Ontogeny and subcellular localization of rat liver mitochondrial branched chain amino-acid aminotransferase

Branched chain amino-acid aminotransferase (BCAT) activity is present in fe tal liver but the developmental pattern of mitochondrial BCAT (BCATm) expre ssion in rat liver has not been studied. The aim of this study was to deter mine the activity, protein and mRNA concentration of BCATm in fetal and pos tnatal rat liver, and to localize this enzyme at the cellular and subcellul ar levels at both developmental stages. Maximal BCAT activity and BCATm mRN A expression occurred at 17 days' gestation in fetal rat liver and then dec lined significantly immediately after birth. This pattern was observed only in liver; rat heart showed a different developmental pattern. Fetal liver showed intense immunostaining to BCATm in the nuclei and mitochondria of he patic cells and blood cell precursors; in contrast, adult liver showed mild immunoreactivity located only in the mitochondria of hepatocytes. BCAT act ivity in isolated fetal liver nuclei was 0.64 mU.mg(-1) protein whereas it was undetectable in adult liver nuclei. By Western blot analysis the BCATm antibody recognized a 41-kDa protein in fetal liver nuclei, and proteins of 41 and 43 kDa in fetal liver supernatant. In adult rat liver supernatant, the BCATm antibody recognized only a 43-kDa protein; however, neither prote in was detected in adult rat liver nuclei. The appearance of the 41-kDa pro tein was associated with the presence of the highly active form of BCATm. T hese results suggest the existence of active and inactive forms of BCAT in rat liver.