反芻動物における非蛋白態窒素化合物の利用-4-牛乳中への移行

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The utilization of urea-N in the synthesis of N components of milk has been studied. In the present experiment (exp. III), in which 15N-urea was administered orally to a cow (as for experimental details, see the former report3)), transference of 15N into the different N components of milk has been measured with the lapse of time (Table 1).In exp. III, 12.9% of 15N was transfered into milk in 6 days after the administration (12.7% in 5.5 days). On the other hand, it was 8.97% in 5.5 days in our former experiment1)) (exp. II). The difference between two experiments may be explained by the difference of contents of protein and carbohydrate between each ration (Table 2).By the way, in the other studies7, 8, 15), it has been proposed that NH3 absorbed through the rumen wall may be used for the synthesis of some non-essential amino acids. Now, Fig. 1 presents the semi-log curve of 15N atom-% excess in the rest-N fraction of milk in exp. III, and the decreasing part of it can be decomposed into two straight lines. This means that some substances labeled with 15N in this fraction may be originated from the different two N pools in the body, and that the fractional turnover rate of each N pool is calculated as 0.248h-1 and 0.023h-1 independently. In Fig. 2, 15N atom-% excess of the protein-fraction and of casein are semi-log plotted. The straight line in it shows that casein is synthesized from N compounds belonging to sole N pool, fractional turnover rate of which is calculated as 0.023 h-1. As casein is known to be synthesized from free amino acids in the blood, the N pool that turns over at the rate of 0.023 per an hour is considered to comtain free amino acids, and therefore, one of the N compounds which enter into the rest-N fraction of milk could also be amino acids. The turnover rate of 0.023 h-1 is in the same magnitude as the rate of flowing out of microbial 15N from the reticulo-rumen, that has already been obtained as 0.029h-, in exp. III. This eems to suggest that the increase of excess 15N in the free amino acids pool is related directly to the outflow of microbial 15N from the rumen. In exp. III, the decreasing rate of NH3-15N in the rumen has been obtained as 0.864 h-1, of which 0.463 h-1 ows to be utilized by bacteria, and 0. 124 h-1 ows to flow out toward the omasum2). The rest 0.277 h-1 corresponds to the rate of absorption of NH3-N through the rumen wall, and this value is nearly the same as another fractional turnover rate 0.248h-1, obtained from Fig. 1. This seems to suggest that another sort of N compounds which enter into the rest-N fraction is associated with the absorption of NH3 through the rumen wall. These substances, however, cannot be amino acids, because no influence is observed in 15N atom-% excess ofcasein (Fig. 2). Other investigators observed higher 15N atom-% excess in the amide fraction both of blood serum protein and of milk at once after the administration of NPN-compounds labeled with 15N. Recently, HOSHINO et al.11) reported that glutamate that is synthesized by the reductive amination of alpha-ketoglutamate will be further converted to glutamine in the rumen mucosa. These facts suggest that those N compounds associated with the absorption of NH3 may be principally amides such as glutamine.These results confirm the governmental role played by rumen microorganisms in the net utilization of NPN-compounds in ruminants.
社団法人日本畜産学会の論文

反芻動物における非蛋白態窒素化合物の利用-4-牛乳中への移行

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