Acetate esterase

The pKa of Zn H20 in the isoenzymes are determined from the pH dependence of 4-nitrophenyl acetate esterase activity and spectroscopic properties of the metal substituted (essentially Co11) enzyme (142 146). In bovine isoenzyme II, the titration behavior of zinc-H20 and His-64 are interdependent and both have similar pKa values. Similar behavior has been reported for human isoenzyme II (147). 

In isoenzyme I, the titration behavior of zinc H20 is complicated due to the presence of three titratable active-site histidines as described in Section VI.D. Lindskog reports a value of 7.1 for the pKa of zinc-H20 (142), but higher values were obtained by other authors. The pH-rate profile for 4-nitrophenyl acetate esterase activity yields pKa = 7.45 (142). 

Lindskog reported that Cl- is a competitive inhibitor of the CO2-HCO3 exchange reaction for isoenzyme I (157c). H NMR experiments (142,157c) and pH dependence of the SCN- inhibition of the 4-nitrophenyl acetate esterase activity have shown that the affinity constants of SCN- for various free enzyme forms follow the order HEH > EH > HE > E (142). The estimated values of the affinity constants are found to be 5x 104 M-1 and < 2 for HEH and E, respectively. Under inhibitory conditions the CA has a finite residual activity (kcai value) which is large for I-, intermediate for N3 and SCN-, and very small for NCO-. This has been explained on the basis of the relative binding affinities of these anions for the enzyme (190). 

Michor et al. (1996a) Batch Transesterification of menthol with isopropenyl acetate Esterase from Pseudomonas marginata... 

Two other enzymes are involved in aldehyde production. The first is an acetate esterase, which is soluble, located in the gland, and hydrolyzes tetradecanyl acetate and its All unsaturated derivatives (Z or E) at the same rate. The second is an alcohol oxidase, which also is soluble and located in the gland. The oxidase requires oxygen, but does not require reducing cofactors (12). The saturated and unsaturated 14-carbon acids all react at similar rates in the reaction catalyzed by this enzyme. 

The specificity of blends of compounds used for pheromone communication by Lepidoptera species is the result of essentially two distinct sets of biosynthetic enzymes which regulate the production of specific olefinic bonds and synthesis of the oxygenated functional moiety, respectively. In Heliothis moths the regulatory systems that are responsible for production of the functional group during the final stages of pheromone biosynthesis consist of cellular acetate esterases and extracellular alcohol oxidases. Evidence indicates that the relative activities of these enzymes differ for each species of Heliothis. Thus, pheromone mediated reproductive isolation between closely related species of Heliothis is probably the result, in large measure, of the fact that some species require only aldehydes for communication while others use acetates, alcohols and aldehydes. 

As indicated above, acetates formed in the pheromone gland of the above species are subsequently converted back to the alcohol via the action of an acetate esterase. Evidence to date suggests that the esterase resides in the soluble fraction of pheromone gland homogenates while the acetyl transferase is found in the microsomal fraction of homogenates (33, 34). 

Figure 1. Percentage of (Z)-11-tetradecen-l-ol acetate (500 ng) converted to alcohol by the action of acetate esterase (solid bars) and subsequently to aldehyde (checked bars) via the oxidase in the intact pheromone glands of H. subflexa. H. virescens and H. zea (n - 10, each species).

The selective inhibition of JHE and omaphthyl acetate esterase(s) by ortho, meta and para substituted compounds showed that meta and para substitution offered selectivity towards JHE, however, the ortho substituted compounds favored inhibition of a-naphthyl acetate esterases (54). It was thought that substitution in the ortho position might be detrimental for inhibition of JHE. Therefore we decided to add Es value for the ortho substituents in the regression analysis to merge the ortho compounds with their meta and para analogs in the same regression, "Equation 37". 

Induction of carboxylesterases and epoxide hydroloases would also affect the toxicity of insecticides. For example, host plant induction of 1-naphthyl acetate esterase would decrease the toxicity of certain insecticides containing an ester linkage such as organo-phosphates, pyrethroids, and some juvenile hormone analogs and, possibly, carbamates. 

It was possible for us to describe this clinical picture in detail from our own observations of a 13-year-old girl. We detected a very high content of cholesterol ester in the biopsy sample and a deficiency of lysosomal a-naphthyl-acetate esterase in the fibroblast culture. (205) (s. figs. 21.5 31.13, 31.14)... 

Based on the above discussion, trifluoromethyl ketones should inhibit proteases such as chymotrypsin (32), and serine esterases, such as acetylcholinesterase (33,24)> carboxylesterases (10), JHE and other esterases with varying selectivity. In a series of some juvenoid-like trifluoromethyl ketones and compounds of the structure A, l,l,l-trifluoro-2-tetradecanone (TFT) was found to be highly active and selective against JHE (I50 lxlO 7M) as compared to a-naphthyl acetate esterase (o-NaE) or trypsin (4iJ>). 

SuTFELD, R. and G.H.N. Towers 5-(4-Acetoxy-l-Butynyl)-2,2 -Bithiophene Acetate Esterase from Tagetes patula. Phytochemistry 21, 111 (1982). 

---