The goat milk protein, a-lactalbumin, contains no methionine residues, hence if 35S label is supplied to the gland in the form of labelled methionine only, the appearance of 35S in a-lactalbumin gives a measure of transsulphuration of methionine to cysteine within the gland. Two lactating Saanen goats were surgically modified with catheters implanted in the mammary milk vein, femoral and external pudic mammary artery, and jugular vein to enable infusion of 35S-methionine, into either the whole body (jugular) or directly into the gland, and repetitive blood sampling. Specific radioactivity of methionine in the blood and milk proteins was determined at hourly intervals for 9 hours. Radioactivity of 35S was determined in a-lactalbumin when 35S-methionine was supplied directly to the gland via the mammary artery, but could not be detected when supplied via the jugular vein. Analysis of mammary tissue from the latter experiment did however show the appearance of 35S label in the intracellular cysteine pool which was derived from methionine. Both approaches suggested that transsulphuration within the gland, although small, may be important especially when supply of methionine to the gland is high. The fractional extraction of plasma methionine by the gland for two goats was 46.4
Proceedings of the New Zealand Society of Animal Production, Volume 56, , 53-57, 1996
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