Acetoin (3-hydroxy-2-butanon) production was investigated in extracts from
suspension cultured cells of carrot, tobacco, maize and rice. Crude extract
s were able to catalyze acetoin synthesis from pyruvate and/or acetaldehyde
at rates ranging from 0.02 to 0.1 mkat kg(-1) protein, while no evidence w
as found for acetolactate-deriving acetoin production. Three acetoin-formin
g enzymes were resolved upon adsorption chromatography. A minor peak of act
ivity was deduced as due to a partial, nonenzymatic decarboxylation of the
acetolactate produced by acetolactate synthase under the same experimental
conditions, being completely abolished by the addition of an acetolactate s
ynthase inhibitor. The other two activities were characterized following fu
rther purification by gel filtration chromatography. A low molar ratio betw
een acetoin production and pyruvate utilization, the capability of producin
g acetaldehyde from pyruvate at higher rate, an optimal activity at acidic
pH values and its increase in extracts from cells grown under hypoxic condi
tion strongly suggested the former as a side reaction of pyruvate decarboxy
lase. The latter activity, which showed maximal rate at neutral pH values,
was on the contrary found to quantitatively convert acetaldehyde and pyruva
te to acetoin. This pyruvate carboligase, which increased in actively proli
ferating cells and declined in a late logarithmic phase and was not induced
under anaerobiosis, was present at similar levels in all four plant specie
s. (C) 1998 Elsevier Science Ltd. All rights reserved.