Kinetics of the degradation of NG-nitro-L-arginine and its methyl ester in human umbilical vein blood and amniotic fluid.

The kinetics of the degradation of the inhibitors of the nitric oxide synthesis, NG-nitro-L-arginine methyl ester and NG-nitro-L-arginine, were examined in human amniotic fluid and umbilical vein blood. The reaction rate constants were calculated or estimated using the time-controlled concentration course of both substances. These concentrations were measured by high-performance liquid chromatography with two different separation systems: ion-exchange chromatography and ion-pair chromatography. Using this method, either NG-nitro-L-arginine methyl ester and/or NG-nitro-L-arginine were added to 18 samples of amniotic fluid, 33 samples of plasma and 21 samples of uncentrifuged umbilical vein blood samples; subsequently these samples were used for measurement. The degradation of the two individual study substances can be described by a uni-unimolecular two-step consecutive reaction. Thereby, NG-nitro-L-arginine methyl ester decomposes to NG-nitro-L-arginine. Although NG-nitro-L-arginine decomposed further, the decomposition product could not be identified. The average of the reaction rate constants for NG-nitro-L-arginine methyl ester/NG-nitro-L-arginine was determined, yielding the following values: 0.032 h-1/0.00047 h-1 in amniotic fluid, 0.029 h-1/0.00384 h-1 and 0.00074 h-1 in plasma and 0.80 h-1/0.00060 h-1 in uncentrifuged umbilical vein blood. During the first hours after sampling, these reaction rate constants could be used to approximate the concentrations of NG-nitro-L-arginine methyl ester and NG-nitro-L-arginine at the time of sampling.