[35S excretion in rats after injection of 35S methionine in relation to protein and amino acid nutrition].

Experimental rats received purified or semisynthetic diets containing 10% or 6% crude protein (Soya protein). These were supplemented with 0.1% or 0.2% L-, D- or DL methionine. After a 5-day feeding period the animals were injected 35S-DL-methionine. For 48 hrs after 35S administration the pattern of urinary 35S excretion was traced. Neither the level of protein does not the configuration of the methionine supplemented nor the level of methionine supplementation were found to influence urinary 35S excretion in any way. Amino acid imbalance as induced by supplementation of 5% DL threonine produced a highly significant increase in the rate of 35S excretion on the 1st day of experiment. The results obtained suggest the conclusion that any determination of methionine requirements based on metabolic data should leave out from consideration values on the level of urinary 35S excretion obtained on the first day of experiment, as these are dependent on amino acid transport rather than on protein metabolism.