Peaches substrate activity

There are marked species differences in A-esterase activity. Birds have very low, often undetectable, levels of activity in plasma toward paraoxon, diazoxon, pirimi-phos-methyl oxon, and chlorpyrifos oxon (Brealey et al. 1980, Mackness et al. 1987, Walker et al. 1991 Figure 2.10). Mammals have much higher plasma A-esterase activities to all of these substrates. The toxicological implications of this are discussed in Chapter 10. Some species of insects have no measurable A-esterase activity, even in strains that have resistance to OPs (Mackness et al. 1982, Walker 1994). These include the peach potato aphid (Myzus persicae Devonshire 1991) and the... 

Hydrolysis. Carboxylesterases are frequently one of the major factors in OP resistance. In some insects, for instance the house fly (28), there are highly substrate specific esterases which attack only one or a very few molecules. "Malathionase", the prominent esterase responsible for many cases of malathion resistance, is highly specific for malathion. It cleaves one or both of the ethyl ester groups leaving malathion mono- or diacid (29). This enzyme is a true serine carboxylesterase that is inhibited by malaoxon (28) and does not hydrolyze any of the phosphoester bonds. In Anopheles stephensi from Pakistan, the malathion resistance decreased with adult age, but there was no concommittant decrease in general esterase activity as measured with 1- and 2-naphthylace-tate as model substrates (301. other mosquitoes have a carboxylesterase with broad substrate specificity that is associated with resistance (31-331. As mentioned above, the green peach aphid has a carboxylesterase, E4, with broad substrate specificity that sequesters toxicants (24). 

The FG activity in ripe freestone peaches is due to two enzymes (42). In contrast to tomatoes which contain two endo-PG s, peaches contain an endo-PG but also an exo-PG (42). The peach enzymes can be separated by chromatography on a Sephadex G-lOO column. The endo-FG has a pH optimum near 4 and is very effective in solubilizing pectin from peach cell walls. The discovery of exo-FG in peaches represented the first time that this enzyme was found in fruit tissue. The exo-FG has a pH optimum near 5.5 and requires Ca for activity with an optimum concentration of 0.4 mM. It removes monomer units from the nonreducing ends of the substrate molecules. This enzyme does not solubilize the pectin from isolated peach cell walls. 

---