Backwell F R, Bequette B J, Wilson D, Metcalf J A, Franklin M F, Beever D E, Lobley G E, MacRae J C
Rowett Research Institute, Aberdeen, United Kingdom.
Am J Physiol. 1996 Oct;271(4 Pt 2):R955-60. doi: 10.1152/ajpregu.1996.271.4.R955.
Precursors for milk protein synthesis have been examined in lactating dairy goats using arteriovenous difference and isotope kinetic techniques. Certain amino acids, such as phenylalanine and histidine, are taken up by the mammary gland in quantities that are insufficient to account for their output in milk protein. Some amino acids have been shown to be present in significant quantities (10-30% of total non-protein-bound amino acids) as peptides (< 1,500 Da) in the arterial supply to the mammary gland, although methodological considerations make it difficult to accurately assess the extent of their uptake across the tissue bed. Indirect evidence for the utilization of peptides for milk protein synthesis in vivo has been obtained, however, by examination of the kinetics of milk casein labeling during long-term (24 h) systemic infusion of [1-13C]phenylalanine and [1-13C]leucine. Comparison of plateau enrichments for blood, plasma, and casein indicate that, although, for leucine, the plasma free pool seems to provide all the leucine for milk protein synthesis, sources other than the labeled plasma free amino acids contribute phenylalanine (10-20%) for casein biosynthesis. These findings raise questions relating to the type and source of amino acid precursors used by tissues for protein synthesis.
利用动静脉差和同位素动力学技术，对泌乳期奶山羊乳蛋白合成的前体物质进行了研究。某些氨基酸，如苯丙氨酸和组氨酸，乳腺摄取的量不足以满足其在乳蛋白中的输出量。已表明某些氨基酸以肽（<1500Da）的形式大量存在于乳腺动脉供应中（占总非蛋白结合氨基酸的10%-30%），不过由于方法学上的考虑，难以准确评估它们跨组织床的摄取程度。然而，通过长期（24小时）全身输注[1-13C]苯丙氨酸和[1-13C]亮氨酸期间检测乳酪蛋白标记的动力学，已获得体内肽用于乳蛋白合成的间接证据。血液、血浆和酪蛋白的平台期富集度比较表明，虽然对于亮氨酸来说，血浆游离池似乎为乳蛋白合成提供了所有的亮氨酸，但除标记的血浆游离氨基酸外的其他来源为酪蛋白生物合成贡献了苯丙氨酸（10%-20%）。这些发现引发了与组织用于蛋白质合成的氨基酸前体的类型和来源相关的问题。
