Nτ-Methylhistidine content of organs and tissues of cattle and an attempt to estimate fractional catabolic and synthetic rates of myofibrillar proteins of skeletal muscle during growth by measuring urinary output of Nτ-methylhistidine

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Abstract
1. Distribution ofNτ-methylhistidine (3-methylhistidine; Me-His) among organs and tissues in cattle was determined. From the amount of Me-His in skeletal muscle protein and daily urinary output of Me-His, fractional catabolic and synthetic rates of myofibrillar proteins of skeletal muscle during growth were calculated.2. More than 93.4% of the total Me-His (35.6 mg/kg body-weight) in the analysed cattle tissues occurred in skeletal muscle protein. The amount of Me-His in tissues other than skeletal muscle was relatively small. Daily urinary excretion of Me-His in the cattle which were fed on hay and concentrate was 135 mg at a growing stage of 217 kg body-weight and 145 mg at a stage of 312 kg. The Me-His content of foodstuffs was also carefully checked in the present investigation. Assuming that absorbed dietary Me-His in quantitatively excreted in the urine without delay, the contribution of Me-His in foodstuffs was calculated to be approximately 30 % of the urinary Me-His. Rumen protozoa and bacteria contained little Me-His.3. From these results, fractional catabolic and synthetic rates of myofibrillar proteins of the cattle at a growing stage of 217 kg body-weight were calculated to be 1.22%/d (half-life 56 d) and 2.73%/d, while these rates at a stage of 312 kg body-weight were 1.02%/d (half-life 67 d) and 1.51%/d respectively. These values were calculated on the same assumptions as those for the rat (Younget al.1972; Funabikiet al.1976). Of the Me-His present in foodstuffs 94% was tentatively assumed to be excreted into urine.

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