Metabolic studies with seleniated compounds: II—Turnover studies with Se 75-methionine in rats

Abstract

Tracer amounts of Se 75-methionine were injected intravenously into adult female rat and the turnover of labeled plasma proteins as well as the distribution of the isotope in subcellular fractions of the liver were studied between 5 min and 6 hr after administration. Plasma proteins incorporate rapidly the labeled amino-acid; the largest amount of Se 75 was found in the α globulin, followed by the γ globulin. The rate of incorporation remains unchanged in animals with sub-total hepatectomy, and even in those with total hepatectomy; moreover, a small proportion of seleno-methionine is bound to plasma proteins after incubation in vitro with total blood. These results suggest that the biosynthesis of plasma proteins can take place by transpeptidation between the circulating proteins and the free amino-acids; this does not preclude the template assembly of amino-acids on the ribosome, but offers the possibility of both mechanisms acting simultaneously in some cases. Supplementary evidence is offered by the fact that the microsomal fraction of the liver did not exhibit any significant increase of Se 75 during the period of time when the amino-acid is supposed to be bound to the ribosome, but not yet released into the circulation. It is not established whether the mechanism of transpeptidation is quantitatively important, and also whether it is peculiar to selenomethionine or a more generalized phenomenon. There is no pulmonary excretion of Se 75 after intravenous injection of Se 75-methionine, in contrast to the significant lung excretion of isotope after administration of selenite; the main excretion route of seleno-methionine is the kidney; fecal and biliary excretion are lower during the first 6 hr after administration. The highest differentiation between pancreas and liver is obtained in rat at 3 hr after injection of Se 75-methionine, while the highest difference between pancreas and blood occurs at 1 hr after injection. Thus, if animal results can be extrapolated to humans, it might be advisable to make an early scanning in cases of severe pancreas congestion.