Induction of expression of branched-chain aminotransferase and alpha-keto acid dehydrogenase in rat tissues during lactation.

This study was designed to determine the effect of lactation and weaning on the gene expression of branched-chain aminotransaminase (BCAT) and branched-chain alpha-keto acid dehydrogenase (BCKD) in different tissues of the lactating rat. BCAT activity increased in mammary tissue during lactation and was 6-fold higher than in virgin rats. This increase was associated with an increase in protein levels measured by immunoblot analysis, and with an increase in BCAT mitochondrial (BCATm) mRNA concentration. Twenty-four hours after weaning, BCAT activity, protein concentration, and mRNA levels in the dam decreased. BCAT activity, protein enzyme levels, and BCATm mRNA concentration in muscle were higher in weaning rats than in lactating rats. BCAT cytosolic (BCATc) mRNA was not expressed in mammary tissue, and there was no BCATc enzyme detected by Western blot in any physiological state. Mammary tissue BCKD activity increased and was active (dephosphorylated) during the lactation period. The level of enzyme also increased and the mRNA level for the E2 subunit in mammary tissue was 10-fold higher than the virgin values. Hepatic enzyme activity increased during weaning, and this was associated with the protein level and with the mRNA level of the E2 subunit. Muscle BCKD activity and protein content were the lowest of all tissues, and the E2 subunit mRNA level was barely detected by Northern blot analysis. The results suggest gene regulation of the two main catabolic enzymes of the branched-chain amino acid metabolism during lactation.

Dietary protein level regulates expression of the mitochondrial branched-chain aminotransferase in rats.

The first step in the degradation of branched-chain amino acids (BCAA) is transamination catalyzed by the branched-chain aminotransferase (BCAT), which is located in extrahepatic tissues. Studies of the effect of dietary protein on BCAT activity have given contradictory results. Therefore, we established the levels of BCAT activity and mitochondrial BCAT (BCATm) mRNA expression in different organs and tissues of rats. We then determined the effect of different levels of dietary protein in well-nourished rats, the effect of feeding a 0.5% casein diet for 5 wk (protein-malnourished rats) and nutritional rehabilitation of these rats with different levels of dietary protein on BCAT activity and BCATm mRNA expression in selected tissues. Finally, the response of tissue BCAT activity and BCATm mRNA levels in rats fed a 10% casein diet and injected with glucagon (4 d) or hydrocortisone (7 d) was determined. The highest concentration of BCATm mRNA was found in stomach, followed by kidney, heart, muscle, brain, skin and lung. Low levels were found in intestine, and no BCATm mRNA was detectable in liver. Although BCAT activity was significantly higher in muscle, kidney and brain from rats adapted to consume a 50% casein diet for 7 h/d for 10 d than in rats fed 6, 18 or 35% casein diets, only muscle had significantly higher levels of BCATm mRNA. In protein-malnourished rats, BCAT activity and BCATm mRNA expression in kidney, muscle and heart were not different from those of rats with free access to an 18% casein diet. Nutritional rehabilitation of the protein-malnourished rats with 50% casein for 21 d significantly increased the BCAT activity and BCATm mRNA expression in muscle. Neither hydrocortisone nor glucagon injection affected BCAT activity or BCATm mRNA concentrations in rat kidney, muscle or heart. We conclude that the nutritional regulation of BCATm is extrahepatic, tissue specific and may involve transcriptional and post-translational mechanisms.