PROTEIN RECYCLING IN RABBITS: INCORPORATION OF MICROBIAL LYSINE IN GROWING RABBITS AS A METHOD OF MEASUREMENT

The incorporation in body tissues of lysine synthesised by the microflora of the rabbit caecum was estimated by 15 N labelling of microbial protein in six animals receiving a conventional diet supplemented with 15 NH4Cl (T1), using a control group of four rabbits fed the same unlabelled diet (T3). An additional group of six rabbits received the 15 Nlabelled diet, but just during the last ten days and they were provided with a neck collar (T2). The latter group was used to estimate caecotrophes production and find out if there is any microbial lysine incorporation in non-caecotrophagic animals. The experimental diets were administered for 35 days. Then animals were slaughtered, and caecal population and tissues were sampled. Lysine was isolated from tissues, microbes and food by ion exchange chromatography and its enrichment determined by Isotope Ratio Mass Spectrometry (IRMS). T1-rabbits incorporated 15 N-lysine into the body tissues in significant amounts (0.11 (SE 0.005) atom % excess (APE)), although 15 N-lysine enrichment was significantly lower than in bacteria (0.21 (SE 0.010) APE), confirming the double origin of this essential amino acid (dietary and microbial). The contribution of microbes to tissular lysine was calculated from the ratio of 15 N-lysine enrichment between tissues and bacteria. So, it was estimated that the contribution of microbial labelled lysine to total lysine absorbed was 0.46. T2-rabbits ingested the labelled diet only for ten days, but showed the same level of enrichment in caecal bacteria (0.22 (SE 0.034) APE) than T1-rabbits. However, tissular 15 N-lysine enrichment was much lower (0.02 (SE 0.003) APE), although still significantly higher than in control group. This fact shows a certain absorption of microbial lysine by non-caecotrophagic routes in rabbits, but it represented only 14 % of total microbial lysine absorption, much lower than that obtained by the reingestion of soft faeces.