The daily rates of synthesis of protein by the whole body and by the individual tissues were determined in two Hereford × Friesian heifers (236 kg and 263 kg live weight), and a dry Friesian cow (628 kg live weight).
The rate of whole-body protein synthesis (g protein/d) was estimated from the total flux through the blood of [3H]leucine and [3H]tyrosine following infusion at a constant rate for 8 h.
The fractional rates of protein synthesis (ks) in the tissues (g synthesized/d per g tissue protein) were obtained after slaughter of the animals at the end of the infusion period. The fractional rate of protein synthesis was calculated assuming that the specific radioactivity of free tyrosine in either the blood (to give ks, b) or the tissue homogenate (to give ks, h) defined closely the specific radioactivity of the amino acid precursor for protein synthesis. Total protein synthesis (As, b or As, h; g/d) in an individual tissue was calculated as the product of ks, b) (or ks, h) × protein content.
Based on the total leucine flux, i.e. without correction for oxidation, 1.6 kg protein were synthesized daily in the heifers; for the cow this value was 2.0 kg/d.
The sum of the daily total synthesis in the major tissues (muscle+bone+brain, gastrointestinal tract (GIT), liver, hide) gave values of 1.4–1.9 kg/d based on As, b, and 2.2–3.0 kg/d based on As, h.
The percentage contributions of the individual tissues to the total protein synthesis were similar in all three animals, for example based on As, h muscle was 12–16; carcass (muscle+bone+brain) 32–33; GIT 38–46; liver 7–8; skin 14–21%.
The contribution of muscle to total synthesis estimated from the leucine flux was 19–22%; this value is in agreement with those calculated on the same basis for other species.
The energy cost of protein synthesis was estimated to account for a maximum of 30% of heat production.
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