[Use of 15N-labeled lysine for the determination of fractional protein synthesis rates by the flooding method].

The fractional synthesis rates (FSR) for liver, pancreas, small intestine, large intestine, skeleton muscle and whole body protein were estimated with the help of the large dose method of McNurlan et al. (1979) and Garlick et.al (1980) using simultaneously [14C]lysine and [15N]lysine as flooding substances in growing rats. The following results and conclusions can be drawn: In the case of lysine as flooding substance the incorporation time (time from the injection up to killing the animals) can be fixed at 20 minutes. On principle lysine is suitable as flooding substance. [14C]lysine and [15N]lysine can yield identical values for FSR in the same animal. The variation coefficients of the FSR values were 7.3% (4... 12%) using [14C]lysine and 13.8% (9.5... 36%) using [15N]lysine (emission-spectrometry) as flooding substance. Using the mass-spectrometric method for measuring 15N-excess it is possible to obtain the same accuracy of result as in the case of estimation [14C]lysin. The main advantage of using 15N-labelled amino acids as flooding substances is the absence of any radioactivity. Therefore this method is also suitable for farm animals and--in combination with the indolent muscle biopsy--it may be used for men.