The intestinal absorption of small peptides was investigated in rats under unrestrained conditions. The peptide utilized in the experiment was hydrolysate of egg white protein with an average molecular weight of about 350 and containing less than 10% of free amino acids. We compared the intestinal absorption of these small peptides with that of an amino acid mixture with the same small peptide amino acid composition by determining the concentration of individual amino acids in portal blood after a single administration of a nitrogen source. The absorptive intensity of each amino acid was calculated from its rate of elevation in the portal blood; it was higher in the small peptides. The proportion of the amount of each amino acid absorbed in portal blood from small peptides was much more like the composition of the administered amino acids than like that from the amino acid mixture. Among the amino acids administered in the mixture, some amino acids such as L-tyrosine, L-threonine, L-serine and L-histidine increased more slowly in the blood than others. These results suggested that the small peptide formula was utilized more effectively than the amino acid mixture and had higher nutritive value.

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1. Critical studies on the distribution of NT-methylhistidine (3-methylhistidine; Me-His) among organs and tissues in adult rats are reported. Adult rats contained 46.5 ± 3.6 mg Me-His/kg body-weight. Almost 90% of the Me-His in the body was recovered from skeletal muscle. These results support the hypothesis that fractional catabolic rates of myosin and actin in skeletal muscle can be estimated by measuring urinary excretion of Me-His.2. Dietary protein level did not affect the total amount of Me-His in the body. However, urinary excretion of Me-His increased as dietary protein intake was increased.3. From these results it was concluded that fractional catabolic rates of myosin and actin increase as dietary protein intake increases.

I . Distribution of N'-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 is 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 (Young er Funabiki et al. 1976).Of the Me-His present in foodstuffs 94 yo was tentatively assumed to be excreted into urine.Estimation of synthetic and catabolic rates of skeletal muscle proteins of domestic animals is very important for studying whole-body protein metabolism.Perry (1974) estimated synthetic and catabolic rates of muscle proteins of piglet. Buttery et al. (1975) showed that the fractional turnover rate of muscle protein in sheep is 1.7%/d (half-life 40.7 d). However, there have been no reports of studies of the turnover rates of myofibrillar proteins of skeletal muscle in cattle.The turnover rate of myofibrillar proteins is often calculated from urinary excretion of N7-methylhistidine (3-methylhistidine; Me-His) in the instance of small animals, as well as man (Young et al. 1972;Long et al. 1975;Haverberg, Deckelbaum et al. 1975;Funabiki et al. 1976;Nishizawa et al. 1977; Nishizawa, Shimbo et al.