Metabolism of Cystine by Merino Sheep Genetically Different in Wool Production III. the Incorporation of Radioactivity Into Wool Fibres During and After Intravenous Infusions of l-[35S]Cystine and its Relationship to Wool Growth and Efficiency of Conversi

Abstract

Twelve mature ewes from a flock selected for high clean fleece weight (Fleece Plus) and twelve from a flock selected for low clean fleece weight (Fleece Minus) were randomly divided between two dietary treatments: 500 or 1100 g per day of chaffed lucerne hay. After the sheep experienced these dietary regimes for 40 days, each ewe was infused intravenously with L-[35Sjcystine for 12-14 hr. Incorporation of 35S activity into fibres during the infusions, and into wool clipped subsequent to the infusions, was measured.During the final 7-9 hr of infusion, the 35S activity in plucked fibres increased linearly with time. The linear regression coefficients relating 35S activity to time did not differ in the genetic or the dietary comparisons, regardless of whether the rate of incorporation of 35S was expressed relative to a 1000 fibres, or to the number of fibres growing from unit area of skin. The mean specific radioactivity of cystine incorporated into wool proteins during the infusions was less in ewes consuming 1100 g per day of lucerne (23 v. 35 nCi/mg : P < 0'05). This difference was less than the difference in the "plateau" specific radioactivity of cystine in the plasma, indicating that the 35S was differentially diluted with "cold" cystine during its transfer from plasma into the wool fibre; dilution being greater in ewes consuming the lesser quantity of feed (4, 0 v. 2·9 :A similar effect of feeding level was evident on the specific radioactivity of cystine in wool clipped from the ewes subsequent to the infusions. In addition, the specific radioactivity of cystine in wool from Fleece Plus ewes was less than that from Fleece Minus ewes (3, 2 v. 3· 6 nCi/mg : P < O· 05), indicating that greater dilution of 35S with "cold" cystine was occurring in these ewes. As judged by the wool production per unit area of skin, the efficiency of conversion of food to wool would have been greater in Fleece Plus ewes and in those consuming 500 g per day of lucerne. Thus, mechanisms controlling the dilution 0[35S between plasma and fibre, presumably arising

Metabolism of Cystine by Merino Sheep Genetically Different in Wool Production IV. Rates of Entry of Cystine Into Plasma, Measured With a Single Intravenous Injection of L- [35S] Cystine, and the Subsequent Incorporation of 35S Into Wool Fibres

“…A significantly greater proportion of the injected radioactivity was recovered in wool clipped from Fleece Plus ewes, and from ewes offered the larger quantity of food (Table 2), confirming previous results (Williams 1973). There, the results were equivocal due to possible bias introduced by variations among treatment groups in emergence times of the fibres (Downes and Sharry 1971).…”

“…(2) The wool grown by Fleece Plus ewes had a lower sulphur content, and the difference was independent of dietary intake (Williams 1973). In the present experiment, the efficiency of conversion of food into wool was not influenced by the level of intake.…”

“…The specific radioactivity of cystine in the wool fibres following the continuous infusion of [ 35 S]cystine was less in Fleece Plus ewes than in Fleece Minus ewes (Williams 1973), despite the similar mean specific radioactivities of cystine in plasma from the two flocks. An intermediate pool of cystine, causing dilution of the label during its passage from plasma into the wool fibre, apparently functioned differently in the two flocks.…”

“…This parameter was not significantly affected by the experimental treatments. In view of the greater proportion of 35S recovered in wool clipped from Fleece Plus ewes, total follicle number, rather than the ability of individual follicles, would be a major factor determining the output of cystine in wool which was related to the proportion of 35S recovered in clipped wool (Williams 1973). While differences in follicle number may account for the results in the genetic comparison, there would have to be 20-30 % fewer follicles in the ewes offered 600 g lucerne/day to account for the similar values of Xl + X 2 + X3 at the two levels of intake, but with a greater recovery of 35S in clipped wool among the ewes offered 1100 g lucerne/day.…”

Sulfur and Methionine Metabolism in Sheep. V. Utilization of Methionine Isomers

“…In addition, if the sulfur metabolism of sheep on the control diet was different from that of sheep receiving the supplements, then estimates of the amount of sulfur excreted from cold supplements might be incorrect. The use of isotopes to trace the metabolism of supplements is also complicated by factors such as differential dilution of radioactive sulfur with cold sulfur as discussed by Williams (1973). In the present experiment, dilution effects could be due to the presence of methionine which was not in isotopic equilibrium with the injected radioactive material, that is feed methionine, methionine from endogenous inputs into the gastrointestinal tract, infused methionine (as mentioned above) and methionine in the sheep's tissues.…”