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Articles

-Alanine as an Ethylene Precursor. Investigations Towards Preparation, and Properties, of a Soluble Enzyme System From a Subcellular Particulate Fraction of Bean Cotyledons ¹

^a Plant Biochemistry, South Laboratory, University of Alberta, Edmonton, Alberta, Canada

A method is described for the preparation, from a subcellular particulate fraction of wax bean cotyledons, of a soluble enzyme system that is capable of converting -alanine to ethylene. In the presence of ATP, CoA, thiamine pyrophosphate, MgSO₄, and pyridoxal phosphate, ethylene production is maximum at a 0.5 mM concentration of -alanine. The system exhibits a pH optimum at 7.0 but when the pH is raised above 8, evolution of the volatile again increases and continues to do so up to pH 12. The enzyme system is stimulated by either NADPH or NADH; the concentration of NADPH necessary to obtain maximum activity is twice that of NADH. The requirement for a reducing agent is in agreement with the proposal that malonate semialdehyde, formed by an aminotransferase reaction from -alanine, is reduced to -hydroxypropionate. Both malonate semialdehyde and -hydroxypropionate are better stimulators of production of the volatile in the soluble system than is -alanine, and -hydroxypropionate is a better stimulator than malonate semialdehyde. This system is also able to incorporate tritium from tritiated water into ethylene; this supports the proposal that ethylene is formed by the decarboxylation of acrylate, the latter being formed from -hydroxypropionate.

Experiments with both cold and labeled malonate suggest that this compound stimulates ethylene production by acting as an end product inhibitor that prevents the loss of malonate semialdehyde from the pathway. Malonate does not appear to serve as a precursor.

Addition of cytoplasmic enzymes to the `soluble system' (prepared from particulate enzymes) results in a considerable boost in ethylene production, but the specific activity (mµ1 / mg protein) is lowered from that of the particulate enzymes alone.

¹ The authors are grateful to the National Research Council of Canada for a grant-in-aid (A-1451) of this research and for a studentship to R.A.S.