The effect of 5-thio-d-glucopyranose (thioglucose) upon protein biosynthesis in vitro was examined in testes from mature rats. Thioglucose in vitro is without demonstrable effect upon incorporation of l-[U-14C]phenylalanine into protein by whole testis but inhibits this incorporation by a purified fraction of immature spermatids (stages 1-8) prepared by centrifugal elutriation; the inhibition is observed with or without glucose added in vitro and is concentration dependent in the range 1-50 mM. Similar inhibition is observed with three other 14C-labeled amino acids (leucine, lysine and glutamate). Mature spermatids (>stage 8) and other heterogeneous fractions of testicular cells prepared by the same method also show inhibition by thioglucose of incorporation of phenylalanine into protein so that it is not possible to say that the effect is confined to spermatids although it is most pronounced in these cells. Inhibition of protein synthesis in vitro is also observed when thioglucose was administered in vivo (33 mg/kg body wt./day). This change occurs at the minimal dose observed by other workers to produce arrest of spermatogenesis and hence infertility.
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