Biochemical evidence for multiple acetoin-forming enzymes in cultured plant cells

Acetoin (3-hydroxy-2-butanon) production was investigated in extracts from suspension cultured cells of carrot, tobacco, maize and rice. Crude extracts 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 was found for acetolactate-deriving acetoin production. Three acetoin-forming enzymes were resolved upon adsorption chromatography. A minor peak of activity 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 synthase inhibitor. The other two activities were characterized following further purification by gel filtration chromatography. A low molar ratio between acetoin production and pyruvate utilization, the capability of producing acetaldehyde from pyruvate at higher rate, an optimal activity at acidic pH values and its increase in extracts from cells grown under hypoxic condition strongly suggested the former as a side reaction of pyruvate decarboxylase. The latter activity, which showed maximal rate at neutral pH values, was on the contrary found to quantitatively convert acetaldehyde and pyruvate to acetoin. This pyruvate carboligase, which increased in actively proliferating cells and declined in a late logarithmic phase and was not induced under anaerobiosis, was present at similar levels in all four plant species.