Sulfur amino acid metabolism in the whole body and mammary gland of the lactating Saanen goat

Five multiparous Saanen goats in late lactation were infused with S-35-cyst eine into the mammary gland via the external pudic artery. A further 2 goat s were infused with S-35-methionine via the same artery and later with S-35 -methionine into the jugular vein. Total uptake of cysteine from the arteri al blood supply by the mammary gland was approximately 6% of the S-35-cyste ine flux past the gland, whereas uptake of methionine was 30-40%. Total mam mary uptake of cysteine was also lower than that of methionine when express ed as a percentage of whole body utilisation (6.5 and 14%, respectively). T he uptake from the blood did not account for output in the milk for either cysteine or methionine. Both amino acids were highly conserved by the gland as shown by little release of any degraded constitutive protein amino acid s and no evidence of oxidation products of either cysteine or methionine be ing released into the blood. Comparison of S-35 activity in the milk from t he infused and non-infused sides of the gland showed up to 10% trans-sulfur ation of methionine to cysteine within the gland, none of which was exporte d in the venous drainage. Total ATP production by one side of the gland was 12.1 mol/day or 13 mmol/min.kg mammary tissue, of which 15% was required f or gland protein synthesis. The experimental measurements from both the cys teine and methionine infusions were used to solve a model of gland amino ac id uptake and partitioning. Modelling radioactivity of both amino acids in the blood, intracellular free pool, and milk protein suggested that a singl e intracellular pool cannot be the only source of amino acid for protein sy nthesis. The model also provides support for the hypothesis that a signific ant proportion of the uptake of at least some amino acids by the mammary gl and is from intracellular hydrolysis of extracellularly derived peptides.