The kinetics and dynamics of three kinds of radioactive methionine after i.v. administration in mice.

In C57BL mice, a tracer amount of methionine (Met) at a concentration approximate to that in the blood was administered in the tail vein (i.v.). The rates of endogenous metabolic decomposition of methionine were obtained by using three specifically labelled compounds, [1-14C]-Met, [m-14C]-Met and [35S]-Met. Results on the kinetics and dynamics after i.v. administration suggested that 5 min after administration, most of the labelled compounds were taken up into the organs and tissues, and only a small portion was excreted in the expired air as CO2, the final metabolite of methionine. At 30 min the radioactive concentration in blood was minimal and was consistent with the maximum of the 14CO2 excretion in expired air. Gradual increase of the radioactive concentration in blood after 30 min might be due to the contribution of the metabolite in the blood. Methionine taken up was endogenously utilized by more than 40% during 48 h and it maintained the order of [1-14C]:[m-14C]:[35S]. Out of the remaining 60%, 40% was metabolized in 6 h after administration, the [1-14C] moiety being excreted mainly into expired air, the [m-14C] moiety into expired air and urine, and the [35S] moiety mainly into urine and partly into faeces. Microautoradiograms revealed that a part of the last 20% was taken up into the enzyme proteins contained in the pancreatic juice and intestinal juice, and was decomposed within 48 h.