In vitro oxidation of branched chain amino acids by porcine mammary tissue

Six (three each of parity 1 and 2) lactating sows (d 10 to 17 of lactation) were used for an in vitro study to determine CO2 production from individual branched-chain amino acids. The first and second productive glands on one side of each sow were biopsied to collect mammary secretory tissue. Uniformly C-14-labeled L-isoleucine, L-leucine, and L-valine were included in the medium at 5 mu L (100 mu Ci/mL) for a 1-h incubation with mammary tissue. Carbon dioxide production rates as percentages of the C-14-labeled amino acid metabolized by the mammary tissue were 2.57, 1.86, and 4.07% for isoleucine, leucine, and valine, respectively (P<.03). Although not different (P<.16), isoleucine appeared to have numerically the greatest uptake rate (1.11 vs .91 and .89 pmol/mg.h leucine and valine, respectively). These results indicate that, in the lactating sow mammary gland, valine has the greatest oxidation rate of the branched-chain amino acids. The branched-chain amino acid content of the diet fed to sows prior to biopsy may have created large pools of branched chain amino acids in the tissues, diluting the isotope uptake and utilization by the mammary gland. Of the previously reported 30 to 80% excess uptake of the branch chain amino acids by the mammary gland beyond secretion as milk protein, only a small fraction appears to be utilized strictly as an energy source. These data suggest that the branched chain amino acids may play a role as C and N donors for synthesis of nonessential amino acids, lactose, or lipid. (C) 1998 Elsevier Science Inc.