Testicular injury to rats fed on soybean protein-based vitamin B ₁₂-deficient diet can be reduced by methionine supplementation

We have previously reported that rats fed on a vitamin B₁₂ (B₁₂)-deficient diet containing 180 g soybean protein per kg diet showed marked histologic damage in their testes. In this paper, we report the effect of B₁₂-deficiency on B₁₂-dependent methionine synthase in the rats' testes and the effect of methionine supplementation of the diet on testicular damage. Rats were fed the soybean protein-based B ₁₂-deficient diet for 120 d. We confirmed that those rats were in serious B₁₂-deficiency by measuring urinary methylmalonic acid excretion and B₁₂ content in tissues. Methionine synthase activity in the testis of the B₁₂-deficient rats was less than 2% of that in B₁₂-supplemented (control) rats. To complement disrupted methionine biosynthesis, methionine was supplied in the diet. A supplement of 5 g D,L-methionine per kg diet to the B₁₂-deficient diet did not affect urinary methylmalonic acid excretion of B₁₂-deficient rats. The testicular histology of rats fed the methionine-supplemented B ₁₂-deficient diet was almost indistinguishable from that of control rats. Thus, we conclude that the lowered testicular methionine synthase activity is the primary cause of the histologic damage due to B₁₂-deficiency and that methionine supplementation to the diet can reduce the damage. These findings would indicate the importance of the methionine synthase activity, especially for testicular function.