A C-13 NUCLEAR-MAGNETIC-RESONANCE INVESTIGATION OF THE METABOLISM OF LEUCINE TO ISOAMYL ALCOHOL IN SACCHAROMYCES-CEREVISIAE

The metabolism of leucine to isoamyl alcohol in yeast was examined by C-13 nuclear magnetic resonance spectroscopy. The product of leucine t ransamination, alpha-ketoisocaproate had four potential routes to isoa myl alcohol. The first, via branched-chain alpha-keto acid dehydrogena se to isovaleryl-CoA with subsequent conversion to isovalerate by acyl -CoA hydrolase operates in wild-type cells where isovalerate appears t o be an end product. This pathway is not required for the synthesis of isoamyl alcohol because abolition of branched-chain alpha-keto acid d ehydrogenase activity in an lpd1 disruption mutant did not prevent the formation of isoamyl alcohol. A second possible route was via pyruvat e decarboxylase; however, elimination of pyruvate decarboxylase activi ty in a pdc1 pdc5 pdc6 triple mutant did not decrease the levels of is oamyl alcohol produced. A third route utilizes alpha-ketoisocaproate r eductase (a novel activity in Saccharomyces cerevisiae) but with no ro le in the formation of isoamyl alcohol from alpha-hydroxyisocaproate b ecause cell homogenates could not convert alpha-hydroxyisocaproate to isoamyl alcohol. The final possibility was that a pyruvate decarboxyla se-like enzyme encoded by YDL080c appears to be the major route of dec arboxylation of alpha-ketoisocaproate to isoamyl alcohol although disr uption of this gene reveals that at least one other unidentified decar boxylase can substitute to a minor extent.