Metabolic perspectives on in vitro toxicity tests.

1. The toxic effects of chemicals arise either from the chemical itself or from metabolites. In either event, the concentration of the molecular species responsible for toxicity is under the control of metabolism. This metabolism is highly variable between animals and humans, and in human populations may exhibit substantial inter-individual differences. The range of human variability can be covered by resorting to a number of animal models. 2. For many purposes, it is desirable to have in vitro procedures to allow detailed examination of mechanisms, but for a long time it was commonly the case that results from in vitro systems were misleading. It was realised that this problem arose at least in part from the absence of appropriate metabolism in many in vitro systems, and to overcome this, it has become common to add metabolizing enzymes in the form of the so-called S9 fraction--the 9000 g supernatant containing both microsomal and cytosolic enzymes--of rat liver. These enzyme preparations contain both the cytochrome P-450 mono-oxygenases and the obligatory NADPH-generating system. This system is thereby optimized for the mono-oxygenase reactions which are frequently responsible for the metabolic activation of toxic chemicals. 3. It is extremely important to appreciate that such systems are highly artificial, being far removed from the situation operating in the intact cell and the whole body. The levels of reactive metabolites in the whole cell are determined by the balance between activating and inactivating enzyme systems. When the S9 fraction is used for metabolism, the detoxication enzymes are essentially inactive, so that a misleading emphasis is thrown upon metabolic activation. Although the inactivating enzymes, such as the glutathione S-transferases, the sulphotransferases and the glucuronosyltransferases, are present in the S9 fraction, their cofactors are diluted so far below the relevant Km values as to render them completely inoperative. This absence of metabolic inactivation in in vitro test systems very commonly underlies observed discrepancies between the results of bacterial mutagenicity tests and carcinogenicity studies in whole animals. 4. Present and future demands upon the toxicologist require the use of rapid in vitro tests for the screening of chemicals, the results of which have predictive value for in vivo adverse effects, e.g. general toxicity, carcinogenicity or mutagenicity. The key role of metabolic information in such procedures is illustrated in this report with reference to the series of methylated derivatives of the polycyclic aromatic hydrocarbon benz(a)anthracene.(ABSTRACT TRUNCATED AT 400 WORDS)