Sesquiterpene α-farnesene synthase:: Partial purification, characterization, and activity in relation to superficial scald development in apples

To decipher the relation between alpha-farnesene metabolism and the development of superficial scald in apples, trans,trans-alpha-farnesene synthase, the enzyme that catalyzes the conversion of farnesyl pyrophosphate to alpha-farnesene, was partially purified from skin tissue of 'Delicious' apples (Malus xdomestica Borkh.) and characterized. Total and specific activities of the enzyme were higher in the cytosolic fraction than in membrane fractions. alpha-Farnesene synthase was purified 70-fold from the cytosolic fraction by ion exchange chromatography and gel permeation, and the native molecular weight was estimated to be 108,000, The enzyme had optimal activity at a pH of 5.6 and absolutely required a divalent metal ion such as Mg2+ or Mn2+ for activity. It exhibited allosteric kinetics, S-(0.5) for farnesyl pyrophosphate being 84 +/- 18 mu mol.L-1, and a Hill coefficient (n(H)) of 2.9, indicating the number of subunits to be two or three. Enzyme activity was highest between 10 and 20 degrees C, while 50% of the maximal activity was retained at 0 degrees C. In vivo alpha-farnesene synthase activity was minimal at harvest, then increased rapidly during 16 weeks storage in air at 0 degrees C, and decreased during further:storage. Activity of alpha-farnesene synthase, alpha-farnesene content, and conjugated triene alcohol (the putative scald-causing oxidation product of alpha-farnesene) content in skin tissue were not correlated to the inherent nature of scald susceptibility or resistance in 11 apple cultivars tested.