The effects of administration of a methionine-free amino acid solution (AO-90) via total parenteral nutrition on the tumor and liver vitamin B₁₂ dependent methionine synthase activity in Yoshida sarcoma-bearing rats

AO-90 is a methionine (Met)-free intravenous amino acid solution that is intended for use in cancer therapy via total parenteral nutrition (TPN) in combination with 5 fluorouracil (5-FU) and mitomycin C. It has been reported that AO-90 enhances the anti-tumor effect of 5-FU by biochemical modulation. This potentiation by AO-90 may start with the reduction of Met levels in the tumor. This reduction will increase the activity of Met synthase (EC2.1.1.13:MS) which forms Met from homocysteine. The conversion of homocysteine into Met is coupled with folic acid metabolism. Folic acid metabolism is activated by MS and as a result the methylenetetrahydrofolate (CH₂FH₄) level in the tumor will increase. It has been reported that the CH₂FH₄ level in the tumor and the formation of a ternary complex (CH₂FH₄:FdUMP:TS) which leads to inhibition of thymidylate synthase (TS) are closely related. We have already reported that the administration of AO- 90 to Yoshida sarcoma-bearing rats decreased the serum Met level and increased the CH₂FH₄ level in tumors. In the present study, we examined the change in the tumor and liver MS activities in Yoshida sarcoma bearing rats administered AO-90 via TPN. Twelve male Donryu rats were inoculated with 1 x 10⁴ Yoshida sarcoma cells. Three days after inoculation, the rats were divided into 3 treatment groups and were administered a TPN solution for 5 days. The AO-90 group received a solution containing AO-90, glucose, electrolytes and vitamins via TPN at a dose of 247 kcal/kg/day. The Pan-S group received the same as the AO-90 group except that a commercially available amino acid solution (Pan-Amin S) containing Met was substituted in place of AO-90. The N-free group received the same as the AO-90 group except that distilled water was substituted in place of AO-90. The administration doses of the Pan-S group and the N-free group were 250 kcal/kg/day and 210 kcal/kg/day respectively. On the 1st day of TPN, the administration dose was reduced to half the full dosage in each group. On day 5 of TPN, the rats were killed and the livers and tumors were removed for measurement of Met levels and MS activities. The tumor MS activity in the AO-90 group (11.97±0.50 pmol/min/mg protein) was significantly higher (p<0.05) than in the Pan-S group and N-free group (5.57±2.94 pmol/min/mg protein and 6.03±4.01 pmol/min/mg protein respectively). The tumor Met level in the AO-90 group (71.0± 31.2 nmol/g) was significantly lower (p<0.05) than in the Pan-S group (132.0±31.1 nmol/g). On the other hand, there was no significant difference in Met levels or MS activities in the liver among the three groups. These results demonstrate that the administration of AO-90 via TPN decreases the Met level and enhances MS activity in the tumor.