食米蛋白質限制胺基酸研究方法之檢討―在等能量、等氮量對飼育條件下以老鼠肝臟細胞發育情形為指標的研究方法

Rice supplies 50% dietary energy and 40% protein in Taiwan Area, and of course it is the most important cereal of the world since it is the staple food of more than half the human race, The nutritive value of rice protein can be improved significantly by supplementation with the first limiting amino acid L-lysine and the second limiging amino acid L-threonine were reported by Pecora and Rosenberg. The results of Pecora’s and Rosenberg's were based on the different level of amino acid supplementation on the rats fed ad libitum with low-protein high-rice diets (diet protein level were 5-6%). In order to shorten the distance between the test dietary protein level and the protein requirement of the rats, high-protein high-rice diet (dietary protein level was 11.3%, composed of 88.5% BPI-76 rice, 5% vegetable oil, 4% salt mixture, 1% vitmain premix and 1.5% fiber) was used as the basal diet and supplemented with 0.3. 0.4, 0.5, 0.6% L-lysine HCI, 0.4% L-lysine HCI+0.2% L-threonine and 0.6% L-lysine HCI+0.3 L-threonine respectively and non-essential amino acid mixture compose of L-aspartic acid, L-glutamic acid, L-alanine and glycine were added so far as to adjusted the 7 test diets isonitrogenously. Male rats of the sprague-Dawley strain from U.S. Naval Medical Research Unit No.2, weighing from 38 to 43 grams were divided into 7 groups of 6 animals each. The test animal in each dietary gropp were pair fed with 7 test diets respectively. Body weight change were measured. weekly and after 80 days isonitrogenous and isocaloric feeding the test rats were anesthetized with ether and exsanguinated. Plasma protein, hemoglonin and hematocrit were determined. The liver specimens were homogenized in ice-cold distilled water, Protein, RNA and DNA contents of the liver homogenates were determined by some modifications of the a1kaline hydrolysis procedure of Wannemacher et al. The results revealed that: (1) L-lysine HCI supplementation to basal high-rice diet remarkable improved body weight increase of the test rats (Table 3). The optimal supplementation of L-lysine HCI was 0.5% during the first 63 days of the test feeding while 0.4% became the optimal in the 64-80 day period. These figures are remarkab1e higher than that of the previous studies (4.6). It assumed that it might owed to the higher protein high-rice diet being used and the longer period being fed in present work than that of the previous studies. (2) 0.6% L-lysine HCI supplemented group showed less body weight increase than that of the 0.4% group. Nevertheless, in 0.6% L-lysine HCI+0.3% L-threonine supplemented group showed the highest body weight increase among the seven test dietary groups. These results indicate that excess L-lysine supplementation gives rise second limiting amino acid imbalance and which can be further balanced by the 0.3%L-threonine supplementation. Therefore L-threonine is the second limiting amino acid of rice demonstrated by the present isocaloric and isonitrogenous feeding method. (3) No considerable influence was observed by the supplemertation of L-lysine or L-lysine plus L-threonine on plasma protein level. Hemoglobin and hematocrit value were increased together with the increase amount of L-sysine HCI supplementation and showed maximum value by its 0.5% supplementation but its 0.6% supplementation become excess and both values significantly' dropped. This excess phenomenon could be corrected by the additional 0.3% L-threonine supplementation besides 0.6% L-lysine HCI and both valuss were enhanced to higher level in comparison with optimal 0.5% L-lysine HCI supplemented group. This can be considered as an another prove that L-threonine is the second limiting amino acid of rice protein. (4) 0.6% L-lysine HCI supplemented group showed expectionally the highest liver total DNA value among the seven test dietary groups. But Liver Protein/DNA and RNA/DNA value was remarkable, dropped in this group. Nevertheless, significantly higher Liver Protein/DNA and RNA/DNA value were observed when 0.3% L-threonine was added instead of sole 0.6%L-lysine HCI supplementation. Such results indicate that excess L-lysine supplementation may give rise metabolic disorder which can be corrected by the second limiting amino acid supplementation. Thus the role of L-threonine as the second limiting amino acid rice of, protein is definitely established.