Relationship between induced arginase activity and the level of free amino acids and polyamines in the excised embryonic axes of germinating soybean seeds

Abstract

Summary Supplying exogenous arginine to excised embryonic axes of germinating soybean (Glycine max L.) seeds resulted in increased arginase (EC 3.5.3.1) activity, which reaches a maximum with 50 mmol/L arginine in the incubation medium. The raised arginase activity appeared 24 h after the administration of arginine and remained high for at least another 30 h. In the presence of exogenous arginine, free amino acids sharply increased in the embryonic axes during the incubation period between 24 and 36 h. The amount of accumulated free amino acids after 36 h was 3 fold over the level of free amino acids in the absence of exogenous arginine. Among the free amino acids accumulating during the 48 h incubation with arginine, asparagine accumulated the most. In contrast, the administration of exogenous arginine did not effect a change in free polyamine (putrescine + spermidine + spermine) levels. Additional nitrogen sources, such as glutamine and NH4Cl, had no effect on the levels of polyamines during the 48 h incubation with arginine, although they did suppress arginase induction by 60 to 80%. The results suggest that induction of arginase in the growing axis of germinating seeds leads to increased arginine degradation and subsequendy to enhanced assimilation of nitrogen into amino acids rather than into polyamines.