The overall objective of this proposal is to elucidate biochemical mechanisms underlying diseases of organic acid metabolism. Biochemical investigations of organic acid metabolism in patients with organic acidemia will be conducted in vivo in two directions. The first is aimed at clarification of the following steps in the metabolism of branched chain amino acids using 13C as a metabolic tracer: Conversion of methylmalonic acid semialdehyde to methylmalonyl CoA; mechanism of butanone formation; and stereospecific dehydrogenation of isobutyrl CoA. The second will employ gas chromatography-mass spectrometry and proton nuclear magnetic resonance to detect and characterize new organic acidemias and to resolve chemical ambiguities in patients with such conditions as Jamaican vomiting sickness, isovaleric acidemia and propionic acidemia by quantitative chemical approaches. Investigations in vitro will be conducted in juxtaposition to the in vivo work. Such investigations will include: development of a new method of assay for isovaleryl CoA dehydrogenease, the enzyme whose deficiency is probably responsible for the chemical abnormalities is isovaleric acidemia; biochemical characterization of isovaleryl CoA dehydrogenase; and investigation of the mechanism of formation of medium chain dicarboxylic acids in hypoglycin intoxication.