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The Effect of Dietary Protein on Nitrogen and Sulfur Metabolism in Portacaval-Shunted Rats¹

Department of Nutritional Sciences, University of Wisconsin, Madison, WI 53706

The effect of varying the amount of protein in the diet on postoperative recovery, plasma ammonia, urinary orotic acid and metabolism of sulfur-containing amino acids was examined in rats with portacaval shunts (PCS). Food intake and weight gain were lower in both PCS and control rats fed a low (6%) casein diet unsupplemented with methionine compared with rats fed an adequate (18%) casein diet. PCS rats fed 60% casein ate slightly less and took longer to recover their preoperative body weight compared to 60% controls. Shunted rats were consistently hyperammonemic and orotic aciduric compared to controls. Increasing protein in the diet elevated plasma ammonia and urinary orotic acid in all rats to levels above those of the rats fed 18% casein, but the effect was greater in rats with PCS. After i.p. injection of L-[³⁵S]methionine or L-[³⁵S]cysteine, urinary ³⁵S and [³⁵S]sulfate excretion increased and [³⁵S]taurine and total taurine excretion decreased in all rats fed 60% casein. These changes are consistent with our observation that hepatic activities of cysteine dioxygenase and cysteine sulfinate:-ketoglutarate aminotransferase increased and that of cysteine sulfinate decarboxylase decreased in rats fed the high protein diet. The effect of dietary treatment on both urinary taurine excretion and decarboxylase activity was greater in PCS rats than in controls. Although PCS rats fed a high protein diet may have a decreased taurine-synthesizing capability compared to controls, their ability to oxidize a methionine or cysteine load to sulfate is not compromised by feeding either an 18 or 60% casein diet.

KEY WORDS: • portacaval shunt • hyperammonemia • orotic acid • methionine • cysteine

¹ This work was supported in part by United States Public Health Service grant AM-32521, the University of Wisconsin College of Agricultural and Life Sciences, and the Graduate School.

² To whom requests for reprints should be sent.

Manuscript received 19 March 1985. Revision accepted 24 September 1985.