Sesquiterpene alpha-farnesene synthase: partial purification, characterization, and activity in relation to superficial scald development in apples.

摘要

To decipher the relation between alpha-farnesene metabolism and the development of superficial scald in apples, trans, trans-alpha-farnesene synthase, the enzyme that catalyses the conversion of farnesyl pyrophosphate to alpha-farnesene, was partially purified from skin tissue of Delicious apples 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 byion 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±18μmol/litre, and a Hill coefficient (nH) of 2.9, indicating the number of subunits to be 2 or 3. Enzyme activity was highest between 10 and 20deg C, while 50% of the maximal activity was retainedat 0deg C. In vivo alpha-farnesene synthase activity was minimal at harvest, then increased rapidly during 16 weeks' storage in air at 0deg C, and decreased during further storage. Activity of alpha-farnesene synthase, alpha-farnesene content, and conjugatedtriene alcohol (the putative scald-causing oxidation production of alpha-farnesene) content in skin tissue were not correlated to the inherent nature of scald susceptibility or resistance in 11 apple cultivars tested.