the exception of the dibenzodioxins and dibenzofurans (6), do not have a high acute toxicity. Most of the problems associated with the use of these compounds are related to their persistence in the environ ment and their tendency to accumulate in the tissues of v i r t u a l l y every type of exposed organism. However, the diverse molecular structure of these compounds and the lack of adequate assay techniques have complicated comparative studies of their metabolism, distribution and excretion. Thus, to date, i t has been possible only to assume, from the existing data, that the rate of metabolism and excretion of chlorinated hydrocarbons was in some way inversely correlated to their degree of chlorination. Thus, we saw the PCB's as a unique opportunity to study a series of chlorinated hydrocarbons having the same basic carbon skeleton and differing only in their degree of chlorination. Such a study would allow us to relate the metabolism, storage and excretion of a series of chlorinated hydrocarbons directly to their chlorination. The adult male white rat was chosen as the subject of the study because of i t s convenience and a v a i l a b i l i t y . Also many of the previous metabolism, distribution and excretion studies with chlorinated hydrocarbons have used this animal. Any v a r i a b i l i t y in intestinal absorption was circumvented by injecting the material into the t a i l vein. The numerous rapid and accurate assays of tissue concentrations necessary for such a study were f a c i l i t a t e d by the use of carbon-14 labeled PCB's and a Biological Material Oxidizer (R. J. Harvey Instrument Corp.). PCB formulations are marketed according to their percent chlorine by weight. The compounds studied to date, a mono-, d i - and pentachlorobiphenyl, (Fig. 1) contain 18.8, 31.8, and 54.5 percent chlorine by weight, respectively. Thus their degrees of chlorination compare to those of Arochlors 1221, 1232 and 1254 which contain approximately 21, 32 and 54 percent chlorine by weight, respectively. Studies of the pentachloro biphenyl at doses of 0.06, 0.6 and 6.0 mg/kg showed that the distribution and excretion of this compound over a 100-fold dose range was similar (7). Mono- and dichlorobiphenyl were studied at doses of 0.6 mg/kg. The data presented here for pentachlorobiphenyl represents the 0.6 mg/kg dose only. Three animals were sacrificed at each of the time points which varied from 5 min to 7 days post treatment; however, in order to simplify the figures only those time points which were most relevant are presented. I n i t i a l l y the PCB content of every major organ and tissue was determined. This l i s t was even tually narrowed down to the blood, l i v e r , muscle, skin and fat as being most important in the distribution of these compounds. Animals held for 24 hrs or longer were held in metabolism cages and urine and feces were collected daily.
Kohn; Mechanism of Pesticide Action ACS Symposium Series; American Chemical Society: Washington, DC, 1974.
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56
MECHANISM OF PESTICIDE ACTION
PCB removal from blood with time i s shown in Fig. 2. The time points shown are 30 min, 1, 2, 4 and 8 hr, 1 and 2 days, and the curves are extrapolated to the next point i n both directions. Greater than 90 percent of each of the PCB's was removed from the blood within the f i r s t 30 min post treatment. The i n i t i a l rate of PCB removal from the blood appeared to be related to the degree of chlorination (Fig. 2). However, this situation reversed Itself at approximately 12 hrs and thereafter. In any study i n which the distribution of a compound i s studied by Intravenous injection 100 percent of the t o t a l dose must be i n the blood at zero time. In order to prove this point and i n an effort to observe the i n i t i a l very fast removal of a PCB from the blood, we Injected pentachlorobiphenyl into the right l a t e r a l t a i l vein, as usual, and withdrew blood samples from the l e f t l a t e r a l t a i l vein at short time periods thereafter. The results of this study are presented i n F i g . 3. A straight l i n e through the points obtained with three animals treated i n this way comes quite close to 100 percent of the total dose at zero time and quite close to the value obtained by drawing blood from the hearts of three other animals sacrificed 10 min post treatment. Thus, the removal of PCB s from the blood appears to follow a decay curve having three components. The i n i t i a l "very fast component represents PCB distribution throughout the body. The second component represents r e d i s t r i bution of the PCB's from tissues of high perfusion but low a f f i n i t y to tissues of lower perfusion but higher a f f i n i t y . Transport of metabolites to the excretory organs may also be involved i n the second component. The third component of the blood decay curve primarily represents a general elimina tion of the PCB's from the body. The major deposition sites at the early time points were the l i v e r and muscle (Figs. 4 and 5). Again, we obtained data for times varying from 5 min to 7 days post treatment but only the 30 min, 1, 2, 4, and 8 hr and 1 and 2 day points were plotted and the curve was extrapolated to the next point i f the PCB had not been effectively removed from the given tissue. PCB removal from the blood and accumulation i n these tissues i n i t i a l l y appeared to be related to the degree of chlorination, and could account for the fact that pentachlorobiphenyl was more rapidly removed from the blood than either the mono- or dichlorobiphenyl. The i n i t i a l peak, which i n the case of pentachlorobiphenyl exceeded 30 percent of the t o t a l dose i n each of these tissues at the earliest time points, was most Impressive In the l i v e r since this tissue accounts for only about 5 percent of the total body weight. The muscle, of course, accounted for a large percentage of the total dose by virtue of i t s relative mass which i n the rat i s 50 percent of the total body weight. Following an i n i t i a l peak concen tration, the amount of PCB i n each of these tissues decreased 1
11
Kohn; Mechanism of Pesticide Action ACS Symposium Series; American Chemical Society: Washington, DC, 1974.
4. MATTHEWS ET AL.
Highly Chlorinated Compounds in Mammals
«CKJ 4-CHL0R0BIPHENYL
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4,4'-DICHL0R0BIPHENYL
