Acetaldehyde formation from threonine

Kobayashi (1989) reported the formation of Sotolon in wines by an aldol condensation of acetaldehyde and a-ketobutiric acid (derived from threonine) followed by lactonization (Fig. 7.11). During aging, ethanol is converted into acetaldehyde, thus allowing the formation of Sotolon (Silva Ferreira et ah, 2003). 

The isotope studies of Adelberg led him at first to propose that the formation of isoleucine was initiated by an aldol condensation of a-keto-butyrate (derived from threonine) and acetaldehyde followed by enoliza-tion and hydration 147). This would be susceptible to pinacol rearrange-... 

Diacetyl, and its reduction products, acetoin and 2,3-butanediol, are also derived from acetaldehyde (Fig 8D.7), providing additional NADH oxidation steps. Diacetyl, which is formed by the decarboxylation of a-acetolactate, is regulated by valine and threonine availability (Dufour 1989). When assimilable nitrogen is low, valine synthesis is activated. This leads to the formation of a-acetolactate, which can be then transformed into diacetyl via spontaneous oxidative decarboxylation. Because valine uptake is suppressed by threonine, sufficient nitrogen availability represses the formation of diacetyl. Moreover, the final concentration of diacetyl is determined by its possible stepwise reduction to acetoin and 2,3-butanediol, both steps being dependent on NADH availability. Branched-chain aldehydes are formed via the Ehrlich pathway (Fig 8D.7) from precursors formed by combination of acetaldehyde with pyruvic acid and a-ketobutyrate (Fig 8D.7). 

Hofmann and Schieberle (1996) suggested hydroxyacetaldehyde and 2,3-butanedione as possible precursors of this odorant lactone. A mechanism of formation (in vin jaune ) has been proposed by Guichard et al. (1998) by transformation of threonine (present in coffee) into 2-oxobutyric acid (which can also be derived from carbohydrates), condensation with acetaldehyde and cyclization. 

Three possible mechanisms (Scheme 7) have been discussed to account for stereoselectivity in the formation of [Co(L)3] (L=threonine) from the condensation of acetaldehyde with [Co(gly)3]. Pathway C is shown to be most likely. ... 

Model reactions of this type have been studied in which the catalyst is pyridoxal plus a metal. The enzymatic reactions all appear to use pyridoxal phosphate as a cofactor, and in the case of a bacterial system, Mn" is also required. A major difference between the enzymatic and the model reactions is the requirement for a folic acid cofactor in the former. The formation of glycine and acetaldehyde from L-threonine and L-allo-threonine has been described by Lin and Greenberg. Their partially purified enzyme, threonine aldolase, was not shown to require any cofactors, and the reaction was not reversed. This is in contrast to the results of nonenzymatic experiments in which pyridoxal and a metal catalyze the reversible cleavage of threonine. 

Methylthiazolidine-4-carboxylic acid, a condensation product of cysteine and acetaldehyde, occurs even in human blood as a consequence of ethanol consumption. Serine and threonine analogously produce C-2 substituted (2J S,4S)-oxazolidine-4-carboxylic acids (2-124). Heterocyclic products, C-2 substituted (2J S,4S)-pyrimidine-4-carboxylic acids, are also produced in the reaction of aldehydes with asparagine (2-125). Phenylalanine yields C-1 substituted (lJ S,3S)-tetrahydroisoquinoline-3-carboxylic acids (2-126) and analogous products arise from tyrosine. Tryptophan reacts with aldehydes under the formation of 9H-pyrido[3,4-b]indole (also known as -carboline or norharmane) derivatives, (lJ S,3S)-l,2,3,4-tetrahydro-fi-carboline-3-carboxylic acids (2-127, R = H or alkyl or residues of other aldehydes and sugars), the reaction of tryptamine yields the corresponding (lRS)-l,2,3,4-tetrahydro-P-carbolines. 

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