Trans-species enzyme systems

While the availability of trans-species enzyme systems has had a major impact, advances in molecular biology have also enabled the query of increasingly sophisticated questions. Molecular biological methods have made it possible to clone and express enzymes to study reactions at a molecular level. This has improved our ability to study enzyme reactions at a fine molecular level, to discern the contributions of individual enzymes in complex systems, and even to employ them as bioreactors to generate small quantities of metabolite standards. 

In the mid-1960s we showed firstly that the natural tolerance of houseflies to cyclodienes resulted mainly from oxidative detoxication (33 55) and secondly that another enzyme system, epoxide hydrase, converted certain dieldrin analogues into the corresponding trans-diols, (56,57) Interspecific differences in ability to attack enzymatically the unchlorinated ring systems of various analogues, either oxidatively and/or hydratively (if appropriate) can confer selective toxicity between insect species and also between insects and mammals (58) ... 

The accumulation of -carotene can be explained in terms of geometrical isomerism. This is based on the following observation [ c] -carotene which accumulated in the dark on incubation with radiolabelled 1.5-cis phytoene as a substrate, is desaturated to lycopene upon illumination (Fig. 2). It was concluded that a photoisomerization of C-carotene is the crucial point in this sequence. This can be demonstrated by analytical HPLC (Fig. 3). The enzymatically formed -carotene species is in the 15-cis configuration and this central cis double bond, deriving from 15-cis phytoene, is photoiso-merized to trans. The resulting all-trans -carotene is then accepted as a substrate in the lycopene-forming enzyme system. 

As is outlined in the following section, it is now apparent that endogenous synthesis of RA occurs in most, if not all, animal species. The classic studies of Mahfouz et al. (1980) and Pollard et al. (1980) showed independently that positional isomers of trans-octadecenoic acids are desaturated by the enzyme A-9 desaturase in rat liver microsomal systems. All tram monoenes, A-4 to A-13, except A-8, 9, and 10, were substrates, with products being trans-A-x, cis-9 dienes. The rate of A-9 desaturation increased as the trans bond was removed further from the A-9 position, so that trans-A-4 and A-13 monoenes were most rapidly desaturated (Mahfouz et al., 1980). The trans-5, cis-9 18 2 was isomerized rapidly to the cis, cis diene without changing bond positions tram-4, cis-9 18 2 was isomerized similarly, but at a slower rate (Mahfouz et al., 1980). Significant amounts of some of the cis/trans dienes were desaturated further at A-6, to yield cis, tram, cis trienes (Pollard et al., 1980). 

Another interesting feature of these enzymes is the formation of an intense purple intermediate upon the addition of excess hydroperoxy product. It has been noted that this chromophore resembles the iron-alkylperoxo complexes previously characterized by Que and co-workers. While no spectroscopic evidence has been obtained for the LO systems, such a complex would not be rationalized by the mechanism put forth in Figure 25 as the peroxy-substrate species would need to be positioned in a trans position to the iron. 

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