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Aspects of Sulfate Utilization by the Microorganisms of the Ovine Rumen^1,2,

Department of Experiment Station Biochemistry, South Dakota State University, Brookings, South Dakota

Strained rumen fluid, obtained from sheep ingesting alfalfa hay of 14% protein content and trace-mineralized salt, was incubated with a small additive portion containing sulfate alone or with other sulfur sources. The incubation was at 39° and for 30, 90 and 180 minutes. With the added sulfate being labeled (with ³⁵S) and introduced in known concentration, its occurrence in protein was followed separately from that of undetermined total sulfate. About maximal utilization of added sulfate-S for protein synthesis occurred when the additive sulfate was present at a 500 µM level. Thus, this concentration of sulfate appeared to furnish the dominant part of the sulfate in the medium. To gain further knowledge of the role of sulfide as an intermediate of sulfate-S in the synthesis of ruminal protein, the occurrence of sulfide within the enclosed fluid was also observed. The rate of production of sulfide was increased materially by adding sulfate (500 µM level). This extra production of sulfide usually exceeded the measured conversion of the added sulfate-S to sulfide. The conversion was not affected by the presence of other sulfur (500 µM level) as casein, methionine, cystine or sulfide. However, the overall production of sulfide was increased mildly by casein or methionine and markedly by cystine and these increases were associated with somewhat commensurate decreases in the incorporation of the added sulfate-S into protein. This effect was not consistent for casein. Added sulfide lowered the utilization of sulfate-S even more than did the presence of methionine or cystine. Thus, the organisms which utilized sulfate upon reduction to sulfide were apparently adapted to utilizing sulfide from other sources. A fluid which represented a different ingesta showed a lesser utilization of added sulfate (500 µM level) for protein synthesis. This was attributed to a higher content of original sulfate in the second fluid.

² Supported in part by an American Veterinary Medical Association Scholar Award to one of the investigators (G. D. W.).

³ Present address: Department of Nutrition and Food Science, MIT, Cambridge, Massachusetts 02139.

⁴ Present address: United States Department of Agriculture, ARS, Metabolism and Radiation Research Laboratory, State University Station, Fargo, North Dakota 58103.

Manuscript received 16 February 1968.