Biology of Reproduction, Vol 25, 466-474, Copyright © 1981 by Society for the Study of Reproduction

Glucose-Stimulated Protein Synthesis in Rat Testis Slices: Substrate Specificity and Effects of Insulin and Substrate Analogs

¹ Division of Nutritional Sciences and the Section of Biochemistry, Cell and Molecular Biology, Division of Biological Sciences, Cornell University, Ithaca, New York 14853.

The effects of substrate, substrate concentration, insulin, and substrate analogs on the incorporation of lysine into rat testis slices were studied. Incorporation was stimulated by 10 mM glucose and 10 mM mannose, to a lesser extent by 10 mM fructose, but not by galactose, glucitol, or xylitol. Half-maximal stimulation occurred at 2 mM glucose or 2 mM mannose and was maximal at concentrations above 4 mM. Fructose at a concentration of 20 mM was about as effective as 2 mM glucose. The glucose-stimulated incorporation was not affected by insulin. Several analogs were tested at a final concentration of 10 mM as inhibitors of glucose- and fructose-stimulated lysine incorporation. At a glucose concentration of 4 mM, only 2-deoxyglucose and 2-deoxygalactose were inhibitory, reducing incorporation to levels not significantly different from basal. When the glucose concentration was reduced to 2 mM, other analogs were also inhibitory; the most effective of these were 5-thioglucose, 3-0-methylglucose, 4,6-ethylideneglucose, and galactosamine. When 10 mM fructose was used as a substrate, the same analogs plus the fructose analogs mannoheptulose, 1-deoxyfructose, 2,5-anhydromannose, and 2,5-anhydromannitol were inhibitory. Differences in analog inhibitions against fructose and glucose suggest different rate-limiting steps for the stimulation by these two sugars. It is proposed that the response to glucose and mannose, but not fructose, is mediated by an uptake system which is limiting at low substrate concentrations. Inhibition by various sugar analogs generally results from uptake system which is limiting at low substrate concentrations. For most of the analogs this occurs under conditions in which a step subsequent to the hexose uptake system, possibly a phosphorylation step, is not saturated.

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ACKNOWLEDGMENTS We wish to thank Drs. Robert Foote and Robert Wall of the Dept. of Animal Science, Cornell University, for helpful discussions. The technical assistance of Tom Covey and Paul Singer is gratefully acknowledged.