Biochemical and regulatory effects of methionine analogues in Saccharomyces cerevisiae

The effect of three methionine analogues, ethionine, selenomethionine, and trifluoromethionine, on the biosynthesis of methionine in Saccharomyces cerevisiae has been investigated. We have found the following to be true. (i) A sharp decrease in the endogenous methionine concentration occurs after the addition of any one of these analogues to growing cells. (ii) All of them can be transferred to methionine transfer ribonucleic acid in vitro as well as in vivo with, as a consequence, their incorporation into proteins. In the absence of radioactive trifluoromethionine, this conclusion results from experiments of an indirect nature and must be taken as an indication rather than a direct demonstration. (iii) Ethionine and selenomethionine can be activated as homologues of S-adenosylmethionine, whereas trifluoromethionine cannot. (iv) All of them can act as repressors of the methionine biosynthetic pathway. This has been shown by measuring the de novo rate of synthesis of methionine in a culture grown in the presence of any one of the three analogues.

[Granulopoiesis: comparison of several methods for studying production and bone marrow cellularity (author's transl)]

Several methods measuring the granulocyte production have been compared in normal subjects, hypoplastic granulopenia, chronic leucopenia and acute leukemia: the granulocyte turnover-rate after injection of in vitro 51Cr labelled autologous granulocytes, the efficiency of 75Se-selenothionine in vivo labelling, the flow-out of the myelocyte compartment after in vitro flash-labelling with 3H-thymidine. The two first methods are correlated and measure the global effective production of granulocytes. The third method is a fractional measure of the bone marrow production and is not correlated with the former ones. The ratio between the granulocyte turnover-rate and the flow-out of the bone marrow proliferative pool is a measure of the bone marrow cellularity. Discrepancies between this measure and the granulocyte turnover-rate has been found only in leukaemic patients.

75Se-selenomethionine excretion in bile and pancreatic juice

The radioactivity in the bile and pancreatic juice of two patients was measured after intravenous injection of 75Se-l-selenomethionine. The concentration of radioactivity in both fluids is almost equal. Thin-layer chromatography shows that the main part of radioactivity in pancreatic juice is due to unbound 75Se-selenomethionine, while free 75Se-selenomethionine is only a minor part of the total radioactivity appearing in the bile.