AROMATIC POLYKETIDE SYNTHASES - PURIFICATION, CHARACTERIZATION, AND ANTIBODY DEVELOPMENT TO BENZALACETONE SYNTHASE FROM RASPBERRY FRUITS

p-Hydroxyphenylbutan-2-one, the characteristic aroma compound of raspb erries (Robes idaeus L.), is synthesized from p-coumaryl-coenzyme A an d malonyl-coenzyme A in a two-step reaction sequence that is catalyzed by benzalacetone synthase and benzalacetone reductase (W. Borejsza-Wy socki and C. Hrazdina [1994] Phytochemistry 35: 623-628). Benzalaceton e synthase condenses one malonate with p-coumarate to form the pathway intermediate p-hydroxyphenylbut-3-ene-2-one (p-hydroxybenzalacetone) in a reaction that is similar to those catalyzed by chalcone and stilb ene synthases. We have obtained an enzyme preparation from ripe raspbe rries that was preferentially enriched in benzalacetone synthase (appr oximately 170-fold) over chalcone synthase (approximately 14-fold) act ivity. This preparation was used to characterize benzalacetone synthas e and to develop polyclonal antibodies in rabbits. Benzalacetone synth ase showed similarity in its molecular properties to chalcone synthase but differed distinctly in its substrate specificity, response to 2-m ercaptoethanol and ethylene glycol, and induction in cell-suspension c ultures. The product of the enzyme, p-hydroxybenzalacetone, inhibited mycelial growth of the raspberry pathogen Phytophthora fragariae var r ubi at 250 mu M. We do not know whether the dual activity in the benza lacetone synthase preparation is the result of a bifunctional enzyme o r is caused by contamination with chalcone synthase that was also pres ent. The rapid induction of the enzyme in cell-suspension cultures upo n addition of yeast extract and the toxicity of its product, p-hydroxy benzalacetone, to phytopathogenic fungi also suggest that the pathway may be part of a plant defense response.