Genetic variants at the H subunit of the lactate dehydrogenase protein in milk of Maiwa and Jiulong yak

M. Yongjiang, Zh. Yucai, Zh. Guanghui, P. Xianwen and J. Mingfeng

Department of Animal Science, Southwest Nationalities College, Chengdu 610041, Sichuan, P.R. China

Summary

The isozymes of the lactate dehydrogenase (LDH) in the skim milk of 109 Maiwa and 100 Jiulong yak were assayed by polyacrylamide gel electrophoresis (PAGE). Usually one band (LDH₁₎ was observed for each sample. However, four genetic variants of LDH₁ were detected and named A, B, C and D according to their electrophoretic mobilities. The D variant was only observed in the Jiulong yak. The C variant was the commonest within the two breeds. There was a significant difference in frequencies of LDH₁ phenotypes between the Maiwa and the Jiulong yak.

Keywords: Isozyme, lactate dehydrogenase, milk, yak

Introduction

Lactate dehydrogenase (LDH) is an oxidoreductase involved in glycolysis in tissues of animals. LDH usually exhibits five types of isozymes (LDH₁ to LDH_5). Amano et al. (1990) reported the presence of phenotypes of LDH₁ in red cells of Tibetan yak. Two genetic variants of LDH₁ were also identified in red cells of Qinghai yak and in plasma of Maiwa yak (Zhang et al. 1994; Zhong et al. 1998). Zheng and Chen (1997) reported three genetic variants (A, B and C) of LDH₁ in serum of the Tibetan yak. Moreover, the LDH isozyme patterns of serum between yak with A or B variants and yak with C variant are also different (Zheng and Chen 1997). In view of the involvement of LDH in glycolysis under anaerobic conditions and the hypoxic habitat of yak, genetic variants of LDH₁ might have physiological significances.

The purpose of this study was to analyse the LDH isozymes in milk of Maiwa yak and Jiulong yak to get a better understanding of biochemical properties and genetic variation of LDH in yak.

Materials and methods

One hundred and nine Maiwa yak, 15 hybrids of male Tibetan Yellow cattle × female Maiwa yak (Hongyuan County), 100 Jiulong yak, 15 Jiulong Yellow cattle, and 8 hybrids of Jiulong Yellow cattle × Jiulong yak were sampled by collecting approximately 30 ml of morning milk from each individual cow. These samples were frozen and then transferred to laboratory for analysis. The phenotypes of LDH isozymes in the skim milk were assayed after gel electrophoresis on a 6.5% PAGE as reported by Zheng and Chen (1997).

Results and discussion

LDH isozymes pattern in the milk of yak, Yellow cattle, and their hybrids show the LDH₁ band (Figure 1), in agreement with our previous results on the Maiwa yak (Zheng and Han 1997). LDH is widely distributed in tissues and fluid of human and animals. The main source of LDH in milk is white cells and epithelial cells of mammary gland. The relative activity of LDH₁ is 93.50 ± 2.45% in normal bovine milk, 70.90 ± 0.82% in mammary gland tissue, and 56.40 ± 6.21% in white cells (Wang and Zou 1995). LDH₂–LDH₅ isozymes were not noticed even after long period of staining of the gels.

1 to 10 represented 10 milk samples of Jiulong yak. The phenotypes of LDH1 were: 3-B, 4-A and 6-D; others are C. 6.5% PAGE gel, 10 ml skim milk were loaded per well.

Figure 1. Electrophoresis of LDH isozymes in yak milk.

LDH₁ shows polymorphism in yak and its hybrids. It includes A, B, C and D phenotypes, named according to their mobilities from fast to slow after gels electrophoresis (Figure 1). The phenotypic frequencies are listed in Table 1. A, B and C phenotypes of LDH₁ were found in both Maiwa and Jiulong yak. LDH₁-C was predominant in both breeds. Only very slight difference of mobility existed between LDH₁-A and LDH₁-B variants. In four milk samples of Jiulong yak, a band (classified temporarily as D) with slower mobility than the C phenotype was observed. It is close to the mobility expected for LDH₂ or LDH₃ bands. However, we think it is a new genetic variant of LDH₁ in regards to its relative high enzymatic activity similar to that the ones observed at other variants of LDH_1.

Table 1. Phenotypes of LDH₁ in milk of yak, Yellow cattle and their hybrids.

There was a significant difference in distribution of the phenotypic frequencies of LDH₁ variants between Maiwa yak and Jiulong yak (Table 1, P<0.05). Also, a special LDH isozyme pattern with a frequency of 5% was detected in the milk of Jiulong yak (data not shown). It consists of two to five narrow bands with the first band migrating approximately to the same distance than the LDH₁-A or the LDH₁-B band. Also, the second and the third band of this pattern have the highest enzymatic activities as indicated by their stronger intensities. Whether these bands are identical or related to any of the LDH₁ to LDH₅ bands previously identified is unknown. In a separate study, we had identified 3 to 4 LDH isozyme bands in the colostrums of Maiwa yak with the LDH₁ bands being the one showing the highest enzymatic activity (Zheng and Han 1997).

Our findings regarding genetic polymorphisms at LDH₁ are identical with that of Zhang et al. (1994) and Zhong et al. (1998), who have identified in Maiwa yak, Amano et al. (1990), who have identified in Tibetan yak, Zhang et al. (1994) and Zhong et al. (1998), who have identified in Qinghai yak. LDH₁-C phenotype frequency of Jiulong yak (53%) is significantly lower than in Maiwa yak (approximately 68.9% to 86.7%). Differences in frequencies of LDH₁ variants between Maiwa and Jiulong yak support their classification in two distinct breeds. Such distinction is also supported by blood protein polymorphism data (Tu et al. 1997).

Acknowledgments

This work was supported with funds from the National Natural Science Foundation of China, and Science Committee of Sichuan Province.

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