1. In the safety evaluation of drugs and other chemicals it is important to evaluate their possible inducing and inhibitory effects on the enzymes of drug metabolism. 2. While many similarities exist between species in their response to inducers and inhibitors, there are also important differences. Possible mechanisms of such variation are considered, with particular reference to the cytochrome P-450 system. 3. Differences in inhibition may be due to differences in inhibitory site of the enzyme involved, which is not always the active site of the enzyme, in competing pathways or in the pharmacokinetics of the inhibitor. 4. Differences in induction could be due to differences in the nature of the induction mechanism, in the isoenzyme induced, in tissue- or age-dependent regulation, in competing pathways for the substrate or its products, or in the pharmacokinetics of the inducing agent. 5. Examples of each of these possible differences are considered, often from our own work on the P450 IA subfamily, and results in animals are compared with those in humans, where possible. 6. At present, the differences between species in their response to inducers and inhibitors make extrapolation to humans from the results of animal studies difficult, so that ultimately such effects should be studied in the species of interest, humans.

The orientation of eukaryotic cytochromes P450, with respect to the membrane of the endoplasmic reticulum, has been investigated. There is now good evidence that the tertiary structure of these proteins is essentially the same as that of the soluble bacterial isoenzyme cytochrome P450CI, with the exception of an extension at the N-terminus which is thought to form a membrane-anchoring sequence. The remainder of the molecule protrudes from the cytosolic face of the membrane so that it can interact with substrates and electron-donating proteins. Two models based on this structure have been considered, in which the plane of the heme of cytochrome P450 is oriented either parallel with or perpendicular to the plane of the membrane of the endoplasmic reticulum. The validity of these models has been assessed from the results of studies involving the binding of antipeptide antibodies directed toward known regions of cytochromes P450, modeling of the interaction of cytochrome P450 with cytochrome b5, proposed intramolecular movements of cytochrome P450 during its catalytic cycle, and the partitioning of substrates for cytochrome P450 between the cytosol and membrane. It is concluded that cytochrome P450 is most likely oriented such that the heme is not fixed horizontal to the plane of the membrane of the endoplasmic reticulum and may well lie with the heme perpendicular to the membrane.

Earlier studies had shown that the viscerotropic NIH 173 strain of cutaneous Leishmania major fails to come under Lsh gene control. Visceral Leishmania donovani LV9 and another viscerotropic cutaneous strain, Leishmania mexicana mexicana LV4, are controlled by Lsh. The results of double-infection experiments presented here show that expression of Lsh resistance against L. mexicana mexicana was enhanced in the presence of L. donovani, whereas L. major still failed to come under Lsh gene control, even in the presence of L. donovani. Prior irradiation (850 rads) of mice showed that in the absence of infiltrating monocytes, Lsh did exert some influence over L. major. The presence of a higher infiltrate of fresh monocytes after L. major infection was confirmed in liver macrophage populations isolated from mice after infection in vivo and in liver cryosections immunostained with monoclonal antibody M1/70 directed against the type 3 complemnent receptor CR3. The results support the hypothesis that Lsh is expressed maximally in the resident tissue macrophages and poorly in the immature macrophages preferentially infected by L. major amastigotes. Recent studies reported by Mock and co-workers (20, 21) demonstrated that the NIH 173 viscerotropic strain of Leishmania major fails to come under Lsh gene control when injected intravenously (i.v.) into congenic BlO.L-Lshr or C.D2-Idh-J,Pep-3 (Lshr) mice. This was surprising considering the broad action of the LshlltylBcg gene in controlling such phylogenetically unrelated macrophage pathogens as Leishmania donovani (5, 6), Salmonella typhimurium (24, 25), Mycobacterium bovis Montreal (15), Mycobacterium lepraemurium (8, 26), and Mycobacterium intracellulare (14). Significant differences in liver parasite loads 15 days after i.v. inoculation of another viscerotropic cutaneous Leishmania species, L. mexicana mexicana LV4, into congenic C57BL/lOScSn or B10 (Lshs) and (BlO x B1O.L-Lshr)F1 mice (3) suggest that this species does come under Lsh gene control. Several explanations for the results obtained with L. major NIH 173 were possible. It may either fail to induce Lsh gene expression or be insensitive to the Lsh-gene-controlled resistance mechanism. Alternatively, this strain of L. major may target to a different macrophage population which fails to express Lsh gene activity in vivo. The results of experiments presented here provide evidence that Lsh gene control of L. major NIH 173 is, in fact, masked by preferential infection of infiltrating monocytes in which the gene is poorly expressed. MATERIALS AND METHODS Mice. CBA/Ca (Lshr) and C57BL/lOScSn or B10 (Lshs) mice were purchased from Harlan Olac Ltd.

Through differential screening of established human leukaemia cell lines, we have identified and molecularly cloned lymphopain, a novel cysteine proteinase of the papain family. Lymphopain exhibits a remarkably restricted cellular pattern of expression, being predominantly expressed in cytotoxic T-lymphocytes and natural killer cells. The human lymphopain locus maps to chromosome 11q13, encodes a polypeptide of 376 amino acids and is conserved in the mouse. Both human and murine forms appear more closely related to protozoan papainlike enzymes than to other mammalian members of the papain family. The cellular distribution of lymphopain expression, together with the functional demonstration of lymphopainassociated proteinase activity in vitro, is suggestive of a role for lymphopain in immune cell-mediated, cell killing.

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