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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.

The aim of our experiments was to identify a restricted lysine bioavailability after heating of wheat gluten by estimating a reduced metabolic 14C-lysine degradation. In two trials, male broiler chickens were fed with six diets based on wheat and wheat gluten (gluten untreated or heated), but differing in lysine content according to lysine supplementation. In trial 1 animals were fed restrictively, in trial 2 they were fed ad libitum. For estimation of metabolic lysine degradation all animals received an additional i.v. injection of 14C-U-L-lysine 3 weeks posthatching, followed by hourly collection of 14CO2 up to 3 h after injection. There were no differences between groups receiving untreated or heated gluten concerning weight gain and N-balance if the lysine supplementation was medium or high. When applying a lysine supply close to the requirement level or above the requirement the lysine degradation to 14CO2 (% of the dose) and the specific radioactivity of CO2 in animals receiving heated gluten was significantly lower compared to the corresponding group with untreated gluten. It can be concluded that reduced bioavailability of lysine due to heat treatment of gluten might be indicated by means of weight gain or N-balance only at lysine supply levels below the requirement. In contrast, measurements of lysine degradation by means of 14CO2-excretion after i.v. lysine injection indicate the heat-damaging effect, especially at lysine levels close to the requirement.