Aldehydes enzyme inactivation

A problem associated with beef sterilized by irradiation at approximately room temperature is the production of an unpleasant flavor and aroma. This paper summarizes knowledge of the volatile components of enzyme-inactivated irradiated and nonirradiated beef, reviews the effects of concurrent and nonconcurrent irradiation procedures and of storage on these components, and presents evidence that methional (3-methylmercaptopropion-aldehyde), 1-nonanal, and phenylacetaldehyde are of primary importance to irradiation off-odor in beef thus processed. 

HA turnover is rapid in the brain, with a half-life of about 30 min. This can change very quickly depending on neuronal activity. There is no high-affinity uptake system for HA once released, HA is inactivated by catabolism. In the brain, released HA is methylated almost exclusively by the enzyme histamine-N-methyltransferase (E.C. 2.1.1.8). The tele-methyl-HA is subsequently degraded by monoamine oxidase-B (MAO-B) and aldehyde dehydrogenase to produce tele-methylimidazoleacetic acid (Brown et ah, 2001). 

The activities of NHases from Rhodococcus sp. Adpl2 and Gordonia sp. BR-1 strains have been partially characterized [25]. In reactions that catalyze the hydration of a-hydroxynitriles such as lactonitrile or glycolonitrile, the substrate can dissociate to produce HCN and the corresponding aldehydes. HCN can inhibit and/or inactivate NHase, and it was determined that these two enzymes remain active in the presence of cyanide ion at concentrations up to 20 him. The dependence of the NHase activity of cell-free extracts of Rhodococcus rhodochrous J1 and Gordonia sp. BR-1 on cyanide ion concentration is illustrated in Figure 8.1, demonstrating the improved cyanide stability of BR-1 NHase relative to that of Jl. 

The detailed mechanism of inhibition of TEM-2 (class A) enzyme with clavulanate has been established (Scheme 1) [23,24], The inhibition is a consequence of the instability of the acyl enzyme formed between the /1-lactam of clavulanate and the active site Ser-70 of the enzyme. In competition with deacylation, the clavulanate acyl-enzyme complex A undergoes an intramolecular fragmentation. This fragmentation initially provides the new acyl enzyme species B, which is at once capable of further reaction, including tautomeriza-tion to an entity C that is much less chemically reactive to deacylation. This species C then undergoes decarboxylation to give another key intermediate enamine D, which is in equilibrium with imine E. The imine E either forms stable cross-linked vinyl ether F, by interacting with Ser-130 or is converted to the hydrated aldehyde G to complete the inactivation. 

Two aldehydic nucleotide derivatives have found use as affinity labels. The magnesium salt of (64), formed by oxidation of ATP with periodate, is a competitive inhibitor of pyruvate carboxylase with respect to [Mg. ATP2-],100 and (65), obtained from the / -anomer of 5-formyluridine-5 -triphosphate on treatment with alkali, is a non-competitive and reversible inhibitor of DNA-dependent RNA polymerase from E. coli.101 In each case, addition of borohydride gives stoicheiometric covalent linkage of the nucleotide to the enzyme, with irreversible inactivation. It is thought that condensation with lysine occurs to give a Schiff s base intermediate, which undergoes subsequent reduction. 

Xanthine oxidase (XO) was the first enzyme studied from the family of enzymes now known as the molybdenum hydroxylases (HiUe 1999). XO, which catalyzes the hydroxylation of xanthine to uric acid is abundant in cow s milk and contains several cofactors, including FAD, two Fe-S centers, and a molybdenum cofactor, all of which are required for activity (Massey and Harris 1997). Purified XO has been shown to use xanthine, hypoxan-thine, and several aldehydes as substrates in the reduction of methylene blue (Booth 1938), used as an electron acceptor. Early studies also noted that cyanide was inhibitory but could only inactivate XO during preincubation, not during the reaction with xanthine (Dixon 1927). The target of cyanide inactivation was identified to be a labile sulfur atom, termed the cyanolyzable sulfur (Wahl and Rajagopalan 1982), which is also required for enzyme activity. 

Other enzymes may also be similarly inactivated by such cyclopropanone adducts generated in situ by catalytic unravelling of some latent precursors. NAD+ as coenzyme favours the electrophilic attack of AGP 17 on the enzymic thiol group, and considerably increases the rate of inhibition [17]. ALDH in brain was also inhibited in rats pretreated with coprine, and aldehyde reductase was slightly inhibited by AGP 17, in vitro [22]. 

To overcome problems of poor acceptor substrate acceptance, high concentrations of aldehyde substrates are required to obtain synthetically useful product yields. Unfortunately, DERA shows rather poor resistance to such high aldehyde concentrations, especially toward CIAA, resulting in rapid, irreversible inactivation of the enzyme. Therefore, the organic synthesis of (3R,5S)-6-chloro-2,4,6-trideoxy-hexapyranoside 1 requires very high amounts of DERA. Thus, despite the synthetic usefulness of DERA to produce chiral intermediates for statin side chains, the large-scale application is seriously hampered by its poor stability at industrially relevant aldehyde concentrations. The production capacity for such 2,4,6-trideoxy-hexoses of wild-type E. coli DERA is rather low [15]. 

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