ISOTOPIC ENRICHMENT KINETICS OF MILK NITROGENOUS FRACTIONS IN GOATS RECEIVING A SINGLE INTRAVENOUS-INJECTION OF EITHER L-(U-C-14)-LEUCINE OR L-(1-C-13)-LEUCINE

Enrichment levels and kinetics of the milk nitrogenous fractions in ei ght goats receiving a single intravenous injection of either L-(1-C-13 )-LEU (500 mg/goat, 90% atom C-13) or L-(U-C-14)-LEU (0.045 mg/goat., 3.6 x 10(6) Bq) were studied. After tracer injection, the goats were m ilked out at 3, 5, 7, 9, 24, 32, 48, 56 and 72 h for C-13 and C-14 enr ichment measurements. In both experiments, a labelling peak was observ ed in all nitrogenous fractions [casein (CN), whey protein (WP), non p rotein (NP) fraction] three hours after the tracer injection, and afte r the peak an exponential decreasing curve was noticed, In the first e xperiment, the maximal C-13 enrichment of CN and WP was -7.27 +/- 2.23 delta parts per thousand C-13 VS. PDB (Pee Dee Belemnite) and -4.63 /- 2.53 delta parts per thousand vs. PDB, respectively. In the second experiment, the radioactivity values measured at the peak were 60.3 +/ - 3.4 Bq/mg C in CN and 55.3 +/- 9.9 Bq/mg C in WP. In spite of the di fference between the LEU doses in both experiments, the labelling kine tics of CN and WP were similar. The overall recovery of the tracer in milk proteins was similar in both experiments: 9.8% vs. 11.6% for C-13 and C-14, respectively. In contrast, a substantial difference in the C-13 and C-14 Skim milk labelling kinetics was observed. The overall C -13 recovery in the NP fraction averaged 8% while it was close to 1% i n the C-14 experiment. These observations indicate that a high dose of intravenously injected C-13-LEU may induce modifications in the mamma ry utilisation of free plasma LEU. The level of C-14 enrichment in cas ein (at peak time point) was tested in monogastric animals. The radioa ctivity recovery in stomach contents of the rats was between 32% and 6 9% and in ileum contents between 0.2% and 0.4% of the ingested dose. I n portal plasma, radioactivity was between 10 and 40 Bq/ml. In all ani mals, radioactivity in the liver was approximately 20% and total radio activity recovered in stomach contents, ileum contents and liver was b etween 53% and 93%. This study shows the possible utilization of these radioactive goat milk proteins for further nutritional studies in mon ogastric animals.