Aldehyde production, enzymes

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. 

Dissociation of the aldehyde product would leave a low-coordinate, Cu(I) redox center associated with two protonated tyrosine phenols in the active site. This complex is known to be very reactive toward dioxygen, the second-order kinetic constant for reoxidation of the reduced enzyme by O2 being nearly 8x10 s (Borman et aL, 1997 Whit-... 

FIGURE 14. Structure of E coli amine oxidase active site in the equilibrium turnover complex. Interactions and labelling as in Figure 12 except that the reduced form of the cofactor is labelled TPR and carries a nitrogen at the 5-position. The aldehyde product, labelled Prod, is retained in the channel to the enzyme surface. Water W4 is positioned for nucleophihc attack on C5 of the cofactor and W2 is retained as a link in a proton shuttle from 04 to dioxygen. 

The enzymes are rather nonspecific, so most amines in solution will be determined. The reaction can be monitored by oxygen uptake, aldehyde production, ammonia production, or hydrogen peroxide. By coupling the reaction to peroxidase-p-hydroxyphenylacetic acid the reaction can be followed fiuorimetrically (37). The formation rate of ammonia can be determined conveniently by use of a Beckman 39137 cation selective electrode which permits the determination of 1-100 /xg/ml of amine (19). 

Enzymes of Aldehyde Production. Lipoxygenases are discussed elsewhere in this volume. The aldehyde-lyase has only recently been obtained even in crude extracts 145), However, use of 13- and 9-hydroper-oxyoctadecanoic acids to form Cg and C9 aldehydes and corresponding w-oxoacids is evidence that the pathway is correct (145). As with alliinase and myrosinase these reactions occur only when leaves or fruit are cut or crushed (138,143,144,145). If they are heated before blending or blended under N2 (138), little carbonyl compound is formed (142). H2O2 inhibits (138) this reaction as expected for lipoxygenase (153). lrarw-2-Hexenal is formed nonenzymatically from m-S-hexenal (138). 

Fig. 12. Cleavage of the O-alkyl linkage in glycerolipids (A) is catalyzed by (1) a Pte-H4-dependent alkyl monooxygenase, a microsomal enzyme found primarily in liver and intestinal tissues. The hemiacetal shown in this reaction has not been isolated because of its instability. The fatty aldehyde product can be either reduced to a long-chain fatty alcohol by (II) a reductase or oxidized to a fatty acid by (III) an oxidoreductase. Removal of the 0-alk-l -enyl moiety from plasmalogens (B) is catalyzed by plasmalogenase. As with the O-alkyl monooxygenase, the fatty aldehyde can be converted to either the corresponding fatty alcohol or the fatty acid. GPE, sn-glycero-3-phosphoethanolamine.

Substances other than enzymes can be immobilized. Examples include the fixing of heparin on polytetrafluoroethylene with the aid of PEI (424), the controUed release of pesticides which are bound to PEI (425), and the inhibition of herbicide suspensions by addition of PEI (426). The uptake of anionic dyes by fabric or paper is improved if the paper is first catonized with PEI (427). In addition, PEI is able to absorb odorizing substances such as fatty acids and aldehydes. Because of its high molecular weight, PEI can be used in cosmetics and body care products, as weU as in industrial elimination of odors, such as the improvement of ambient air quaHty in sewage treatment plants (428). 

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