Short-term changes in whole body and skin sulfur amino acid metabolism of sheep in response to supplementary cysteine

Abstract

Two groups of five 40-kg, 12–15-month-old, cryptorchid Romney sheep were housed indoors in metabolism crates at 18°C and fed lucerne (Medicago sativa) chaff at above-maintenance intakes. At 2-weekly intervals, 1 sheep from each group underwent surgery to insert arteriovenous catheters across the abdominal skin patch. One group of sheep was continuously infused with 2 g/day of cysteine (Cys) in sterile physiological saline via the jugular, and the other group received an infusion of saline only. After 5 days of infusion, measurements were made in each sheep comparing the whole body metabolism of Cys and skin metabolism of Cys and methionine (Met). The groups were then reversed in treatment 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 Cys-supplemented sheep but there were no significant treatment differences in glutathione or Met concentration. The irreversible loss rate (ILR) of Cys was increased significantly (P < 0·01) in the Cys-supplemented 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 reduced in the Cys-supplemented group, resulting in an unchanged flux of Cys 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 was no net efflux of oxidation products of Cys or Met in the venous drainage of either treatment group.

The effect of short-term supplementation with Cys directly into the peripheral circulation of the sheep was an increase in whole body catabolism of both Cys and Met that would reduce 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 for protein synthesis. The skin does not export oxidation products of either Cys or Met, and so all the enhanced skin uptake of Cys in response to supplementation is directed towards protein synthesis, but this is probably predominantly used to change the expression of individual proteins rather than to enhance total protein synthesis.