| Yak (Bos grunniens) is the only type of bovine which can live and reproduce at the altitude above 3,500 m with strong resistance to high altitude, dramatic temperature change, coldness and hypoxia. Myoglobin stores oxygen temporarily as oxymyoglobin and releases oxygen to meet the urgent need when muscles make strong contraction. For any certain OPP, Mb has a higher oxygen saturation degree than Hb, and oxygen molecules bound by Mb are 6-fold higher than those by Hb. So, myoglobin plays a key role of transporting and storing oxygen in muscle for the animals living on the plateau. In this paper, the cDNA sequence of yak myoglobin (Mb), as well as Mb concentration and oxidation related to lipid oxidation in yak skeletal and cardiac muscles, was investigated and compared with bovine Mb in order to understand the mechanism of yak's adaptation to hypoxia environment. Comparison of Mb cDNA sequence in yak and bovine revealed that there were only two different bases (cac→cat and gcc→gct, at the 89th and 91th amino acids) but the amino acid sequence had no difference due to the degeneracy of codons, so, there were no difference on their protein sequences and structures. The concentration of yak Mb in skeletal muscles was 488.3 nmol/g and had no significant difference with that of bovine skeletal muscles (410.96nmol/g), however, the concentration of yak Mb in cardiac muscles was 823.4 nmol/g, being 61.2％(P＜0.05) higher than that of bovine cardiac muscles (510.7nmol/g). During storage at 4℃, the accumulations of metmyoglobin in skeletal and cardiac muscles were studied to compare yak and bovine myoglobin oxidation rate. At the end of the experiment, the accumulations of metmyoglobin in bovine and yak cardiac muscles were 23.3％and 17.0％, respectively, and there was no significant difference between them (P＞0.05). However, the accumulation of metmyoglobin in bovine skeletal muscles was 25.8％(P＞0.05) higher than that of yak, which meant that the oxidation rate of yak Mb in skeletal muscles was slower by 25.8％than that of bovine. To further understand the mechanism by which yaks protect their myoglobin from oxidation in hypoxia condition, lipid oxidation in both yak and bovine muscles were determined during storage. During the storage for 6 days, TBARS values of both yak and bovine muscles increased steadily with storage time, with this value in yak increasing from 0.021 on day 0 to 0.266 on day 6 at the increasing rate of 0.04 per day, while in bovine TBARS increased from 0.017 to 0.435 at the increasing rate of 0.07 per day, almost doubled to the value in yak (P＜0.05). The thiobarbituric acid-reactive substance (TBARS) value in yak skeletal muscles was slower by 63.5％(P＜0.05) than that of bovine. Our study demonstrated that yak's adaptation to hypoxia environment was not due to the change of the amino acid sequence thus protein structure of yak's myoglobin through evolution, but by the increase of its Mb concentration and the slower Mb and lipid oxidation rate, which meant that there was more myoglobin for yak to keep the abilty of binding oxygen to meet the needs of oxygen under the hypoxia environment.
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