Branched-chain fatty acids of the milk fat of goats were analyzed by high resolution gas chromatography-mass spectrometry. Iso-and anteiso-acids predominated, but a range of other monomethyl-branched components, mostly with methyl-substitution on carbons 4 and 6, was present. Analysis of the milk fat of cows revealed the presence of iso-and anteiso-fatty acid; other monomethyl-substituted fatty acids, as found in the milk fat of the goat, were virtually absent. Only a trace amount of 6-methylhexadecanoate was detected. The difference between goats and cows in the effectiveness with which these animals metabolize propionyl-CoA and methylmalonyl-CoA is discussed.

A lactating mammary gland of a goat was perfused for 9 h in the presence of [U-14 C]-L-threonine and received adequate quantities of glucose, acetate and amino acids. Two lactating sheep udders were likewise perfused in the presence of [U-14 C]-Lphenylalanine: the plasma levels of phenylalanine in the first of these experiments were 4 times higher than in the second.In the P 4 C]threonine experiment, 4 % of the casein and 0-4 % of the expired CO 2 were derived from threonine; 85 % of the threonine and 1-6 % of the glycine residues in casein originated from plasma threonine. Small 14 C levels were found in glutamic acid, aspartic acid and serine of casein. The relative specific activities amongst the casein amino acids and the appearance of appreciable labelling in plasma glycine are consistent with the view that threonine is split by threonine aldolase.In the [ 14 C]phenylalanine experiments virtually no radioactivity was detected in CO 2 , lactose or citric acid, indicating that this substrate is not broken down by mammary tissue. In the second experiment, 96 % of the phenylalanine and 0-3 % of the tyrosine of casein originated from plasma phenylalanine. In the first experiment, a 30-fold higher 14 C incorporation into casein tyrosine relative to phenylalanine was observed. The possible significance of the phenylalanine concentration in the plasma on the degree of conversion of phenylalanine into tyrosine within the mammary gland is discussed.Most milk proteins are synthesized in the mammary gland from free amino acids. Studies on the uptake of free plasma amino acids by the lactating udder have been carried out by the technique of arterio-venous differences across the udder on cows (Verbeke & Peeters, 1965) and goats (Mepham & Linzell, 1966). In both species an important uptake of phenylalanine, tyrosine and threonine by the udder was observed. In the experiments on cows the mammary blood flow was not measured, but calculations were made on the basis of the differences observed, using average values from the literature for the blood flow through the udder. It was calculated that in the cow the uptake of these amino-acids was probably large enough to provide all the corresponding amino-acid residues in milk proteins. Mepham & Linzell (1966) measured arterio-venous differences in lactating goats, simultaneously estimating mammary blood flow by a direct method. These authors calculated that there was a good agreement between the uptake of phenylalanine, tyrosine and threonine and output of these amino acids in the milk.

SUMMARYSeven lactating goat mammary glands from 6 goats were perfused for several hours in the presence of [U-14C]L-leucine (4 experiments) or [2-3H; l-14C]DL-leucine (3 experiments) and received adequate quantities of glucose, acetate and amino acids. Radioactivity in casein was mainly recovered in leucine and 90% of casein leucine was derived from free plasma leucine. About 64% of the leucine molecules were used for casein synthesis. Up to 12% of the molecules were channelled into lipid synthesis, while the remaining (up to 24%) were metabolized to CO2. From the 3H/14C ratio of casein and casein leucine, it was calculated that 70–80% of the leucine molecules were reversibly transaminated before their incorporation into milk protein. However, only 4–8% of the plasma leucine molecules were transaminated during passage through the udder. Different pools for oxidation and for protein synthesis may be present in the goat mammary gland.

TWO lactating mammary glands excised from 2 goats were perfused for several hours in the presence of [U-14 C; 2,3-3 H]-L-valine and received adequate quantities of glucose, acetate and amino acids. In the synthesized milk 96 and 89 % respectively of the casein valine was derived from free plasma valine. Valine was extensively catabolized by mammary tissue, resulting in a considerable 14 CO 2 production and in the incorporation of 14 C into milk citric acid and to a lesser extent into casein aspartic acid and glutamic acid. About 30 % of the valine molecules which were taken up by the mammary gland were oxidized to CO 2 and 70 % were incorporated in casein as valine residues. About 10 % of the plasma valine molecules were reversibly transaminated during one passage through the udder. An important amount of radioactivity of plasma was present in unknown metabolites. Only 7 % of this activity was localized in isobutyrate. The radioactivity of total milk fat was very low. Mainly iso-14:0, iso-16:0 and 15:0 were labelled.