3-Methylcrotonyl coenzyme A carboxylase is a component of the mitochondrial leucine catabolic pathway in plants

3-Methylcrotonyl-coenzyme A carboxylase (MCCase) is a mitochondrial biotin- containing enzyme whose metabolic function is not well understood in plants . In soybean (Glycine max) seedlings the organ-specific and developmentally induced changes in MCCase expression are regulated by mechanisms that cont rol the accumulation of MCCase mRNA and the activity of the enzyme. During soybean cotyledon development, when seed-storage proteins are degraded, leu cine (Leu) accumulation peaks transiently at 8 d after planting. The coinci dence between peak MCCase expression and the decline in Leu content provide s correlative evidence that MCCase is involved in the mitochondrial catabol ism of Leu. Direct evidence for this conclusion was obtained from radiotrac er metabolic studies using extracts from isolated mitochondria. These exper iments traced the metabolic fate of [U-C-14]Leu and (NaHCO3)-C-14, the latt er of which was incorporated into methylglutaconyl-coenzyme A (CoA) via MCC ase. These studies directly demonstrate that plant mitochondria can catabol ize Leu via the following scheme: Leu --> alpha-ketoisocaproate --> isovale ryl-CoA --> 3-methylcrotonyl-CoA --> 3-methylglutaconyl-CoA --> 3-hydroxy-3 -methylglutaryl-CoA --> acetoacetate + acetyl-CoA. These findings demonstra te For the first time, to our knowledge, that the enzymes responsible for L eu catabolism are present in plant mitochondria. We conclude that a primary metabolic role of MCCase in plants is the catabolism of Leu.