Amino Acids Supply and Milk Protein in Lactating Dairy Cattle

摘要

The sufficient and well-balanced amino acids (AA) are extremely important in ruminant nutrition. Both dietary protein and AA supply have effects on milk protein content and yield. Methionine, isoleucine, leucine and valine have been identified as the limiting AAs in most cases. Protection of methionine from rumen degradation by physical or chemical techniques may improve the bioavailability of methionine. The AA which is short for milk protein synthesis appears to activate their withdrawal from the peptide-bound amino acids (PBAA) pool. It has been found that the molecular bases for the uptake of intact oligopeptide into mammary gland epithelial cells are the apically located H/peptide cotransporters, peptide transporter 1 and 2. Not only amount of dietary AA, but also does AA balance have a great influence on milk protein synthesis. In some cases, dietary AA profiles are more important than crud protein content for milk protein synthesis in mammary gland. Appropriate ratio of essential AA can increase the synthesis of milk protein and improved AA balance is good for enhancing milk protein yield and dairy cow performance. There exist many models to predict AA requirement for milk protein yield. However, the AA metabolism in the mammary gland is regulated by various factors such as dietary ingredient and AA composition, feed additive, rumen function, etc. On the other hand, the expression of milk protein genes is regulated at multiple steps including transcription, posttranscriptional processing, nuclear export, stability, and translation of mature mRNA molecules into protein. Whereas, the translation step itself is regulated by a diverse array of mechanisms that act not only on initiation. Massive production of milk proteins in the mammary gland could be regulated at the initiation step of translation during the lactation period. There is increasing recognition of the need to understand the role of AA as signaling molecules in the regulation of milk protein synthesis.