Effects Of The Proportion Of Peptide Binding And Free Amino Acids On Casein Synthesis In The Mammary Gland Explants

The study confirmed that circulating peptides can partially replace the free amino acids as thesource of casein protein synthesis. Some scholars have estimated that2%-16%ofphenylalanine,12%-19%of lysine and17%of methionine came from the small peptide inthe blood of casein synthesis, meanwhile Leucine and valine almost came from free aminoacids. There are different transporters for small peptides and free amino acids to enter theorganization. The transporters for small peptides and free amino acids saturated, and thusmammary tissue achieved the highest expression of casein. The proportion of small peptidesand free amino acids to highest casein expression remains controversial.This study cultured mammary gland explants in vitro, culture medium conteined all theessential amino acids, Meanwhile Met-Met and Arg-His-Ile insteaded of free amino acids indifferent proportions (0%,5%,10%,15%,20%,40%,60%,80%,100%) respectively.Normal medium cultured mammary gland explants2days, starvation medium culturedmammary gland explants1day and then treatment medium cultured mammary gland explants2days in vitro. In the end of the research, we examined casein, peptide transporter2andaminopeptidase using RT-PCR and Western-Blot.The results of this study show that:(1) Met-Met significantly promoted the casein in themRNA expression and protein expression (P<0.05), that Met-Met can be as the source of theamino acid to be used by the mammary tissue. However, Arg-His-Ile improved the mRNAexpression and protein expression of casein in mammary gland explants, the difference is notsignificant (P>0.05), showed that Arg-His-Ile can’t increase the expression of casein.(2)When FMet: Met-Met=80-85:20-15, the mRNA expression and protein expression of caseinwere in the peak, and then increasing the proportion of free methionine and Met-Met, themRNA expression and protein expression of casein expressed a trend of declining. Thatsuggested that only appropriate proportion can improve the yield of milk.(3) Met-Met andHis-Arg-Ile promoted the expression of PepT2(P<0.05). Circulating peptides can be transported into the mammary tissue by the PepT2.