Lower methionine/cystine ratio in low-protein diet improves animal reproductive performance by modulating methionine cycle

Background As the extend of low-protein diets in many countries including China, more researches of amino acid nutrition (AA) have been carried out. But the ideal AA pattern, especially the reasonable proportion of amino acids such as the desired methionine (Met): cystine (Cys) ratios are not yet clear. Objectives In this article, the experiments of low-protein diet with varying ratios of Met:Cys (low ratio of 1:3, medium ratio of 1:1, and high ratio of 3:1) were conducted to investigate the effect on mice growth and reproductive performance. Results The result indicated that the lower Met:Cys ratio improves reproductive performance in male mice, but the growth performance were no change in all groups. Meanwhile, the abnormality rate of sperm increased as the ratio of methionine and cystine increased. Quantitative analysis showed that the low Met:Cys ratio has obviously decreased the expression of Bax protein and the concentrations of testosterone in male serum, but Prm2, Pgk2, Bcl-2, Bak1, and AR gene were made no difference. Furthermore, different Met:Cys ratios have significant effects on the modulated methionine cycle by increasing the expressions of MAT2B, GNMT, and BHMT in low Met:Cys ratio. On the other hand, it was also found that there were increases in GSH-Px enzyme activities and decreases in MDA levels in male serum. Conclusions Overall, the present study showed that a dietary lower Met:Cys ratio in a low-protein diet had a positive influence on the reproductive performance of male animal through obviously improving sperm quality and antioxidant capacity, and inhibiting apoptosis. And the study provided new pieces of evidence to re-evaluate the role of precise sulfur amino acid nutrition in a low-protein diet.