SHORT-TERM CHANGES IN WHOLE-BODY AND SKIN SULFUR AMINO-ACID-METABOLISM OF SHEEP IN RESPONSE TO SUPPLEMENTARY CYSTEINE

Two groups of five 40-kg, 12-15-month-old, cryptorchid Romney sheep we re housed indoors in metabolism crates at 18 degrees C and fed lucerne (Medicago sativa) chaff at above-maintenance intakes. At 2-weekly int ervals, 1 sheep from each group underwent surgery to insert arterioven ous catheters across the abdominal skin patch. One group of sheep was continuously infused with 2 g/day of cysteine (Cys) in sterile physiol ogical saline via the jugular, and the other group received an infusio n of saline only. After 5 days of infusion, measurements were made in each sheep comparing the whole body metabolism of Cys and skin metabol ism of Cys and methionine (Met). The groups were then reversed in trea tment and 2 days later the same measurements were repeated followed by a comparison of whole body and skin Met metabolism. There was a trend (P = 0 . 07) for arterial Cys concentration to be increased in the Cy s-supplemented sheep but there were no significant treatment differenc es in glutathione or Met concentration. The irreversible loss rate (IL R) of Cys was increased significantly (P < 0 . 01) in the Cys-suppleme nted sheep. Proportional oxidation of Cys was very significantly (P < 0 . 001) increased across the Cys-supplemented sheep. Met ILR was not related to the Cys ILR. Blood flow to the skin was significantly reduc ed in the Cys-supplemented group, resulting in an unchanged flux of Cy s past the skin. There was a trend (P = 0 . 06) for both net and total uptake of Cys to be increased in the Cys-supplemented sheep. There wa s no net efflux of oxidation products of Cys or Met in the venous drai nage of either treatment group. The effect of short-term supplementati on with Cys directly into the peripheral circulation of the sheep was an increase in whole body catabolism of both Cys and Met that would re duce the efficacy of the supplement. Of the 2 g/day of additional Cys supplied, about 25-30% was able to be taken up by the skin and used fo r protein synthesis. The skin does not export oxidation products of ei ther Cys or Met, and so all the enhanced skin uptake of Cys in respons e to supplementation is directed towards protein synthesis, but this i s probably predominantly used to change the expression of individual p roteins rather than to enhance total protein synthesis.