Utilisation of methionine by the mammary gland of the lactating goat
J. Lee, B.P. Treloar, B.R. Sinclair, C.G. Prosser, S.R. Davis and P.M.
Harris
AgResearch, Private Bag 11-008, Palmerston North, New Zealand.
Proceedings of the New Zealand Society of Animal Production, 1996,
56: 53-57
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±2.7 % and 31.5±2.3 %, which is 11.5% and 13.5% of the whole
body entry rate. The relative plateau methionine specific radioactivities
(SRA; dpm/nmol methionine) at 9 hours in blood (612±32; 494±27), milk
casein (449±2, 229±13) and individual whey proteins (eg þ-lactoglobulin;
830±50; 320±23), together with the rates of approach to plateau for the SRA
of each pool, indicated that casein and whey proteins derive the methionine
used in their synthesis from different precursor pools. The rise in casein -
and þ-lactoglobulin - bound methionine specific radioactivity indicated half-
livesí of 5.9±0.54 and 4.4 ±0.13 hours respectively.
Keywords: NZSAPAB;
transsulphuration; milk protein; net uptake.
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