Comparative Biochemical Research On Milks From Half-lactating And Total-lactating Yaks

Yaks are mainly distributed in the Qinghai-Tibetan Plateau region of China and have various economic values such as milk,meat and draft,and are one of the important daily production resources for the plateau herders.The main objective of this study was to investigate the functional changes in the mammary glands of half-lactating(HL)yaks by comparing the composition characteristics of milk of HL yaks with that of totallactating(TL)yaks.The whole milk of 34 HL yaks and 67 total-lactating yaks was collected in Hongyuan county,China,and was compared in terms of conventional fractions,protein composition,contents of two whey proteins,proteomics and metabolomics.The main contents and results were as follows.(1)Comparison of milk conventional fractions and protein composition between HL and TL yaks.Whole milk samples from HL yaks(n=34)and TL yaks(n=67)were compared for conventional fractions,milk protein composition and whey protein quantification.The results showed that the daily milk yield of HL yaks was only 53% of that of TL yaks,and the electrical conductivity,milk fat and milk protein concentrations were higher than those of TL yaks;the proportions and contents of lactoferrin(LF)andβ-lactoglobulin(β-LG)were higher than those of TL yaks,the proportions and contents of α-lactalbumin(α-LA)were lower than those of TL yaks,and no differences were observed in the lactose content and casein(CN)proportions.The results indicated that the milk composition of HL yaks differed from that of TL yaks and had the lactation characteristics of cows with naturally prolonged lactation.(2)Comparative proteomics of milk between HL and TL yaks.Proteomics analysis of skim milk samples from HL yaks(n=15)and TL yaks(n=15)was performed using i TRAQ technology.The results showed that a total of 767 proteins were identified.A total of 202 differentially expressed proteins were found in HL yak milk compared to TL yak milk,of which 109 proteins were up-regulated and 93 were down-regulated.The functional proteins such as vitamin D-binding protein,retinol-binding protein 4,lactotransferrin and serum transferrin were significantly up-regulated in HL yak milk.A number of hematogenous proteins were significantly up-regulated in HL yak milk,indicating that the permeability of mammary cells is altered and the pathways of carbohydrate metabolism,amino acid biosynthesis and fat digestion and absorption are different from those of TL yaks.It is presumed that HL yak mammary glands are in a degenerative stage.(3)Comparative metabolomics of milk between HL and TL yaks.Non-targeted metabolomic analysis of skim milk samples from HL yaks(n=34)and TL yaks(n=67)was performed by UHPLC-TOF-MS coupling technique.A total of 694 metabolites were detected,which could be classified into 39 major groups.A total of 324 differential metabolites were found in TL yak and HL yak milk,of which 176 metabolites were significantly down-regulated and 148 up-regulated in HL yak milk.This study characterized the metabolite profile of yak skim milk and provided important information for scientific research on yak lactation;the differential metabolites in HL yak milk showed altered mammary gland tight junction permeability and significant differences in glucose metabolism from TL yaks.