Research, in both New Zealand and Australia, is being directed towards enhancing the sulphur amino acid supply of sheep by either plant or animal modification. This study investigated the effects of such improved systeine supply in well-fed sheep on both whole body and skin metabolism of cysteine.. The level of cysteine supplementation chosen (2g/d) represented about a 50% increase in absorbed cysteine. Cysteine was supplied by jugular infusion to avoid loss during gut absorption. At this level of cysteine supplementation a preliminary trial on sheep fed either 700 g or 1250 g lucerne chaff showed no significant effect on wool production but a significant increase in wool sulfur (S) concentration over a one month period of treatment. In a second trial, two groups of five, 40kg, cryptorchid Romney sheep were offered 1200 g/d of lucerne chaff and one group was supplemented with 2g/d cysteine for five days prior to measurement, with the other group receiving saline only. Cysteine supplementation increased whole body cysteine utilization (irreversible loss of 35S-cysteine) from 4.7 ± 0.4 to 6.4 ± 0.3 g/d, while appearance of 35S-cysteine in the oxidation pool increased by 37%. Cysteine supplementation resulted in an increase in circulating cysteine concentration (33.8 ± 5.6 to 51.1 ± 12.0 µM) but blood flow to the skin patch (measured by dye dilution) was reduced from 0.29 ± 0.03 to 0.21 ± 0.02ml. min-1.g-1 skin. Hence the cysteine flux through the skin (blood flow times concentration) was unchanged. Nevertheless uptake of 35S-cysteine by the skin increased with supplementation from 12.8 ± 2.7 to 22.8 ± 2.7%, resulting in a significant increase in skin utilization of cysteine for protein synthesis. The results suggest that enhancing cysteine supply within the sheep does not proportionately improve cysteine availability for protein synthesis. Instead a large proportion of the extra supply is oxidised. Additionally, although enhanced whole body cysteine supply does not improve skin cysteine supply (or flux), increased arterial cysteine concentration appears to stimulate skin cysteine transport into cells so that increased cysteine utilization for protein synthesis is still achieved.
Proceedings of the New Zealand Society of Animal Production, Volume 54, , 139-142, 1994
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